text: /* bcm2835.h
    
     C and C++ support for Broadcom BCM 2835 as used in Raspberry Pi
    
     Author: Mike McCauley
     Copyright (C) 2011-2013 Mike McCauley
     $Id: bcm2835.h,v 1.20 2015/03/31 04:55:41 mikem Exp mikem $
  */
  
  /*! \mainpage C library for Broadcom BCM 2835 as used in Raspberry Pi
    
    This is a C library for Raspberry Pi (RPi). It provides access to 
    GPIO and other IO functions on the Broadcom BCM 2835 chip,
    allowing access to the GPIO pins on the
    26 pin IDE plug on the RPi board so you can control and interface with various external devices.
    
    It provides functions for reading digital inputs and setting digital outputs, using SPI and I2C,
    and for accessing the system timers.
    Pin event detection is supported by polling (interrupts are not supported).
    
    It is C++ compatible, and installs as a header file and non-shared library on 
    any Linux-based distro (but clearly is no use except on Raspberry Pi or another board with 
    BCM 2835).
    
    The version of the package that this documentation refers to can be downloaded 
    from http://www.airspayce.com/mikem/bcm2835/bcm2835-1.45.tar.gz
    You can find the latest version at http://www.airspayce.com/mikem/bcm2835
    
    Several example programs are provided.
    
    Based on data in http://elinux.org/RPi_Low-level_peripherals and 
    http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf
    and http://www.scribd.com/doc/101830961/GPIO-Pads-Control2
    
    You can also find online help and discussion at http://groups.google.com/group/bcm2835
    Please use that group for all questions and discussions on this topic. 
    Do not contact the author directly, unless it is to discuss commercial licensing.
    Before asking a question or reporting a bug, please read http://www.catb.org/esr/faqs/smart-questions.html
    
    Tested on debian6-19-04-2012, 2012-07-15-wheezy-raspbian, 2013-07-26-wheezy-raspbian
    and Occidentalisv01
    CAUTION: it has been observed that when detect enables such as bcm2835_gpio_len() 
    are used and the pin is pulled LOW
    it can cause temporary hangs on 2012-07-15-wheezy-raspbian, 2013-07-26-wheezy-raspbian
    and Occidentalisv01.
    Reason for this is not yet determined, but we suspect that an interrupt handler is
    hitting a hard loop on those OSs.
    If you must use bcm2835_gpio_len() and friends, make sure you disable the pins with 
    bcm2835_gpio_clr_len() and friends after use. 
    
    \par Installation
    
    This library consists of a single non-shared library and header file, which will be
    installed in the usual places by make install
    
    \code
    # download the latest version of the library, say bcm2835-1.xx.tar.gz, then:
    tar zxvf bcm2835-1.xx.tar.gz
    cd bcm2835-1.xx
    ./configure
    make
    sudo make check
    sudo make install
    \endcode
    
    \par Physical Addresses
    
    The functions bcm2835_peri_read(), bcm2835_peri_write() and bcm2835_peri_set_bits() 
    are low level peripheral register access functions. They are designed to use
    physical addresses as described in section 1.2.3 ARM physical addresses
    of the BCM2835 ARM Peripherals manual. 
    Physical addresses range from 0x20000000 to 0x20FFFFFF for peripherals. The bus
    addresses for peripherals are set up to map onto the peripheral bus address range starting at
    0x7E000000. Thus a peripheral advertised in the manual at bus address 0x7Ennnnnn is available at
    physical address 0x20nnnnnn.
    
    On RPI 2, the peripheral addresses are different and the bcm2835 library gets them 
    from reading /proc/device-tree/soc/ranges. This is only availble with recent versions of the kernel on RPI 2.
    
    After initialisation, the base address of the various peripheral 
    registers are available with the following
    externals:
    bcm2835_gpio
    bcm2835_pwm
    bcm2835_clk
    bcm2835_pads
    bcm2835_spio0
    bcm2835_st
    bcm2835_bsc0
    bcm2835_bsc1
  
    \par Raspberry Pi 2 (RPI2)
  
    For this library to work correctly on RPI2, you MUST have the device tree support enabled in the kernel.
    You should also ensure you are using the latest version of Linux. The library has been tested on RPI2
    with 2015-02-16-raspbian-wheezy and ArchLinuxARM-rpi-2 as of 2015-03-29.
  
    When device tree suport is enabled, the file /proc/device-tree/soc/ranges will appear in the file system, 
    and the bcm2835 module relies on its presence to correctly run on RPI2 (it is optional for RPI1). 
    Without device tree support enabled and the presence of this file, it will not work on RPI2.
  
    To enable device tree support:
  
    \code
    sudo raspi-config
     under Advanced Options - enable Device Tree
     Reboot.
    \endcode
    
    \par Pin Numbering
    
    The GPIO pin numbering as used by RPi is different to and inconsistent with the underlying 
    BCM 2835 chip pin numbering. http://elinux.org/RPi_BCM2835_GPIOs
     
    RPi has a 26 pin IDE header that provides access to some of the GPIO pins on the BCM 2835,
    as well as power and ground pins. Not all GPIO pins on the BCM 2835 are available on the 
    IDE header.
    
    RPi Version 2 also has a P5 connector with 4 GPIO pins, 5V, 3.3V and Gnd.
    
    The functions in this library are designed to be passed the BCM 2835 GPIO pin number and _not_ 
    the RPi pin number. There are symbolic definitions for each of the available pins
    that you should use for convenience. See \ref RPiGPIOPin.
    
    \par SPI Pins
     
    The bcm2835_spi_* functions allow you to control the BCM 2835 SPI0 interface, 
    allowing you to send and received data by SPI (Serial Peripheral Interface).
    For more information about SPI, see http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus
    
    When bcm2835_spi_begin() is called it changes the bahaviour of the SPI interface pins from their 
    default GPIO behaviour in order to support SPI. While SPI is in use, you will not be able 
    to control the state of the SPI pins through the usual bcm2835_spi_gpio_write().
    When bcm2835_spi_end() is called, the SPI pins will all revert to inputs, and can then be
    configured and controled with the usual bcm2835_gpio_* calls.
    
    The Raspberry Pi GPIO pins used for SPI are:
     
    - P1-19 (MOSI)
    - P1-21 (MISO) 
    - P1-23 (CLK) 
    - P1-24 (CE0) 
    - P1-26 (CE1)
    
    \par I2C Pins
    
    The bcm2835_i2c_* functions allow you to control the BCM 2835 BSC interface,
    allowing you to send and received data by I2C ("eye-squared cee"; generically referred to as "two-wire interface") .
    For more information about I?C, see http://en.wikipedia.org/wiki/I%C2%B2C
    
    The Raspberry Pi V2 GPIO pins used for I2C are:
    
    - P1-03 (SDA)
    - P1-05 (SLC)
    
    \par PWM
    
    The BCM2835 supports hardware PWM on a limited subset of GPIO pins. This bcm2835 library provides 
    functions for configuring and controlling PWM output on these pins.
    
    The BCM2835 contains 2 independent PWM channels (0 and 1), each of which be connnected to a limited subset of 
    GPIO pins. The following GPIO pins may be connected to the following PWM channels (from section 9.5):
    \code
    GPIO PIN    RPi pin  PWM Channel    ALT FUN
    12                    0            0
    13                    1            0
    18         1-12       0            5
    19                    1            5
    40                    0            0
    41                    1            0
    45                    1            0
    52                    0            1
    53                    1            1
    \endcode
    In order for a GPIO pin to emit output from its PWM channel, it must be set to the Alt Function given above.
    Note carefully that current versions of the Raspberry Pi only expose one of these pins (GPIO 18 = RPi Pin 1-12)
    on the IO headers, and therefore this is the only IO pin on the RPi that can be used for PWM.
    Further it must be set to ALT FUN 5 to get PWM output.
    
    Both PWM channels are driven by the same PWM clock, whose clock dvider can be varied using 
    bcm2835_pwm_set_clock(). Each channel can be separately enabled with bcm2835_pwm_set_mode().
    The average output of the PWM channel is determined by the ratio of DATA/RANGE for that channel.
    Use bcm2835_pwm_set_range() to set the range and bcm2835_pwm_set_data() to set the data in that ratio
    
    Each PWM channel can run in either Balanced or Mark-Space mode. In Balanced mode, the hardware 
    sends a combination of clock pulses that results in an overall DATA pulses per RANGE pulses.
    In Mark-Space mode, the hardware sets the output HIGH for DATA clock pulses wide, followed by 
    LOW for RANGE-DATA clock pulses. 
    
    The PWM clock can be set to control the PWM pulse widths. The PWM clock is derived from 
    a 19.2MHz clock. You can set any divider, but some common ones are provided by the BCM2835_PWM_CLOCK_DIVIDER_*
    values of \ref bcm2835PWMClockDivider.
     
    For example, say you wanted to drive a DC motor with PWM at about 1kHz, 
    and control the speed in 1/1024 increments from 
    0/1024 (stopped) through to 1024/1024 (full on). In that case you might set the 
    clock divider to be 16, and the RANGE to 1024. The pulse repetition frequency will be
    1.2MHz/1024 = 1171.875Hz.
    
    \par SPI
  
    In order for bcm2835 library SPI to work, you may need to disable the SPI kernel module using:
  
    \code
    sudo raspi-config
     under Advanced Options - enable Device Tree
     under Advanced Options - disable SPI
     Reboot.
    \endcode
  
    \par Real Time performance constraints
    
    The bcm2835 is a library for user programs (i.e. they run in 'userland'). 
    Such programs are not part of the kernel and are usually
    subject to paging and swapping by the kernel while it does other things besides running your program. 
    This means that you should not expect to get real-time performance or 
    real-time timing constraints from such programs. In particular, there is no guarantee that the 
    bcm2835_delay() and bcm2835_delayMicroseconds() will return after exactly the time requested. 
    In fact, depending on other activity on the host, IO etc, you might get significantly longer delay times
    than the one you asked for. So please dont expect to get exactly the time delay you request.
    
    Arjan reports that you can prevent swapping on Linux with the following code fragment:
    
    \code
    struct sched_param sp;
    memset(&sp, 0, sizeof(sp));
    sp.sched_priority = sched_get_priority_max(SCHED_FIFO);
    sched_setscheduler(0, SCHED_FIFO, &sp);
    mlockall(MCL_CURRENT | MCL_FUTURE);
    \endcode
    
    \par Bindings to other languages
    
    mikem has made Perl bindings available at CPAN:
    http://search.cpan.org/~mikem/Device-BCM2835-1.9/lib/Device/BCM2835.pm
    Matthew Baker has kindly made Python bindings available at:
    https:  github.com/mubeta06/py-libbcm2835
    Gary Marks has created a Serial Peripheral Interface (SPI) command-line utility 
    for Raspberry Pi, based on the bcm2835 library. The 
    utility, spincl, is licensed under Open Source GNU GPLv3 by iP Solutions (http://ipsolutionscorp.com), as a 
    free download with source included: http://ipsolutionscorp.com/raspberry-pi-spi-utility/
    
    \par Open Source Licensing GPL V2
    
    This is the appropriate option if you want to share the source code of your
    application with everyone you distribute it to, and you also want to give them
    the right to share who uses it. If you wish to use this software under Open
    Source Licensing, you must contribute all your source code to the open source
    community in accordance with the GPL Version 2 when your application is
    distributed. See http://www.gnu.org/copyleft/gpl.html and COPYING
    
    \par Acknowledgements
    
    Some of this code has been inspired by Dom and Gert.
    The I2C code has been inspired by Alan Barr.
     
