SysTick

SysTick Address Definitions

The code snippet below are address definitions used for the SysTick examples in this guide. Visit the STCTRL, STRELOAD, and the STCURRENT section of the TM4C123G datasheet for more info.

SysTick is a timer/counter feature available on all Cortex-M microcontrollers

• SysTick Base Address: 0xE000.E000

  • STCTRL offset: 0x010
  • STRELOAD offset: 0x014
  • STCURRENT offset: 0x018

#define NVIC_ST_CTRL_R          (*((volatile unsigned long *)0xE000E010))
#define NVIC_ST_RELOAD_R        (*((volatile unsigned long *)0xE000E014))
#define NVIC_ST_CURRENT_R       (*((volatile unsigned long *)0xE000E018))

#define NVIC_ST_CTRL_COUNT      0x00010000  // Count flag
#define NVIC_ST_CTRL_INTEN      0x00000002  // Interrupt enable

SysTick Initialization

1. Clear the enable bit in the STCTRL register to turn off SysTick during initialization:

   • ` NVIC_ST_CTRL_R = 0x00; `

2. Set the reload (STRELOAD) register to your desired specifications (this value gets passed/copied to STCURRENT when STCURRENT's value is fully decremented to 0). Setting the STRELOAD value to its maximum value allows it to behave like a clock:

   • ` NVIC_ST_RELOAD_R = 0x00FFFFFF `

3. Write any value to the STCURRENT register to clear the counter (this value is decremented once every bus cycle):

   • ` NVIC_ST_CURRENT_R = 0; `

4. Write to your desired specification in the STCTRL register (check datasheet). In this example I will set [CLK_SRC] = 1 so the counter runs off the 16Mhz system clock, and set [ENABLE] = 1 to enable the SysTick.

   • ` NVIC_ST_CTRL_R = 0x01 + 0x04; ` or ` NVIC_ST_CTRL_R = 0x05; `

void SysTick_Init(void){
  NVIC_ST_CTRL_R = 0x00;                // disable SysTick during setup
  NVIC_ST_RELOAD_R = 0x00FFFFFF;  		// maximum reload value
  NVIC_ST_CURRENT_R = 0;                // any write to current clears it         
  NVIC_ST_CTRL_R = 0x01 + 0x04;			// enable SysTick with core clock
}

Using SysTick to Determine Time

Note: When measuring elapse time with SysTick, the system can only be acurate as long as the elapse time is less than 0.209 seconds (at 80MHz), 1.05 seconds (at 16MHz), and 0.336 seconds (at 50MHz). Acuracy can be calculated by: 2^24 * 12.5ns (at 80MHz).

1. Calculate how long it would take (in seconds) for the value of STCURRENT to decrement once every bus cycle.

   • 1 tick = (1/System Clock) * 10^9 nano seconds

     • PLL activated: (1/80MHz) * 10^9 = 12.5 nano seconds
     • PLL not activated: (1/16MHz) * 10^9 = 62.5 nano seconds
     • Core Clock: (1/50MHz) * 10^9 = 20 nano seconds

SysTick Busy-Wait function

void SysTick_Wait(unsigned long delay){
  volatile unsigned long elapsedTime;
  unsigned long startTime = NVIC_ST_CURRENT_R;
  do{
    elapsedTime = (startTime-NVIC_ST_CURRENT_R)&0x00FFFFFF;
  }
  while(elapsedTime <= delay);
}