text: // SysTick.c
  // Runs on MSP432
  // Provide functions that initialize the SysTick module, wait at least a
  // designated number of clock cycles, and wait approximately a multiple
  // of 10 milliseconds using busy wait.  After a power-on-reset, the
  // MSP432 gets its clock from the internal digitally controlled
  // oscillator, which is set to 3 MHz by default.  One distinct advantage
  // of the MSP432 is that it has low-power clock options to reduce power
  // consumption by reducing clock frequency.  This matters for the
  // function SysTick_Wait10ms(), which will wait longer than 10 ms if the
  // clock is slower.
  // Daniel Valvano
  // February 18, 2017
  
  /* This example accompanies the books
     "Embedded Systems: Introduction to the MSP432 Microcontroller",
         ISBN: 978-1512185676, Jonathan Valvano, copyright (c) 2017
     "Embedded Systems: Real-Time Interfacing to the MSP432 Microcontroller",
         ISBN: 978-1514676585, Jonathan Valvano, copyright (c) 2017
     "Embedded Systems: Real-Time Operating Systems for ARM Cortex-M Microcontrollers",
         ISBN: 978-1466468863, , Jonathan Valvano, copyright (c) 2017
   For more information about my classes, my research, and my books, see
   http://users.ece.utexas.edu/~valvano/
  
  Simplified BSD License (FreeBSD License)
  Copyright (c) 2017, Jonathan Valvano, All rights reserved.
  
  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.
  
  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS 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.
  
  The views and conclusions contained in the software and documentation are
  those of the authors and should not be interpreted as representing official
  policies, either expressed or implied, of the FreeBSD Project.
  */
  
  #include <stdint.h>
  #include "msp.h"
  
  
  // Initialize SysTick with busy wait running at bus clock.
  void SysTick_Init(void){
    SysTick->LOAD = 0x00FFFFFF;           // maximum reload value
    SysTick->CTRL = 0x00000005;           // enable SysTick with no interrupts
  }
  // Time delay using busy wait.
  // The delay parameter is in units of the core clock.
  // assumes 48 MHz bus clock
  void SysTick_Wait(uint32_t delay){
    // method #1: set Reload Value Register, clear Current Value Register, poll COUNTFLAG in Control and Status Register
    if(delay <= 1){
      // without this step:
      // if delay == 0, this function will wait 0x00FFFFFF cycles
      // if delay == 1, this function will never return (because COUNTFLAG is set on 1->0 transition)
      return;                   // do nothing; at least 1 cycle has already passed anyway
    }
    SysTick->LOAD = (delay - 1);// count down to zero
    SysTick->VAL = 0;          // any write to CVR clears it and COUNTFLAG in CSR
    while(( SysTick->CTRL&0x00010000) == 0){};
    // method #2: repeatedly evaluate elapsed time
  /*  volatile uint32_t elapsedTime;
    uint32_t startTime = SYSTICK->VAL;
    do{
      elapsedTime = (startTime-SYSTICK->VAL)&0x00FFFFFF;
    }
    while(elapsedTime <= delay);*/
  }
  // Time delay using busy wait.
  // assumes 48 MHz bus clock
  void SysTick_Wait10ms(uint32_t delay){
    uint32_t i;
    for(i=0; i<delay; i++){
      SysTick_Wait(480000);  // wait 10ms (assumes 48 MHz clock)
    }
  }
  