    \par Revision History
    
    \version 1.0 Initial release
  
    \version 1.1 Minor bug fixes
  
    \version 1.2 Added support for SPI
  
    \version 1.3 Added bcm2835_spi_transfern()
  
    \version 1.4 Fixed a problem that prevented SPI CE1 being used. Reported by David Robinson.
  
    \version 1.5 Added bcm2835_close() to deinit the library. Suggested by C?sar Ortiz
  
    \version 1.6 Document testing on 2012-07-15-wheezy-raspbian and Occidentalisv01
    Functions bcm2835_gpio_ren(), bcm2835_gpio_fen(), bcm2835_gpio_hen()
    bcm2835_gpio_len(), bcm2835_gpio_aren() and bcm2835_gpio_afen() now 
    changes only the pin specified. Other pins that were already previously
    enabled stay enabled.
    Added  bcm2835_gpio_clr_ren(), bcm2835_gpio_clr_fen(), bcm2835_gpio_clr_hen()
    bcm2835_gpio_clr_len(), bcm2835_gpio_clr_aren(), bcm2835_gpio_clr_afen() 
    to clear the enable for individual pins, suggested by Andreas Sundstrom.
  
    \version 1.7 Added bcm2835_spi_transfernb to support different buffers for read and write.
  
    \version 1.8 Improvements to read barrier, as suggested by maddin.
  
    \version 1.9 Improvements contributed by mikew: 
    I noticed that it was mallocing memory for the mmaps on /dev/mem.
    It's not necessary to do that, you can just mmap the file directly,
    so I've removed the mallocs (and frees).
    I've also modified delayMicroseconds() to use nanosleep() for long waits,
    and a busy wait on a high resolution timer for the rest. This is because
    I've found that calling nanosleep() takes at least 100-200 us.
    You need to link using '-lrt' using this version.
    I've added some unsigned casts to the debug prints to silence compiler
    warnings I was getting, fixed some typos, and changed the value of
    BCM2835_PAD_HYSTERESIS_ENABLED to 0x08 as per Gert van Loo's doc at
    http://www.scribd.com/doc/101830961/GPIO-Pads-Control2
    Also added a define for the passwrd value that Gert says is needed to
    change pad control settings.
  
    \version 1.10 Changed the names of the delay functions to bcm2835_delay() 
    and bcm2835_delayMicroseconds() to prevent collisions with wiringPi.
    Macros to map delay()-> bcm2835_delay() and
    Macros to map delayMicroseconds()-> bcm2835_delayMicroseconds(), which
    can be disabled by defining BCM2835_NO_DELAY_COMPATIBILITY
  
    \version 1.11 Fixed incorrect link to download file
  
    \version 1.12 New GPIO pin definitions for RPi version 2 (which has a different GPIO mapping)             
  
    \version 1.13 New GPIO pin definitions for RPi version 2 plug P5
    Hardware base pointers are now available (after initialisation) externally as bcm2835_gpio
    bcm2835_pwm bcm2835_clk bcm2835_pads bcm2835_spi0.
  
    \version 1.14 Now compiles even if CLOCK_MONOTONIC_RAW is not available, uses CLOCK_MONOTONIC instead.
    Fixed errors in documentation of SPI divider frequencies based on 250MHz clock. 
    Reported by Ben Simpson.
  
    \version 1.15 Added bcm2835_close() to end of examples as suggested by Mark Wolfe.
  
    \version 1.16 Added bcm2835_gpio_set_multi, bcm2835_gpio_clr_multi and bcm2835_gpio_write_multi
    to allow a mask of pins to be set all at once. Requested by Sebastian Loncar.
  
    \version 1.17  Added bcm2835_gpio_write_mask. Requested by Sebastian Loncar.
  
    \version 1.18 Added bcm2835_i2c_* functions. Changes to bcm2835_delayMicroseconds: 
    now uses the RPi system timer counter, instead of clock_gettime, for improved accuracy. 
    No need to link with -lrt now. Contributed by Arjan van Vught.
    \version 1.19 Removed inlines added by previous patch since they don't seem to work everywhere. 
    Reported by olly.
  
    \version 1.20 Patch from Mark Dootson to close /dev/mem after access to the peripherals has been granted.
  
    \version 1.21 delayMicroseconds is now not susceptible to 32 bit timer overruns. 
    Patch courtesy Jeremy Mortis.
  
    \version 1.22 Fixed incorrect definition of BCM2835_GPFEN0 which broke the ability to set 
    falling edge events. Reported by Mark Dootson.
  
    \version 1.23 Added bcm2835_i2c_set_baudrate and bcm2835_i2c_read_register_rs. 
    Improvements to bcm2835_i2c_read and bcm2835_i2c_write functions
    to fix ocasional reads not completing. Patched by Mark Dootson.
  
    \version 1.24 Mark Dootson p[atched a problem with his previously submitted code
    under high load from other processes. 
  
    \version 1.25 Updated author and distribution location details to airspayce.com
  
    \version 1.26 Added missing unmapmem for pads in bcm2835_close to prevent a memory leak. 
    Reported by Hartmut Henkel.
  
    \version 1.27 bcm2835_gpio_set_pad() no longer needs BCM2835_PAD_PASSWRD: it is
    now automatically included.
    Added suport for PWM mode with bcm2835_pwm_* functions.
  
    \version 1.28 Fixed a problem where bcm2835_spi_writenb() would have problems with transfers of more than
    64 bytes dues to read buffer filling. Patched by Peter Würtz.
  
    \version 1.29 Further fix to SPI from Peter Würtz.
  
    \version 1.30 10 microsecond delays from bcm2835_spi_transfer and bcm2835_spi_transfern for
    significant performance improvements, Patch by Alan Watson.
  
    \version 1.31 Fix a GCC warning about dummy variable, patched by Alan Watson. Thanks.
  
    \version 1.32 Added option I2C_V1 definition to compile for version 1 RPi. 
    By default I2C code is generated for the V2 RPi which has SDA1 and SCL1 connected.
    Contributed by Malcolm Wiles based on work by Arvi Govindaraj.
  
    \version 1.33 Added command line utilities i2c and gpio to examples. Contributed by Shahrooz Shahparnia.
  
    \version 1.34 Added bcm2835_i2c_write_read_rs() which writes an arbitrary number of bytes, 
    sends a repeat start, and reads from the device. Contributed by Eduardo Steinhorst.
  
    \version 1.35 Fix build errors when compiled under Qt. Also performance improvements with SPI transfers. Contributed b Udo Klaas.
  
    \version 1.36 Make automake's test runner detect that we're skipping tests when not root, the second
    one makes us skip the test when using fakeroot (as used when building
    Debian packages). Contributed by Guido Günther.
  
    \version 1.37 Moved confiure.in to configure.ac as receommnded by autoreconf.<br>
    Improvements to bcm2835_st_read to account for possible timer overflow, contributed by 'Ed'.<br>
    Added definitions for Raspberry Pi B+ J8 header GPIO pins.<br>
  
    \version 1.38 Added bcm2835_regbase for the benefit of C# wrappers, patch by Frank Hommers <br>
  
    \version 1.39 Beta version of RPi2 compatibility. Not tested here on RPi2 hardware. 
    Testers please confirm correct operation on RPi2.<br>
    Unneccessary 'volatile' qualifiers removed from all variables and signatures.<br>
    Removed unsupportable PWM dividers, based on a report from Christophe Cecillon.<br>
    Minor improvements to spi.c example.<br>
  
    \version 1.40 Correct operation on RPi2 has been confirmed.<br>
    Fixed a number of compiler errors and warnings that occur when bcm2835.h is included
    in code compiled with -Wall -Woverflow -Wstrict-overflow -Wshadow -Wextra -pedantic.
    Reported by tlhackque.<br>
    Fixed a problem where calling bcm2835_delayMicroseconds loops forever when debug is set. Reported by tlhackque.<br>
    Reinstated use of volatile in 2 functions where there was a danger of lost reads or writes. Reported by tlhackque.<br>
    
    \version 1.41 Added BCM2835_VERSION macro and new function bcm2835_version(); Requested by tlhackque.<br>
    Improvements to peripheral memory barriers as suggested by tlhackque.<br>
    Reinstated some necessary volatile declarations as requested by tlhackque.<br>
  
    \version 1.42 Further improvements to memory barriers with the patient assistance and patches of tlhackque.<br>
  
    \version 1.43 Fixed problems with compiling barriers on RPI 2 with Arch Linux and gcc 4.9.2. 
    Reported and patched by Lars Christensen.<br>
    Testing on RPI 2, with ArchLinuxARM-rpi-2-latest and 2015-02-16-raspbian-wheezy.<br>
  
    \version 1.44 Added documention about the need for device tree to be enabled on RPI2.<br>
    Improvements to detection of availablity of DMB instruction based on value of __ARM_ARCH macro.<br>
  
    \version 1.45 Fixed an error in the pad group offsets that would prevent bcm2835_gpio_set_pad() 
    and bcm2835_gpio_pad() working correctly with non-0 pad groups. Reported by Guido.
  
