text: /**
   * \file
   *
   * \brief SAM USART Quick Start
   *
   * Copyright (C) 2012-2014 Atmel Corporation. All rights reserved.
   *
   * \asf_license_start
   *
   * \page License
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions are met:
   *
   * 1. Redistributions of source code must retain the above copyright notice,
   *    this list of conditions and the following disclaimer.
   *
   * 2. Redistributions in binary form must reproduce the above copyright notice,
   *    this list of conditions and the following disclaimer in the documentation
   *    and/or other materials provided with the distribution.
   *
   * 3. The name of Atmel may not be used to endorse or promote products derived
   *    from this software without specific prior written permission.
   *
   * 4. This software may only be redistributed and used in connection with an
   *    Atmel microcontroller product.
   *
   * THIS SOFTWARE IS PROVIDED BY ATMEL "AS IS" AND ANY EXPRESS OR IMPLIED
   * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
   * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT ARE
   * EXPRESSLY AND SPECIFICALLY DISCLAIMED. IN NO EVENT SHALL ATMEL BE LIABLE FOR
   * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
   * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   * POSSIBILITY OF SUCH DAMAGE.
   *
   * \asf_license_stop
   *
   */
  
  /**
   * \page asfdoc_sam0_sercom_usart_callback_use_case Quick Start Guide for SERCOM USART - Callback
   *
   * This quick start will echo back characters typed into the terminal, using
   * asynchronous TX and RX callbacks from the USART peripheral. In this use case
   * the USART will be configured with the following settings:
   * - Asynchronous mode
   * - 9600 Baudrate
   * - 8-bits, No Parity and one Stop Bit
   * - TX and RX enabled and connected to the Xplained Pro Embedded Debugger virtual COM port
   *
   * \section asfdoc_sam0_sercom_usart_callback_use_case_setup Setup
   *
   * \subsection asfdoc_sam0_sercom_usart_callback_use_case_prereq Prerequisites
   * There are no special setup requirements for this use-case.
   *
   * \subsection asfdoc_sam0_usart_callback_use_case_setup_code Code
   * Add to the main application source file, outside of any functions:
   * \snippet qs_usart_callback.c module_inst
   * \snippet qs_usart_callback.c rx_buffer_var
   *
   * Copy-paste the following callback function code to your user application:
   * \snippet qs_usart_callback.c callback_funcs
   *
   * Copy-paste the following setup code to your user application:
   * \snippet qs_usart_callback.c setup
   *
   * Add to user application initialization (typically the start of \c main()):
   * \snippet qs_usart_callback.c setup_init
   *
   * \subsection asfdoc_sam0_usart_callback_use_case_setup_flow Workflow
   * -# Create a module software instance structure for the USART module to store
   *    the USART driver state while it is in use.
   *    \snippet qs_usart_callback.c module_inst
   *    \note This should never go out of scope as long as the module is in use.
   *          In most cases, this should be global.
   *
   * -# Configure the USART module.
   *  -# Create a USART module configuration struct, which can be filled out to
   *     adjust the configuration of a physical USART peripheral.
   *     \snippet qs_usart_callback.c setup_config
   *  -# Initialize the USART configuration struct with the module's default values.
   *     \snippet qs_usart_callback.c setup_config_defaults
   *     \note This should always be performed before using the configuration
   *           struct to ensure that all values are initialized to known default
   *           settings.
   *
   *  -# Alter the USART settings to configure the physical pinout, baudrate, and
   *     other relevant parameters.
   *     \snippet qs_usart_callback.c setup_change_config
   *  -# Configure the USART module with the desired settings, retrying while the
   *     driver is busy until the configuration is stressfully set.
   *     \snippet qs_usart_callback.c setup_set_config
   *  -# Enable the USART module.
   *     \snippet qs_usart_callback.c setup_enable
   * -# Configure the USART callbacks.
   *  -# Register the TX and RX callback functions with the driver.
   *     \snippet qs_usart_callback.c setup_register_callbacks
   *  -# Enable the TX and RX callbacks so that they will be called by the driver
   *     when appropriate.
   *     \snippet qs_usart_callback.c setup_enable_callbacks
   *
   * \section asfdoc_sam0_usart_callback_use_case_main Use Case
   *
   * \subsection asfdoc_sam0_usart_callback_use_case_main_code Code
   * Copy-paste the following code to your user application:
   * \snippet qs_usart_callback.c main
   *
   * \subsection asfdoc_sam0_usart_callback_use_case_main_flow Workflow
   * -# Enable global interrupts, so that the callbacks can be fired.
   *    \snippet qs_usart_callback.c enable_global_interrupts
   * -# Send a string to the USART to show the demo is running, blocking until
   *    all characters have been sent.
   *    \snippet qs_usart_callback.c main_send_string
   * -# Enter an infinite loop to continuously echo received values on the USART.
   *    \snippet qs_usart_callback.c main_loop
   * -# Perform an asynchronous read of the USART, which will fire the registered
   *    callback when characters are received.
   *    \snippet qs_usart_callback.c main_read
   */
   /**
   * Support and FAQ: visit <a href="http://www.atmel.com/design-support/">Atmel Support</a>
   */
  
  #include <asf.h>
  #include <conf_clocks.h>
  
  