    \author  Mike McCauley (mikem@airspayce.com) DO NOT CONTACT THE AUTHOR DIRECTLY: USE THE LISTS
  */
  
  
  /* Defines for BCM2835 */
  #ifndef BCM2835_H
  #define BCM2835_H
  
  #include <stdint.h>
  
  #define BCM2835_VERSION 10042 /* Version 1.42 */
  
  /* RPi 2 is ARM v7, and has DMB instruction for memory barriers.
     Older RPis are ARM v6 and don't, so a coprocessor instruction must be used instead.
     However, not all versions of gcc in all distros support the dmb assembler instruction even on conmpatible processors.
     This test is so any ARMv7 or higher processors with suitable GCC will use DMB.
  */
  #if __ARM_ARCH >= 7
  #define BCM2835_HAVE_DMB
  #endif
  
  /*! \defgroup constants Constants for passing to and from library functions
    The values here are designed to be passed to various functions in the bcm2835 library.
    @{
  */
  
  /*! This means pin HIGH, true, 3.3volts on a pin. */
  #define HIGH 0x1
  /*! This means pin LOW, false, 0volts on a pin. */
  #define LOW  0x0
  
  /*! Speed of the core clock core_clk */
  #define BCM2835_CORE_CLK_HZ		250000000	/*!< 250 MHz */
  
  /*! On RPi2 with BCM2836, and all recent OSs, the base of the peripherals is read from a /proc file */
  #define BMC2835_RPI2_DT_FILENAME "/proc/device-tree/soc/ranges"
  /*! Offset into BMC2835_RPI2_DT_FILENAME for the peripherals base address */
  #define BMC2835_RPI2_DT_PERI_BASE_ADDRESS_OFFSET 4
  /*! Offset into BMC2835_RPI2_DT_FILENAME for the peripherals size address */
  #define BMC2835_RPI2_DT_PERI_SIZE_OFFSET 8
  
  /*! Physical addresses for various peripheral register sets
    Base Physical Address of the BCM 2835 peripheral registers
    Note this is different for the RPi2 BCM2836, where this is derived from /proc/device-tree/soc/ranges
    If /proc/device-tree/soc/ranges exists on a RPi 1 OS, it would be expected to contain the
    following numbers:
  */
  /*! Peripherals block base address on RPi 1 */
  #define BCM2836_PERI_BASE               0x3F000000
  #define BCM2835_PERI_BASE               0x20000000
  /*! Size of the peripherals block on RPi 1 */
  #define BCM2835_PERI_SIZE               0x01000000
  
  /*! Offsets for the bases of various peripherals within the peripherals block
    /   Base Address of the System Timer registers
  */
  #define BCM2835_ST_BASE			0x3000
  /*! Base Address of the Pads registers */
  #define BCM2835_GPIO_PADS               0x100000
  /*! Base Address of the Clock/timer registers */
  #define BCM2835_CLOCK_BASE              0x101000
  /*! Base Address of the GPIO registers */
  #define BCM2835_GPIO_BASE               0x200000
  /*! Base Address of the SPI0 registers */
  #define BCM2835_SPI0_BASE               0x204000
  /*! Base Address of the BSC0 registers */
  #define BCM2835_BSC0_BASE 		0x205000
  /*! Base Address of the PWM registers */
  #define BCM2835_GPIO_PWM                0x20C000
  /*! Base Address of the BSC1 registers */
  #define BCM2835_BSC1_BASE		0x804000
  
  /*! Physical address and size of the peripherals block
    May be overridden on RPi2
  */
  extern uint32_t *bcm2835_peripherals_base;
  /*! Size of the peripherals block to be mapped */
  extern uint32_t bcm2835_peripherals_size;
  
  /*! Virtual memory address of the mapped peripherals block */
  extern uint32_t *bcm2835_peripherals;
  
  /*! Base of the ST (System Timer) registers.
    Available after bcm2835_init has been called
  */
  extern volatile uint32_t *bcm2835_st;
  
  /*! Base of the GPIO registers.
    Available after bcm2835_init has been called
  */
  extern volatile uint32_t *bcm2835_gpio;
  
  /*! Base of the PWM registers.
    Available after bcm2835_init has been called
  */
  extern volatile uint32_t *bcm2835_pwm;
  
  /*! Base of the CLK registers.
    Available after bcm2835_init has been called
  */
  extern volatile uint32_t *bcm2835_clk;
  
  /*! Base of the PADS registers.
    Available after bcm2835_init has been called
  */
  extern volatile uint32_t *bcm2835_pads;
  
  /*! Base of the SPI0 registers.
    Available after bcm2835_init has been called
  */
  extern volatile uint32_t *bcm2835_spi0;
  
  /*! Base of the BSC0 registers.
    Available after bcm2835_init has been called
  */
  extern volatile uint32_t *bcm2835_bsc0;
  
  /*! Base of the BSC1 registers.
    Available after bcm2835_init has been called
  */
  extern volatile uint32_t *bcm2835_bsc1;
  
  /*! \brief bcm2835RegisterBase
    Register bases for bcm2835_regbase()
  */
  typedef enum
  {
      BCM2835_REGBASE_ST   = 1, /*!< Base of the ST (System Timer) registers. */
      BCM2835_REGBASE_GPIO = 2, /*!< Base of the GPIO registers. */
      BCM2835_REGBASE_PWM  = 3, /*!< Base of the PWM registers. */
      BCM2835_REGBASE_CLK  = 4, /*!< Base of the CLK registers. */
      BCM2835_REGBASE_PADS = 5, /*!< Base of the PADS registers. */
      BCM2835_REGBASE_SPI0 = 6, /*! Base of the SPI0 registers. */
      BCM2835_REGBASE_BSC0 = 7, /*!< Base of the BSC0 registers. */
      BCM2835_REGBASE_BSC1 = 8  /*!< Base of the BSC1 registers. */
  } bcm2835RegisterBase;
  
  /*! Size of memory page on RPi */
  #define BCM2835_PAGE_SIZE               (4*1024)
  /*! Size of memory block on RPi */
  #define BCM2835_BLOCK_SIZE              (4*1024)
  
  
  /* Defines for GPIO
     The BCM2835 has 54 GPIO pins.
     BCM2835 data sheet, Page 90 onwards.
  */
  /*! GPIO register offsets from BCM2835_GPIO_BASE. 
    Offsets into the GPIO Peripheral block in bytes per 6.1 Register View 
  */
  #define BCM2835_GPFSEL0                      0x0000 /*!< GPIO Function Select 0 */
  #define BCM2835_GPFSEL1                      0x0004 /*!< GPIO Function Select 1 */
  #define BCM2835_GPFSEL2                      0x0008 /*!< GPIO Function Select 2 */
  #define BCM2835_GPFSEL3                      0x000c /*!< GPIO Function Select 3 */
  #define BCM2835_GPFSEL4                      0x0010 /*!< GPIO Function Select 4 */
  #define BCM2835_GPFSEL5                      0x0014 /*!< GPIO Function Select 5 */
  #define BCM2835_GPSET0                       0x001c /*!< GPIO Pin Output Set 0 */
  #define BCM2835_GPSET1                       0x0020 /*!< GPIO Pin Output Set 1 */
  #define BCM2835_GPCLR0                       0x0028 /*!< GPIO Pin Output Clear 0 */
  #define BCM2835_GPCLR1                       0x002c /*!< GPIO Pin Output Clear 1 */
  #define BCM2835_GPLEV0                       0x0034 /*!< GPIO Pin Level 0 */
  #define BCM2835_GPLEV1                       0x0038 /*!< GPIO Pin Level 1 */
  #define BCM2835_GPEDS0                       0x0040 /*!< GPIO Pin Event Detect Status 0 */
  #define BCM2835_GPEDS1                       0x0044 /*!< GPIO Pin Event Detect Status 1 */
  #define BCM2835_GPREN0                       0x004c /*!< GPIO Pin Rising Edge Detect Enable 0 */
  #define BCM2835_GPREN1                       0x0050 /*!< GPIO Pin Rising Edge Detect Enable 1 */
  #define BCM2835_GPFEN0                       0x0058 /*!< GPIO Pin Falling Edge Detect Enable 0 */
  #define BCM2835_GPFEN1                       0x005c /*!< GPIO Pin Falling Edge Detect Enable 1 */
  #define BCM2835_GPHEN0                       0x0064 /*!< GPIO Pin High Detect Enable 0 */
  #define BCM2835_GPHEN1                       0x0068 /*!< GPIO Pin High Detect Enable 1 */
  #define BCM2835_GPLEN0                       0x0070 /*!< GPIO Pin Low Detect Enable 0 */
  #define BCM2835_GPLEN1                       0x0074 /*!< GPIO Pin Low Detect Enable 1 */
  #define BCM2835_GPAREN0                      0x007c /*!< GPIO Pin Async. Rising Edge Detect 0 */
  #define BCM2835_GPAREN1                      0x0080 /*!< GPIO Pin Async. Rising Edge Detect 1 */
  #define BCM2835_GPAFEN0                      0x0088 /*!< GPIO Pin Async. Falling Edge Detect 0 */
  #define BCM2835_GPAFEN1                      0x008c /*!< GPIO Pin Async. Falling Edge Detect 1 */
  #define BCM2835_GPPUD                        0x0094 /*!< GPIO Pin Pull-up/down Enable */
  #define BCM2835_GPPUDCLK0                    0x0098 /*!< GPIO Pin Pull-up/down Enable Clock 0 */
  #define BCM2835_GPPUDCLK1                    0x009c /*!< GPIO Pin Pull-up/down Enable Clock 1 */
  
  /*!   \brief bcm2835PortFunction
    Port function select modes for bcm2835_gpio_fsel()
  */
  typedef enum
  {
      BCM2835_GPIO_FSEL_INPT  = 0x00,   /*!< Input 0b000 */
      BCM2835_GPIO_FSEL_OUTP  = 0x01,   /*!< Output 0b001 */
      BCM2835_GPIO_FSEL_ALT0  = 0x04,   /*!< Alternate function 0 0b100 */
      BCM2835_GPIO_FSEL_ALT1  = 0x05,   /*!< Alternate function 1 0b101 */
      BCM2835_GPIO_FSEL_ALT2  = 0x06,   /*!< Alternate function 2 0b110, */
      BCM2835_GPIO_FSEL_ALT3  = 0x07,   /*!< Alternate function 3 0b111 */
      BCM2835_GPIO_FSEL_ALT4  = 0x03,   /*!< Alternate function 4 0b011 */
      BCM2835_GPIO_FSEL_ALT5  = 0x02,   /*!< Alternate function 5 0b010 */
      BCM2835_GPIO_FSEL_MASK  = 0x07    /*!< Function select bits mask 0b111 */
  } bcm2835FunctionSelect;
  
  /*! \brief bcm2835PUDControl
    Pullup/Pulldown defines for bcm2835_gpio_pud()
  */
  typedef enum
  {
      BCM2835_GPIO_PUD_OFF     = 0x00,   /*!< Off ? disable pull-up/down 0b00 */
      BCM2835_GPIO_PUD_DOWN    = 0x01,   /*!< Enable Pull Down control 0b01 */
      BCM2835_GPIO_PUD_UP      = 0x02    /*!< Enable Pull Up control 0b10  */
  } bcm2835PUDControl;
  
  /*! Pad control register offsets from BCM2835_GPIO_PADS */
  #define BCM2835_PADS_GPIO_0_27               0x002c /*!< Pad control register for pads 0 to 27 */
  #define BCM2835_PADS_GPIO_28_45              0x0030 /*!< Pad control register for pads 28 to 45 */
  #define BCM2835_PADS_GPIO_46_53              0x0034 /*!< Pad control register for pads 46 to 53 */
  
  /*! Pad Control masks */
  #define BCM2835_PAD_PASSWRD                  (0x5A << 24)  /*!< Password to enable setting pad mask */
  #define BCM2835_PAD_SLEW_RATE_UNLIMITED      0x10 /*!< Slew rate unlimited */
  #define BCM2835_PAD_HYSTERESIS_ENABLED       0x08 /*!< Hysteresis enabled */
  #define BCM2835_PAD_DRIVE_2mA                0x00 /*!< 2mA drive current */
  #define BCM2835_PAD_DRIVE_4mA                0x01 /*!< 4mA drive current */
  #define BCM2835_PAD_DRIVE_6mA                0x02 /*!< 6mA drive current */
  #define BCM2835_PAD_DRIVE_8mA                0x03 /*!< 8mA drive current */
  #define BCM2835_PAD_DRIVE_10mA               0x04 /*!< 10mA drive current */
  #define BCM2835_PAD_DRIVE_12mA               0x05 /*!< 12mA drive current */
  #define BCM2835_PAD_DRIVE_14mA               0x06 /*!< 14mA drive current */
  #define BCM2835_PAD_DRIVE_16mA               0x07 /*!< 16mA drive current */
  
  /*! \brief bcm2835PadGroup
    Pad group specification for bcm2835_gpio_pad()
  */
  typedef enum
  {
      BCM2835_PAD_GROUP_GPIO_0_27         = 0, /*!< Pad group for GPIO pads 0 to 27 */
      BCM2835_PAD_GROUP_GPIO_28_45        = 1, /*!< Pad group for GPIO pads 28 to 45 */
      BCM2835_PAD_GROUP_GPIO_46_53        = 2  /*!< Pad group for GPIO pads 46 to 53 */
  } bcm2835PadGroup;
  
  /*! \brief GPIO Pin Numbers
    
    Here we define Raspberry Pin GPIO pins on P1 in terms of the underlying BCM GPIO pin numbers.
    These can be passed as a pin number to any function requiring a pin.
    Not all pins on the RPi 26 bin IDE plug are connected to GPIO pins
    and some can adopt an alternate function.
    RPi version 2 has some slightly different pinouts, and these are values RPI_V2_*.
    RPi B+ has yet differnet pinouts and these are defined in RPI_BPLUS_*.
    At bootup, pins 8 and 10 are set to UART0_TXD, UART0_RXD (ie the alt0 function) respectively
    When SPI0 is in use (ie after bcm2835_spi_begin()), SPI0 pins are dedicated to SPI
    and cant be controlled independently.
    If you are using the RPi Compute Module, just use the GPIO number: there is no need to use one of these
    symbolic names
  */
  typedef enum
  {
      RPI_GPIO_P1_03        =  0,  /*!< Version 1, Pin P1-03 */
      RPI_GPIO_P1_05        =  1,  /*!< Version 1, Pin P1-05 */
      RPI_GPIO_P1_07        =  4,  /*!< Version 1, Pin P1-07 */
      RPI_GPIO_P1_08        = 14,  /*!< Version 1, Pin P1-08, defaults to alt function 0 UART0_TXD */
      RPI_GPIO_P1_10        = 15,  /*!< Version 1, Pin P1-10, defaults to alt function 0 UART0_RXD */
      RPI_GPIO_P1_11        = 17,  /*!< Version 1, Pin P1-11 */
      RPI_GPIO_P1_12        = 18,  /*!< Version 1, Pin P1-12, can be PWM channel 0 in ALT FUN 5 */
      RPI_GPIO_P1_13        = 21,  /*!< Version 1, Pin P1-13 */
      RPI_GPIO_P1_15        = 22,  /*!< Version 1, Pin P1-15 */
      RPI_GPIO_P1_16        = 23,  /*!< Version 1, Pin P1-16 */
      RPI_GPIO_P1_18        = 24,  /*!< Version 1, Pin P1-18 */
      RPI_GPIO_P1_19        = 10,  /*!< Version 1, Pin P1-19, MOSI when SPI0 in use */
      RPI_GPIO_P1_21        =  9,  /*!< Version 1, Pin P1-21, MISO when SPI0 in use */
      RPI_GPIO_P1_22        = 25,  /*!< Version 1, Pin P1-22 */
      RPI_GPIO_P1_23        = 11,  /*!< Version 1, Pin P1-23, CLK when SPI0 in use */
      RPI_GPIO_P1_24        =  8,  /*!< Version 1, Pin P1-24, CE0 when SPI0 in use */
      RPI_GPIO_P1_26        =  7,  /*!< Version 1, Pin P1-26, CE1 when SPI0 in use */
  
      /* RPi Version 2 */
      RPI_V2_GPIO_P1_03     =  2,  /*!< Version 2, Pin P1-03 */
      RPI_V2_GPIO_P1_05     =  3,  /*!< Version 2, Pin P1-05 */
      RPI_V2_GPIO_P1_07     =  4,  /*!< Version 2, Pin P1-07 */
      RPI_V2_GPIO_P1_08     = 14,  /*!< Version 2, Pin P1-08, defaults to alt function 0 UART0_TXD */
      RPI_V2_GPIO_P1_10     = 15,  /*!< Version 2, Pin P1-10, defaults to alt function 0 UART0_RXD */
      RPI_V2_GPIO_P1_11     = 17,  /*!< Version 2, Pin P1-11 */
      RPI_V2_GPIO_P1_12     = 18,  /*!< Version 2, Pin P1-12, can be PWM channel 0 in ALT FUN 5 */
      RPI_V2_GPIO_P1_13     = 27,  /*!< Version 2, Pin P1-13 */
      RPI_V2_GPIO_P1_15     = 22,  /*!< Version 2, Pin P1-15 */
      RPI_V2_GPIO_P1_16     = 23,  /*!< Version 2, Pin P1-16 */
      RPI_V2_GPIO_P1_18     = 24,  /*!< Version 2, Pin P1-18 */
      RPI_V2_GPIO_P1_19     = 10,  /*!< Version 2, Pin P1-19, MOSI when SPI0 in use */
      RPI_V2_GPIO_P1_21     =  9,  /*!< Version 2, Pin P1-21, MISO when SPI0 in use */
      RPI_V2_GPIO_P1_22     = 25,  /*!< Version 2, Pin P1-22 */
      RPI_V2_GPIO_P1_23     = 11,  /*!< Version 2, Pin P1-23, CLK when SPI0 in use */
      RPI_V2_GPIO_P1_24     =  8,  /*!< Version 2, Pin P1-24, CE0 when SPI0 in use */
      RPI_V2_GPIO_P1_26     =  7,  /*!< Version 2, Pin P1-26, CE1 when SPI0 in use */
  
      /* RPi Version 2, new plug P5 */
      RPI_V2_GPIO_P5_03     = 28,  /*!< Version 2, Pin P5-03 */
      RPI_V2_GPIO_P5_04     = 29,  /*!< Version 2, Pin P5-04 */
      RPI_V2_GPIO_P5_05     = 30,  /*!< Version 2, Pin P5-05 */
      RPI_V2_GPIO_P5_06     = 31,  /*!< Version 2, Pin P5-06 */
  
      /* RPi B+ J8 header */
      RPI_BPLUS_GPIO_J8_03     =  2,  /*!< B+, Pin J8-03 */
      RPI_BPLUS_GPIO_J8_05     =  3,  /*!< B+, Pin J8-05 */
      RPI_BPLUS_GPIO_J8_07     =  4,  /*!< B+, Pin J8-07 */
      RPI_BPLUS_GPIO_J8_08     = 14,  /*!< B+, Pin J8-08, defaults to alt function 0 UART0_TXD */
      RPI_BPLUS_GPIO_J8_10     = 15,  /*!< B+, Pin J8-10, defaults to alt function 0 UART0_RXD */
      RPI_BPLUS_GPIO_J8_11     = 17,  /*!< B+, Pin J8-11 */
      RPI_BPLUS_GPIO_J8_12     = 18,  /*!< B+, Pin J8-12, can be PWM channel 0 in ALT FUN 5 */
      RPI_BPLUS_GPIO_J8_13     = 27,  /*!< B+, Pin J8-13 */
      RPI_BPLUS_GPIO_J8_15     = 22,  /*!< B+, Pin J8-15 */
      RPI_BPLUS_GPIO_J8_16     = 23,  /*!< B+, Pin J8-16 */
      RPI_BPLUS_GPIO_J8_18     = 24,  /*!< B+, Pin J8-18 */
      RPI_BPLUS_GPIO_J8_19     = 10,  /*!< B+, Pin J8-19, MOSI when SPI0 in use */
      RPI_BPLUS_GPIO_J8_21     =  9,  /*!< B+, Pin J8-21, MISO when SPI0 in use */
      RPI_BPLUS_GPIO_J8_22     = 25,  /*!< B+, Pin J8-22 */
      RPI_BPLUS_GPIO_J8_23     = 11,  /*!< B+, Pin J8-23, CLK when SPI0 in use */
      RPI_BPLUS_GPIO_J8_24     =  8,  /*!< B+, Pin J8-24, CE0 when SPI0 in use */
      RPI_BPLUS_GPIO_J8_26     =  7,  /*!< B+, Pin J8-26, CE1 when SPI0 in use */
      RPI_BPLUS_GPIO_J8_29     =  5,  /*!< B+, Pin J8-29,  */
      RPI_BPLUS_GPIO_J8_31     =  6,  /*!< B+, Pin J8-31,  */
      RPI_BPLUS_GPIO_J8_32     =  12, /*!< B+, Pin J8-32,  */
      RPI_BPLUS_GPIO_J8_33     =  13, /*!< B+, Pin J8-33,  */
      RPI_BPLUS_GPIO_J8_35     =  19, /*!< B+, Pin J8-35,  */
      RPI_BPLUS_GPIO_J8_36     =  16, /*!< B+, Pin J8-36,  */
      RPI_BPLUS_GPIO_J8_37     =  26, /*!< B+, Pin J8-37,  */
      RPI_BPLUS_GPIO_J8_38     =  20, /*!< B+, Pin J8-38,  */
      RPI_BPLUS_GPIO_J8_40     =  21  /*!< B+, Pin J8-40,  */
  } RPiGPIOPin;
  
  /* Defines for SPI
     GPIO register offsets from BCM2835_SPI0_BASE. 
     Offsets into the SPI Peripheral block in bytes per 10.5 SPI Register Map
  */
  #define BCM2835_SPI0_CS                      0x0000 /*!< SPI Master Control and Status */
  #define BCM2835_SPI0_FIFO                    0x0004 /*!< SPI Master TX and RX FIFOs */
  #define BCM2835_SPI0_CLK                     0x0008 /*!< SPI Master Clock Divider */
  #define BCM2835_SPI0_DLEN                    0x000c /*!< SPI Master Data Length */
  #define BCM2835_SPI0_LTOH                    0x0010 /*!< SPI LOSSI mode TOH */
  #define BCM2835_SPI0_DC                      0x0014 /*!< SPI DMA DREQ Controls */
  
  /* Register masks for SPI0_CS */
  #define BCM2835_SPI0_CS_LEN_LONG             0x02000000 /*!< Enable Long data word in Lossi mode if DMA_LEN is set */
  #define BCM2835_SPI0_CS_DMA_LEN              0x01000000 /*!< Enable DMA mode in Lossi mode */
  #define BCM2835_SPI0_CS_CSPOL2               0x00800000 /*!< Chip Select 2 Polarity */
  #define BCM2835_SPI0_CS_CSPOL1               0x00400000 /*!< Chip Select 1 Polarity */
  #define BCM2835_SPI0_CS_CSPOL0               0x00200000 /*!< Chip Select 0 Polarity */
  #define BCM2835_SPI0_CS_RXF                  0x00100000 /*!< RXF - RX FIFO Full */
  #define BCM2835_SPI0_CS_RXR                  0x00080000 /*!< RXR RX FIFO needs Reading (full) */
  #define BCM2835_SPI0_CS_TXD                  0x00040000 /*!< TXD TX FIFO can accept Data */
  #define BCM2835_SPI0_CS_RXD                  0x00020000 /*!< RXD RX FIFO contains Data */
  #define BCM2835_SPI0_CS_DONE                 0x00010000 /*!< Done transfer Done */
  #define BCM2835_SPI0_CS_TE_EN                0x00008000 /*!< Unused */
  #define BCM2835_SPI0_CS_LMONO                0x00004000 /*!< Unused */
  #define BCM2835_SPI0_CS_LEN                  0x00002000 /*!< LEN LoSSI enable */
  #define BCM2835_SPI0_CS_REN                  0x00001000 /*!< REN Read Enable */
  #define BCM2835_SPI0_CS_ADCS                 0x00000800 /*!< ADCS Automatically Deassert Chip Select */
  #define BCM2835_SPI0_CS_INTR                 0x00000400 /*!< INTR Interrupt on RXR */
  #define BCM2835_SPI0_CS_INTD                 0x00000200 /*!< INTD Interrupt on Done */
  #define BCM2835_SPI0_CS_DMAEN                0x00000100 /*!< DMAEN DMA Enable */
  #define BCM2835_SPI0_CS_TA                   0x00000080 /*!< Transfer Active */
  #define BCM2835_SPI0_CS_CSPOL                0x00000040 /*!< Chip Select Polarity */
  #define BCM2835_SPI0_CS_CLEAR                0x00000030 /*!< Clear FIFO Clear RX and TX */
  #define BCM2835_SPI0_CS_CLEAR_RX             0x00000020 /*!< Clear FIFO Clear RX  */
  #define BCM2835_SPI0_CS_CLEAR_TX             0x00000010 /*!< Clear FIFO Clear TX  */
  #define BCM2835_SPI0_CS_CPOL                 0x00000008 /*!< Clock Polarity */
  #define BCM2835_SPI0_CS_CPHA                 0x00000004 /*!< Clock Phase */
  #define BCM2835_SPI0_CS_CS                   0x00000003 /*!< Chip Select */
  
  /*! \brief bcm2835SPIBitOrder SPI Bit order
    Specifies the SPI data bit ordering for bcm2835_spi_setBitOrder()
  */
  typedef enum
  {
      BCM2835_SPI_BIT_ORDER_LSBFIRST = 0,  /*!< LSB First */
      BCM2835_SPI_BIT_ORDER_MSBFIRST = 1   /*!< MSB First */
  }bcm2835SPIBitOrder;
  
  /*! \brief SPI Data mode
    Specify the SPI data mode to be passed to bcm2835_spi_setDataMode()
  */
  typedef enum
  {
      BCM2835_SPI_MODE0 = 0,  /*!< CPOL = 0, CPHA = 0 */
      BCM2835_SPI_MODE1 = 1,  /*!< CPOL = 0, CPHA = 1 */
      BCM2835_SPI_MODE2 = 2,  /*!< CPOL = 1, CPHA = 0 */
      BCM2835_SPI_MODE3 = 3   /*!< CPOL = 1, CPHA = 1 */
  }bcm2835SPIMode;
  
  /*! \brief bcm2835SPIChipSelect
    Specify the SPI chip select pin(s)
  */
  typedef enum
  {
      BCM2835_SPI_CS0 = 0,     /*!< Chip Select 0 */
      BCM2835_SPI_CS1 = 1,     /*!< Chip Select 1 */
      BCM2835_SPI_CS2 = 2,     /*!< Chip Select 2 (ie pins CS1 and CS2 are asserted) */
      BCM2835_SPI_CS_NONE = 3  /*!< No CS, control it yourself */
  } bcm2835SPIChipSelect;
  
  /*! \brief bcm2835SPIClockDivider
    Specifies the divider used to generate the SPI clock from the system clock.
    Figures below give the divider, clock period and clock frequency.
    Clock divided is based on nominal base clock rate of 250MHz
    It is reported that (contrary to the documentation) any even divider may used.
    The frequencies shown for each divider have been confirmed by measurement
  */
  typedef enum
  {
      BCM2835_SPI_CLOCK_DIVIDER_65536 = 0,       /*!< 65536 = 262.144us = 3.814697260kHz */
      BCM2835_SPI_CLOCK_DIVIDER_32768 = 32768,   /*!< 32768 = 131.072us = 7.629394531kHz */
      BCM2835_SPI_CLOCK_DIVIDER_16384 = 16384,   /*!< 16384 = 65.536us = 15.25878906kHz */
      BCM2835_SPI_CLOCK_DIVIDER_8192  = 8192,    /*!< 8192 = 32.768us = 30/51757813kHz */
      BCM2835_SPI_CLOCK_DIVIDER_4096  = 4096,    /*!< 4096 = 16.384us = 61.03515625kHz */
      BCM2835_SPI_CLOCK_DIVIDER_2048  = 2048,    /*!< 2048 = 8.192us = 122.0703125kHz */
      BCM2835_SPI_CLOCK_DIVIDER_1024  = 1024,    /*!< 1024 = 4.096us = 244.140625kHz */
      BCM2835_SPI_CLOCK_DIVIDER_512   = 512,     /*!< 512 = 2.048us = 488.28125kHz */
      BCM2835_SPI_CLOCK_DIVIDER_256   = 256,     /*!< 256 = 1.024us = 976.5625MHz */
      BCM2835_SPI_CLOCK_DIVIDER_128   = 128,     /*!< 128 = 512ns = = 1.953125MHz */
      BCM2835_SPI_CLOCK_DIVIDER_64    = 64,      /*!< 64 = 256ns = 3.90625MHz */
      BCM2835_SPI_CLOCK_DIVIDER_32    = 32,      /*!< 32 = 128ns = 7.8125MHz */
      BCM2835_SPI_CLOCK_DIVIDER_16    = 16,      /*!< 16 = 64ns = 15.625MHz */
      BCM2835_SPI_CLOCK_DIVIDER_8     = 8,       /*!< 8 = 32ns = 31.25MHz */
      BCM2835_SPI_CLOCK_DIVIDER_4     = 4,       /*!< 4 = 16ns = 62.5MHz */
      BCM2835_SPI_CLOCK_DIVIDER_2     = 2,       /*!< 2 = 8ns = 125MHz, fastest you can get */
      BCM2835_SPI_CLOCK_DIVIDER_1     = 1        /*!< 1 = 262.144us = 3.814697260kHz, same as 0/65536 */
  } bcm2835SPIClockDivider;
  
  /* Defines for I2C
     GPIO register offsets from BCM2835_BSC*_BASE.
     Offsets into the BSC Peripheral block in bytes per 3.1 BSC Register Map
  */
  #define BCM2835_BSC_C 							0x0000 /*!< BSC Master Control */
  #define BCM2835_BSC_S 			0x0004 /*!< BSC Master Status */
  #define BCM2835_BSC_DLEN		0x0008 /*!< BSC Master Data Length */
  #define BCM2835_BSC_A 			0x000c /*!< BSC Master Slave Address */
  #define BCM2835_BSC_FIFO		0x0010 /*!< BSC Master Data FIFO */
  #define BCM2835_BSC_DIV			0x0014 /*!< BSC Master Clock Divider */
  #define BCM2835_BSC_DEL			0x0018 /*!< BSC Master Data Delay */
  #define BCM2835_BSC_CLKT		0x001c /*!< BSC Master Clock Stretch Timeout */
  
  /* Register masks for BSC_C */
  #define BCM2835_BSC_C_I2CEN 		0x00008000 /*!< I2C Enable, 0 = disabled, 1 = enabled */
  #define BCM2835_BSC_C_INTR 		0x00000400 /*!< Interrupt on RX */
  #define BCM2835_BSC_C_INTT 		0x00000200 /*!< Interrupt on TX */
  #define BCM2835_BSC_C_INTD 		0x00000100 /*!< Interrupt on DONE */
  #define BCM2835_BSC_C_ST 		0x00000080 /*!< Start transfer, 1 = Start a new transfer */
  #define BCM2835_BSC_C_CLEAR_1 		0x00000020 /*!< Clear FIFO Clear */
  #define BCM2835_BSC_C_CLEAR_2 		0x00000010 /*!< Clear FIFO Clear */
  #define BCM2835_BSC_C_READ 		0x00000001 /*!<	Read transfer */
  
  /* Register masks for BSC_S */
  #define BCM2835_BSC_S_CLKT 		0x00000200 /*!< Clock stretch timeout */
  #define BCM2835_BSC_S_ERR 		0x00000100 /*!< ACK error */
  #define BCM2835_BSC_S_RXF 		0x00000080 /*!< RXF FIFO full, 0 = FIFO is not full, 1 = FIFO is full */
  #define BCM2835_BSC_S_TXE 		0x00000040 /*!< TXE FIFO full, 0 = FIFO is not full, 1 = FIFO is full */
  #define BCM2835_BSC_S_RXD 		0x00000020 /*!< RXD FIFO contains data */
  #define BCM2835_BSC_S_TXD 		0x00000010 /*!< TXD FIFO can accept data */
  #define BCM2835_BSC_S_RXR 		0x00000008 /*!< RXR FIFO needs reading (full) */
  #define BCM2835_BSC_S_TXW 		0x00000004 /*!< TXW FIFO needs writing (full) */
  #define BCM2835_BSC_S_DONE 		0x00000002 /*!< Transfer DONE */
  #define BCM2835_BSC_S_TA 		0x00000001 /*!< Transfer Active */
  
  #define BCM2835_BSC_FIFO_SIZE   	16 /*!< BSC FIFO size */
  
  /*! \brief bcm2835I2CClockDivider
    Specifies the divider used to generate the I2C clock from the system clock.
    Clock divided is based on nominal base clock rate of 250MHz
  */
  typedef enum
  {
      BCM2835_I2C_CLOCK_DIVIDER_2500   = 2500,      /*!< 2500 = 10us = 100 kHz */
      BCM2835_I2C_CLOCK_DIVIDER_626    = 626,       /*!< 622 = 2.504us = 399.3610 kHz */
      BCM2835_I2C_CLOCK_DIVIDER_150    = 150,       /*!< 150 = 60ns = 1.666 MHz (default at reset) */
      BCM2835_I2C_CLOCK_DIVIDER_148    = 148        /*!< 148 = 59ns = 1.689 MHz */
  } bcm2835I2CClockDivider;
  
  /*! \brief bcm2835I2CReasonCodes
    Specifies the reason codes for the bcm2835_i2c_write and bcm2835_i2c_read functions.
  */
  typedef enum
  {
      BCM2835_I2C_REASON_OK   	     = 0x00,      /*!< Success */
      BCM2835_I2C_REASON_ERROR_NACK    = 0x01,      /*!< Received a NACK */
      BCM2835_I2C_REASON_ERROR_CLKT    = 0x02,      /*!< Received Clock Stretch Timeout */
      BCM2835_I2C_REASON_ERROR_DATA    = 0x04       /*!< Not all data is sent / received */
  } bcm2835I2CReasonCodes;
  
  /* Defines for ST
     GPIO register offsets from BCM2835_ST_BASE.
     Offsets into the ST Peripheral block in bytes per 12.1 System Timer Registers
     The System Timer peripheral provides four 32-bit timer channels and a single 64-bit free running counter.
     BCM2835_ST_CLO is the System Timer Counter Lower bits register.
     The system timer free-running counter lower register is a read-only register that returns the current value
     of the lower 32-bits of the free running counter.
     BCM2835_ST_CHI is the System Timer Counter Upper bits register.
     The system timer free-running counter upper register is a read-only register that returns the current value
     of the upper 32-bits of the free running counter.
  */
  #define BCM2835_ST_CS 			0x0000 /*!< System Timer Control/Status */
  #define BCM2835_ST_CLO 			0x0004 /*!< System Timer Counter Lower 32 bits */
  #define BCM2835_ST_CHI 			0x0008 /*!< System Timer Counter Upper 32 bits */
  
  /*! @} */
  
  
  /* Defines for PWM, word offsets (ie 4 byte multiples) */
  #define BCM2835_PWM_CONTROL 0
  #define BCM2835_PWM_STATUS  1
  #define BCM2835_PWM_DMAC    2
  #define BCM2835_PWM0_RANGE  4
  #define BCM2835_PWM0_DATA   5
  #define BCM2835_PWM_FIF1    6
  #define BCM2835_PWM1_RANGE  8
  #define BCM2835_PWM1_DATA   9
  
  /* Defines for PWM Clock, word offsets (ie 4 byte multiples) */
  #define BCM2835_PWMCLK_CNTL     40
  #define BCM2835_PWMCLK_DIV      41
  #define BCM2835_PWM_PASSWRD     (0x5A << 24)  /*!< Password to enable setting PWM clock */
  
  #define BCM2835_PWM1_MS_MODE    0x8000  /*!< Run in Mark/Space mode */
  #define BCM2835_PWM1_USEFIFO    0x2000  /*!< Data from FIFO */
  #define BCM2835_PWM1_REVPOLAR   0x1000  /*!< Reverse polarity */
  #define BCM2835_PWM1_OFFSTATE   0x0800  /*!< Ouput Off state */
  #define BCM2835_PWM1_REPEATFF   0x0400  /*!< Repeat last value if FIFO empty */
  #define BCM2835_PWM1_SERIAL     0x0200  /*!< Run in serial mode */
  #define BCM2835_PWM1_ENABLE     0x0100  /*!< Channel Enable */
  
  #define BCM2835_PWM0_MS_MODE    0x0080  /*!< Run in Mark/Space mode */
  #define BCM2835_PWM_CLEAR_FIFO  0x0040  /*!< Clear FIFO */
  #define BCM2835_PWM0_USEFIFO    0x0020  /*!< Data from FIFO */
  #define BCM2835_PWM0_REVPOLAR   0x0010  /*!< Reverse polarity */
  #define BCM2835_PWM0_OFFSTATE   0x0008  /*!< Ouput Off state */
  #define BCM2835_PWM0_REPEATFF   0x0004  /*!< Repeat last value if FIFO empty */
  #define BCM2835_PWM0_SERIAL     0x0002  /*!< Run in serial mode */
  #define BCM2835_PWM0_ENABLE     0x0001  /*!< Channel Enable */
  
  /*! \brief bcm2835PWMClockDivider
    Specifies the divider used to generate the PWM clock from the system clock.
    Figures below give the divider, clock period and clock frequency.
    Clock divided is based on nominal PWM base clock rate of 19.2MHz
    The frequencies shown for each divider have been confirmed by measurement
  */
  typedef enum
  {
      BCM2835_PWM_CLOCK_DIVIDER_2048  = 2048,    /*!< 2048 = 9.375kHz */
      BCM2835_PWM_CLOCK_DIVIDER_1024  = 1024,    /*!< 1024 = 18.75kHz */
      BCM2835_PWM_CLOCK_DIVIDER_512   = 512,     /*!< 512 = 37.5kHz */
      BCM2835_PWM_CLOCK_DIVIDER_256   = 256,     /*!< 256 = 75kHz */
      BCM2835_PWM_CLOCK_DIVIDER_128   = 128,     /*!< 128 = 150kHz */
      BCM2835_PWM_CLOCK_DIVIDER_64    = 64,      /*!< 64 = 300kHz */
      BCM2835_PWM_CLOCK_DIVIDER_32    = 32,      /*!< 32 = 600.0kHz */
      BCM2835_PWM_CLOCK_DIVIDER_16    = 16,      /*!< 16 = 1.2MHz */
      BCM2835_PWM_CLOCK_DIVIDER_8     = 8,       /*!< 8 = 2.4MHz */
      BCM2835_PWM_CLOCK_DIVIDER_4     = 4,       /*!< 4 = 4.8MHz */
      BCM2835_PWM_CLOCK_DIVIDER_2     = 2,       /*!< 2 = 9.6MHz, fastest you can get */
      BCM2835_PWM_CLOCK_DIVIDER_1     = 1        /*!< 1 = 4.6875kHz, same as divider 4096 */
  } bcm2835PWMClockDivider;
  
  /* Historical name compatibility */
  #ifndef BCM2835_NO_DELAY_COMPATIBILITY
  #define delay(x) bcm2835_delay(x)
  #define delayMicroseconds(x) bcm2835_delayMicroseconds(x)
  #endif
  
  #ifdef __cplusplus
  extern "C" {
  #endif
  
      /*! \defgroup init Library initialisation and management
        These functions allow you to intialise and control the bcm2835 library
        @{
      */
  
      /*! Initialise the library by opening /dev/mem and getting pointers to the 
        internal memory for BCM 2835 device registers. You must call this (successfully)
        before calling any other 
        functions in this library (except bcm2835_set_debug). 
        If bcm2835_init() fails by returning 0, 
        calling any other function may result in crashes or other failures.
        Prints messages to stderr in case of errors.
        \return 1 if successful else 0
      */
      extern int bcm2835_init(void);
  
      /*! Close the library, deallocating any allocated memory and closing /dev/mem
        \return 1 if successful else 0
      */
      extern int bcm2835_close(void);
  
      /*! Sets the debug level of the library.
        A value of 1 prevents mapping to /dev/mem, and makes the library print out
        what it would do, rather than accessing the GPIO registers.
        A value of 0, the default, causes normal operation.
        Call this before calling bcm2835_init();
        \param[in] debug The new debug level. 1 means debug
      */
      extern void  bcm2835_set_debug(uint8_t debug);
  
      /*! Returns the version number of the library, same as BCM2835_VERSION
         \return the current library version number
      */
      extern unsigned int bcm2835_version(void);
  
      /*! @} */
  
      /*! \defgroup lowlevel Low level register access
        These functions provide low level register access, and should not generally
        need to be used 
         
        @{
      */
  
      /*! Gets the base of a register
        \param[in] regbase You can use one of the common values BCM2835_REGBASE_*
        in \ref bcm2835RegisterBase
        \return the register base
        \sa Physical Addresses
      */
      extern uint32_t* bcm2835_regbase(uint8_t regbase);
  
      /*! Reads 32 bit value from a peripheral address WITH a memory barrier before and after each read.
        This is safe, but slow.  The MB before protects this read from any in-flight reads that didn't
        use a MB.  The MB after protects subsequent reads from another peripheral.
  
        \param[in] paddr Physical address to read from. See BCM2835_GPIO_BASE etc.
        \return the value read from the 32 bit register
        \sa Physical Addresses
      */
      extern uint32_t bcm2835_peri_read(volatile uint32_t* paddr);
  
      /*! Reads 32 bit value from a peripheral address WITHOUT the read barriers
        You should only use this when:
        o your code has previously called bcm2835_peri_read() for a register
        within the same peripheral, and no read or write to another peripheral has occurred since.
        o your code has called bcm2835_memory_barrier() since the last access to ANOTHER peripheral.
  
        \param[in] paddr Physical address to read from. See BCM2835_GPIO_BASE etc.
        \return the value read from the 32 bit register
        \sa Physical Addresses
      */
      extern uint32_t bcm2835_peri_read_nb(volatile uint32_t* paddr);
  
  
      /*! Writes 32 bit value from a peripheral address WITH a memory barrier before and after each write
        This is safe, but slow.  The MB before ensures that any in-flight write to another peripheral
        completes before this write is issued.  The MB after ensures that subsequent reads and writes
        to another peripheral will see the effect of this write.
  
        This is a tricky optimization; if you aren't sure, use the barrier version.
  
        \param[in] paddr Physical address to read from. See BCM2835_GPIO_BASE etc.
        \param[in] value The 32 bit value to write
        \sa Physical Addresses
      */
      extern void bcm2835_peri_write(volatile uint32_t* paddr, uint32_t value);
  
      /*! Writes 32 bit value from a peripheral address without the write barrier
        You should only use this when:
        o your code has previously called bcm2835_peri_write() for a register
        within the same peripheral, and no other peripheral access has occurred since.
        o your code has called bcm2835_memory_barrier() since the last access to ANOTHER peripheral.
  
        This is a tricky optimization; if you aren't sure, use the barrier version.
  
        \param[in] paddr Physical address to read from. See BCM2835_GPIO_BASE etc.
        \param[in] value The 32 bit value to write
        \sa Physical Addresses
      */
      extern void bcm2835_peri_write_nb(volatile uint32_t* paddr, uint32_t value);
  
      /*! Alters a number of bits in a 32 peripheral regsiter.
        It reads the current valu and then alters the bits defines as 1 in mask, 
        according to the bit value in value. 
        All other bits that are 0 in the mask are unaffected.
        Use this to alter a subset of the bits in a register.
        Memory barriers are used.  Note that this is not atomic; an interrupt
        routine can cause unexpected results.
        \param[in] paddr Physical address to read from. See BCM2835_GPIO_BASE etc.
        \param[in] value The 32 bit value to write, masked in by mask.
        \param[in] mask Bitmask that defines the bits that will be altered in the register.
        \sa Physical Addresses
      */
      extern void bcm2835_peri_set_bits(volatile uint32_t* paddr, uint32_t value, uint32_t mask);
      /*! @}    end of lowlevel */
  
      /*! \defgroup gpio GPIO register access
        These functions allow you to control the GPIO interface. You can set the 
        function of each GPIO pin, read the input state and set the output state.
        @{
      */
  
      /*! Sets the Function Select register for the given pin, which configures
        the pin as Input, Output or one of the 6 alternate functions.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
        \param[in] mode Mode to set the pin to, one of BCM2835_GPIO_FSEL_* from \ref bcm2835FunctionSelect
      */
      extern void bcm2835_gpio_fsel(uint8_t pin, uint8_t mode);
  
      /*! Sets the specified pin output to 
        HIGH.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
        \sa bcm2835_gpio_write()
      */
      extern void bcm2835_gpio_set(uint8_t pin);
  
      /*! Sets the specified pin output to 
        LOW.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
        \sa bcm2835_gpio_write()
      */
      extern void bcm2835_gpio_clr(uint8_t pin);
  
      /*! Sets any of the first 32 GPIO output pins specified in the mask to 
        HIGH.
        \param[in] mask Mask of pins to affect. Use eg: (1 << RPI_GPIO_P1_03) | (1 << RPI_GPIO_P1_05)
        \sa bcm2835_gpio_write_multi()
      */
      extern void bcm2835_gpio_set_multi(uint32_t mask);
  
      /*! Sets any of the first 32 GPIO output pins specified in the mask to 
        LOW.
        \param[in] mask Mask of pins to affect. Use eg: (1 << RPI_GPIO_P1_03) | (1 << RPI_GPIO_P1_05)
        \sa bcm2835_gpio_write_multi()
      */
      extern void bcm2835_gpio_clr_multi(uint32_t mask);
  
      /*! Reads the current level on the specified 
        pin and returns either HIGH or LOW. Works whether or not the pin
        is an input or an output.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
        \return the current level  either HIGH or LOW
      */
      extern uint8_t bcm2835_gpio_lev(uint8_t pin);
  
      /*! Event Detect Status.
        Tests whether the specified pin has detected a level or edge
        as requested by bcm2835_gpio_ren(), bcm2835_gpio_fen(), bcm2835_gpio_hen(), 
        bcm2835_gpio_len(), bcm2835_gpio_aren(), bcm2835_gpio_afen().
        Clear the flag for a given pin by calling bcm2835_gpio_set_eds(pin);
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
        \return HIGH if the event detect status for the given pin is true.
      */
      extern uint8_t bcm2835_gpio_eds(uint8_t pin);
  
      /*! Sets the Event Detect Status register for a given pin to 1, 
        which has the effect of clearing the flag. Use this afer seeing
        an Event Detect Status on the pin.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
      */
      extern void bcm2835_gpio_set_eds(uint8_t pin);
  
      /*! Enable Rising Edge Detect Enable for the specified pin.
        When a rising edge is detected, sets the appropriate pin in Event Detect Status.
        The GPRENn registers use
        synchronous edge detection. This means the input signal is sampled using the
        system clock and then it is looking for a ?011? pattern on the sampled signal. This
        has the effect of suppressing glitches.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
      */
      extern void bcm2835_gpio_ren(uint8_t pin);
  
      /*! Disable Rising Edge Detect Enable for the specified pin.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
      */
      extern void bcm2835_gpio_clr_ren(uint8_t pin);
  
      /*! Enable Falling Edge Detect Enable for the specified pin.
        When a falling edge is detected, sets the appropriate pin in Event Detect Status.
        The GPRENn registers use
        synchronous edge detection. This means the input signal is sampled using the
        system clock and then it is looking for a ?100? pattern on the sampled signal. This
        has the effect of suppressing glitches.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
      */
      extern void bcm2835_gpio_fen(uint8_t pin);
  
      /*! Disable Falling Edge Detect Enable for the specified pin.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
      */
      extern void bcm2835_gpio_clr_fen(uint8_t pin);
  
      /*! Enable High Detect Enable for the specified pin.
        When a HIGH level is detected on the pin, sets the appropriate pin in Event Detect Status.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
      */
      extern void bcm2835_gpio_hen(uint8_t pin);
  
      /*! Disable High Detect Enable for the specified pin.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
      */
      extern void bcm2835_gpio_clr_hen(uint8_t pin);
  
      /*! Enable Low Detect Enable for the specified pin.
        When a LOW level is detected on the pin, sets the appropriate pin in Event Detect Status.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
      */
      extern void bcm2835_gpio_len(uint8_t pin);
  
      /*! Disable Low Detect Enable for the specified pin.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
      */
      extern void bcm2835_gpio_clr_len(uint8_t pin);
  
      /*! Enable Asynchronous Rising Edge Detect Enable for the specified pin.
        When a rising edge is detected, sets the appropriate pin in Event Detect Status.
        Asynchronous means the incoming signal is not sampled by the system clock. As such
        rising edges of very short duration can be detected.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
      */
      extern void bcm2835_gpio_aren(uint8_t pin);
  
      /*! Disable Asynchronous Rising Edge Detect Enable for the specified pin.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
      */
      extern void bcm2835_gpio_clr_aren(uint8_t pin);
  
      /*! Enable Asynchronous Falling Edge Detect Enable for the specified pin.
        When a falling edge is detected, sets the appropriate pin in Event Detect Status.
        Asynchronous means the incoming signal is not sampled by the system clock. As such
        falling edges of very short duration can be detected.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
      */
      extern void bcm2835_gpio_afen(uint8_t pin);
  
      /*! Disable Asynchronous Falling Edge Detect Enable for the specified pin.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
      */
      extern void bcm2835_gpio_clr_afen(uint8_t pin);
  
      /*! Sets the Pull-up/down register for the given pin. This is
        used with bcm2835_gpio_pudclk() to set the  Pull-up/down resistor for the given pin.
        However, it is usually more convenient to use bcm2835_gpio_set_pud().
        \param[in] pud The desired Pull-up/down mode. One of BCM2835_GPIO_PUD_* from bcm2835PUDControl
        \sa bcm2835_gpio_set_pud()
      */
      extern void bcm2835_gpio_pud(uint8_t pud);
  
      /*! Clocks the Pull-up/down value set earlier by bcm2835_gpio_pud() into the pin.
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
        \param[in] on HIGH to clock the value from bcm2835_gpio_pud() into the pin. 
        LOW to remove the clock. 
        \sa bcm2835_gpio_set_pud()
      */
      extern void bcm2835_gpio_pudclk(uint8_t pin, uint8_t on);
  
      /*! Reads and returns the Pad Control for the given GPIO group.
        \param[in] group The GPIO pad group number, one of BCM2835_PAD_GROUP_GPIO_*
        \return Mask of bits from BCM2835_PAD_* from \ref bcm2835PadGroup
      */
      extern uint32_t bcm2835_gpio_pad(uint8_t group);
  
      /*! Sets the Pad Control for the given GPIO group.
        \param[in] group The GPIO pad group number, one of BCM2835_PAD_GROUP_GPIO_*
        \param[in] control Mask of bits from BCM2835_PAD_* from \ref bcm2835PadGroup. Note 
        that it is not necessary to include BCM2835_PAD_PASSWRD in the mask as this
        is automatically included.
      */
      extern void bcm2835_gpio_set_pad(uint8_t group, uint32_t control);
  
      /*! Delays for the specified number of milliseconds.
        Uses nanosleep(), and therefore does not use CPU until the time is up.
        However, you are at the mercy of nanosleep(). From the manual for nanosleep():
        If the interval specified in req is not an exact multiple of the granularity  
        underlying  clock  (see  time(7)),  then the interval will be
        rounded up to the next multiple. Furthermore, after the sleep completes, 
        there may still be a delay before the CPU becomes free to once
        again execute the calling thread.
        \param[in] millis Delay in milliseconds
      */
      extern void bcm2835_delay (unsigned int millis);
  
      /*! Delays for the specified number of microseconds.
        Uses a combination of nanosleep() and a busy wait loop on the BCM2835 system timers,
        However, you are at the mercy of nanosleep(). From the manual for nanosleep():
        If the interval specified in req is not an exact multiple of the granularity  
        underlying  clock  (see  time(7)),  then the interval will be
        rounded up to the next multiple. Furthermore, after the sleep completes, 
        there may still be a delay before the CPU becomes free to once
        again execute the calling thread.
        For times less than about 450 microseconds, uses a busy wait on the System Timer.
        It is reported that a delay of 0 microseconds on RaspberryPi will in fact
        result in a delay of about 80 microseconds. Your mileage may vary.
        \param[in] micros Delay in microseconds
      */
      extern void bcm2835_delayMicroseconds (uint64_t micros);
  
      /*! Sets the output state of the specified pin
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
        \param[in] on HIGH sets the output to HIGH and LOW to LOW.
      */
      extern void bcm2835_gpio_write(uint8_t pin, uint8_t on);
  
      /*! Sets any of the first 32 GPIO output pins specified in the mask to the state given by on
        \param[in] mask Mask of pins to affect. Use eg: (1 << RPI_GPIO_P1_03) | (1 << RPI_GPIO_P1_05)
        \param[in] on HIGH sets the output to HIGH and LOW to LOW.
      */
      extern void bcm2835_gpio_write_multi(uint32_t mask, uint8_t on);
  
      /*! Sets the first 32 GPIO output pins specified in the mask to the value given by value
        \param[in] value values required for each bit masked in by mask, eg: (1 << RPI_GPIO_P1_03) | (1 << RPI_GPIO_P1_05)
        \param[in] mask Mask of pins to affect. Use eg: (1 << RPI_GPIO_P1_03) | (1 << RPI_GPIO_P1_05)
      */
      extern void bcm2835_gpio_write_mask(uint32_t value, uint32_t mask);
  
      /*! Sets the Pull-up/down mode for the specified pin. This is more convenient than
        clocking the mode in with bcm2835_gpio_pud() and bcm2835_gpio_pudclk().
        \param[in] pin GPIO number, or one of RPI_GPIO_P1_* from \ref RPiGPIOPin.
        \param[in] pud The desired Pull-up/down mode. One of BCM2835_GPIO_PUD_* from bcm2835PUDControl
      */
      extern void bcm2835_gpio_set_pud(uint8_t pin, uint8_t pud);
  
      /*! @}  */
  
      /*! \defgroup spi SPI access
        These functions let you use SPI0 (Serial Peripheral Interface) to 
        interface with an external SPI device.
        @{
      */
  
      /*! Start SPI operations.
        Forces RPi SPI0 pins P1-19 (MOSI), P1-21 (MISO), P1-23 (CLK), P1-24 (CE0) and P1-26 (CE1)
        to alternate function ALT0, which enables those pins for SPI interface.
        You should call bcm2835_spi_end() when all SPI funcitons are complete to return the pins to 
        their default functions
        \sa  bcm2835_spi_end()
      */
      extern void bcm2835_spi_begin(void);
  
      /*! End SPI operations.
        SPI0 pins P1-19 (MOSI), P1-21 (MISO), P1-23 (CLK), P1-24 (CE0) and P1-26 (CE1)
        are returned to their default INPUT behaviour.
      */
      extern void bcm2835_spi_end(void);
  
      /*! Sets the SPI bit order
        NOTE: has no effect. Not supported by SPI0.
        Defaults to 
        \param[in] order The desired bit order, one of BCM2835_SPI_BIT_ORDER_*, 
        see \ref bcm2835SPIBitOrder
      */
      extern void bcm2835_spi_setBitOrder(uint8_t order);
  
      /*! Sets the SPI clock divider and therefore the 
        SPI clock speed. 
        \param[in] divider The desired SPI clock divider, one of BCM2835_SPI_CLOCK_DIVIDER_*, 
        see \ref bcm2835SPIClockDivider
      */
      extern void bcm2835_spi_setClockDivider(uint16_t divider);
  
      /*! Sets the SPI data mode
        Sets the clock polariy and phase
        \param[in] mode The desired data mode, one of BCM2835_SPI_MODE*, 
        see \ref bcm2835SPIMode
      */
      extern void bcm2835_spi_setDataMode(uint8_t mode);
  
      /*! Sets the chip select pin(s)
        When an bcm2835_spi_transfer() is made, the selected pin(s) will be asserted during the
        transfer.
        \param[in] cs Specifies the CS pins(s) that are used to activate the desired slave. 
        One of BCM2835_SPI_CS*, see \ref bcm2835SPIChipSelect
      */
      extern void bcm2835_spi_chipSelect(uint8_t cs);
  
      /*! Sets the chip select pin polarity for a given pin
        When an bcm2835_spi_transfer() occurs, the currently selected chip select pin(s) 
        will be asserted to the 
        value given by active. When transfers are not happening, the chip select pin(s) 
        return to the complement (inactive) value.
        \param[in] cs The chip select pin to affect
        \param[in] active Whether the chip select pin is to be active HIGH
      */
      extern void bcm2835_spi_setChipSelectPolarity(uint8_t cs, uint8_t active);
  
      /*! Transfers one byte to and from the currently selected SPI slave.
        Asserts the currently selected CS pins (as previously set by bcm2835_spi_chipSelect) 
        during the transfer.
        Clocks the 8 bit value out on MOSI, and simultaneously clocks in data from MISO. 
        Returns the read data byte from the slave.
        Uses polled transfer as per section 10.6.1 of the BCM 2835 ARM Peripherls manual
        \param[in] value The 8 bit data byte to write to MOSI
        \return The 8 bit byte simultaneously read from  MISO
        \sa bcm2835_spi_transfern()
      */
      extern uint8_t bcm2835_spi_transfer(uint8_t value);
      
      /*! Transfers any number of bytes to and from the currently selected SPI slave.
        Asserts the currently selected CS pins (as previously set by bcm2835_spi_chipSelect) 
        during the transfer.
        Clocks the len 8 bit bytes out on MOSI, and simultaneously clocks in data from MISO. 
        The data read read from the slave is placed into rbuf. rbuf must be at least len bytes long
        Uses polled transfer as per section 10.6.1 of the BCM 2835 ARM Peripherls manual
        \param[in] tbuf Buffer of bytes to send. 
        \param[out] rbuf Received bytes will by put in this buffer
        \param[in] len Number of bytes in the tbuf buffer, and the number of bytes to send/received
        \sa bcm2835_spi_transfer()
      */
      extern void bcm2835_spi_transfernb(char* tbuf, char* rbuf, uint32_t len);
  
      /*! Transfers any number of bytes to and from the currently selected SPI slave
        using bcm2835_spi_transfernb.
        The returned data from the slave replaces the transmitted data in the buffer.
        \param[in,out] buf Buffer of bytes to send. Received bytes will replace the contents
        \param[in] len Number of bytes int eh buffer, and the number of bytes to send/received
        \sa bcm2835_spi_transfer()
      */
      extern void bcm2835_spi_transfern(char* buf, uint32_t len);
  
      /*! Transfers any number of bytes to the currently selected SPI slave.
        Asserts the currently selected CS pins (as previously set by bcm2835_spi_chipSelect)
        during the transfer.
        \param[in] buf Buffer of bytes to send.
        \param[in] len Number of bytes in the tbuf buffer, and the number of bytes to send
      */
      extern void bcm2835_spi_writenb(char* buf, uint32_t len);
  
      /*! @} */
  
      /*! \defgroup i2c I2C access
        These functions let you use I2C (The Broadcom Serial Control bus with the Philips
        I2C bus/interface version 2.1 January 2000.) to interface with an external I2C device.
        @{
      */
  
      /*! Start I2C operations.
        Forces RPi I2C pins P1-03 (SDA) and P1-05 (SCL)
        to alternate function ALT0, which enables those pins for I2C interface.
        You should call bcm2835_i2c_end() when all I2C functions are complete to return the pins to
        their default functions
        \sa  bcm2835_i2c_end()
      */
      extern void bcm2835_i2c_begin(void);
  
      /*! End I2C operations.
        I2C pins P1-03 (SDA) and P1-05 (SCL)
        are returned to their default INPUT behaviour.
      */
      extern void bcm2835_i2c_end(void);
  
      /*! Sets the I2C slave address.
        \param[in] addr The I2C slave address.
      */
      extern void bcm2835_i2c_setSlaveAddress(uint8_t addr);
  
      /*! Sets the I2C clock divider and therefore the I2C clock speed.
        \param[in] divider The desired I2C clock divider, one of BCM2835_I2C_CLOCK_DIVIDER_*,
        see \ref bcm2835I2CClockDivider
      */
      extern void bcm2835_i2c_setClockDivider(uint16_t divider);
  
      /*! Sets the I2C clock divider by converting the baudrate parameter to
        the equivalent I2C clock divider. ( see \sa bcm2835_i2c_setClockDivider)
        For the I2C standard 100khz you would set baudrate to 100000
        The use of baudrate corresponds to its use in the I2C kernel device
        driver. (Of course, bcm2835 has nothing to do with the kernel driver)
      */
      extern void bcm2835_i2c_set_baudrate(uint32_t baudrate);
  
      /*! Transfers any number of bytes to the currently selected I2C slave.
        (as previously set by \sa bcm2835_i2c_setSlaveAddress)
        \param[in] buf Buffer of bytes to send.
        \param[in] len Number of bytes in the buf buffer, and the number of bytes to send.
        \return reason see \ref bcm2835I2CReasonCodes
      */
      extern uint8_t bcm2835_i2c_write(const char * buf, uint32_t len);
  
      /*! Transfers any number of bytes from the currently selected I2C slave.
        (as previously set by \sa bcm2835_i2c_setSlaveAddress)
        \param[in] buf Buffer of bytes to receive.
        \param[in] len Number of bytes in the buf buffer, and the number of bytes to received.
        \return reason see \ref bcm2835I2CReasonCodes
      */
      extern uint8_t bcm2835_i2c_read(char* buf, uint32_t len);
  
      /*! Allows reading from I2C slaves that require a repeated start (without any prior stop)
        to read after the required slave register has been set. For example, the popular
        MPL3115A2 pressure and temperature sensor. Note that your device must support or
        require this mode. If your device does not require this mode then the standard
        combined:
        \sa bcm2835_i2c_write
        \sa bcm2835_i2c_read
        are a better choice.
        Will read from the slave previously set by \sa bcm2835_i2c_setSlaveAddress
        \param[in] regaddr Buffer containing the slave register you wish to read from.
        \param[in] buf Buffer of bytes to receive.
        \param[in] len Number of bytes in the buf buffer, and the number of bytes to received.
        \return reason see \ref bcm2835I2CReasonCodes
      */
      extern uint8_t bcm2835_i2c_read_register_rs(char* regaddr, char* buf, uint32_t len);
  
      /*! Allows sending an arbitrary number of bytes to I2C slaves before issuing a repeated
        start (with no prior stop) and reading a response.
        Necessary for devices that require such behavior, such as the MLX90620.
        Will write to and read from the slave previously set by \sa bcm2835_i2c_setSlaveAddress
        \param[in] cmds Buffer containing the bytes to send before the repeated start condition.
        \param[in] cmds_len Number of bytes to send from cmds buffer
        \param[in] buf Buffer of bytes to receive.
        \param[in] buf_len Number of bytes to receive in the buf buffer.
        \return reason see \ref bcm2835I2CReasonCodes
      */
      extern uint8_t bcm2835_i2c_write_read_rs(char* cmds, uint32_t cmds_len, char* buf, uint32_t buf_len);
  
      /*! @} */
  
      /*! \defgroup st System Timer access
        Allows access to and delays using the System Timer Counter.
        @{
      */
  
      /*! Read the System Timer Counter register.
        \return the value read from the System Timer Counter Lower 32 bits register
      */
      extern uint64_t bcm2835_st_read(void);
  
      /*! Delays for the specified number of microseconds with offset.
        \param[in] offset_micros Offset in microseconds
        \param[in] micros Delay in microseconds
      */
      extern void bcm2835_st_delay(uint64_t offset_micros, uint64_t micros);
  
      /*! @}  */
  
      /*! \defgroup pwm Pulse Width Modulation
        Allows control of 2 independent PWM channels. A limited subset of GPIO pins
        can be connected to one of these 2 channels, allowing PWM control of GPIO pins.
        You have to set the desired pin into a particular Alt Fun to PWM output. See the PWM
        documentation on the Main Page.
        @{
      */
  
      /*! Sets the PWM clock divisor, 
        to control the basic PWM pulse widths.
        \param[in] divisor Divides the basic 19.2MHz PWM clock. You can use one of the common
        values BCM2835_PWM_CLOCK_DIVIDER_* in \ref bcm2835PWMClockDivider
      */
      extern void bcm2835_pwm_set_clock(uint32_t divisor);
      
      /*! Sets the mode of the given PWM channel,
        allowing you to control the PWM mode and enable/disable that channel
        \param[in] channel The PWM channel. 0 or 1.
        \param[in] markspace Set true if you want Mark-Space mode. 0 for Balanced mode.
        \param[in] enabled Set true to enable this channel and produce PWM pulses.
      */
      extern void bcm2835_pwm_set_mode(uint8_t channel, uint8_t markspace, uint8_t enabled);
  
      /*! Sets the maximum range of the PWM output.
        The data value can vary between 0 and this range to control PWM output
        \param[in] channel The PWM channel. 0 or 1.
        \param[in] range The maximum value permitted for DATA.
      */
      extern void bcm2835_pwm_set_range(uint8_t channel, uint32_t range);
      
      /*! Sets the PWM pulse ratio to emit to DATA/RANGE, 
        where RANGE is set by bcm2835_pwm_set_range().
        \param[in] channel The PWM channel. 0 or 1.
        \param[in] data Controls the PWM output ratio as a fraction of the range. 
        Can vary from 0 to RANGE.
      */
      extern void bcm2835_pwm_set_data(uint8_t channel, uint32_t data);
  
      /*! @}  */
  #ifdef __cplusplus
  }
  #endif
  
  #endif /* BCM2835_H */
  
  /*! @example blink.c
    Blinks RPi GPIO pin 11 on and off
  */
  
  /*! @example input.c
    Reads the state of an RPi input pin
  */
  
  /*! @example event.c
    Shows how to use event detection on an input pin
  */
  
  /*! @example spi.c
    Shows how to use SPI interface to transfer a byte to and from an SPI device
  */
  
  /*! @example spin.c
    Shows how to use SPI interface to transfer a number of bytes to and from an SPI device
  */
  
  /*! @example pwm.c
    Shows how to use PWM to control GPIO pins
  */
  
  /*! @example i2c.c
  Command line utility for executing i2c commands with the 
  Broadcom bcm2835. Contributed by Shahrooz Shahparnia.
  */
  
  /*! example gpio.c
    Command line utility for executing gpio commands with the 
    Broadcom bcm2835. Contributed by Shahrooz Shahparnia.
  */
  