SYSTICK

SYSTICK - Introduction

• Found in all ARM Cortex-Microcontrollers, regardless of silicon manufacturer.
  Used for taking actions periodically.
• Often used as time-base for real-time operating systems.
• The Systick is a 24-bit down counter driven by the processor clock.

SYSTICK - Counting

• Counts from initial value down to zero
• 24-bits imply maximum initial value of:
  2²⁴ = 0xFFFFFF = 16,777,216
• Initial value can be set to a value between 0x000000 to 0xFFFFFF

SYSTICK - Registers

• Systick Current Value Register (STCVR)

  This register contains the current count value

• Systick Control & Status Register (STCSR)

  This register allows us to configure the systick clock source, enable/disable interrupts and enable/disable the systick counter

• Systick Reload Value Register (STRVR)

  This is where the initial count value is placed

SYSTICK - Count value computations

Compute the delay achieved by loading 10 in the Systick Reload Value Register (STRVR) given system clock = 16Mhz

Written as:

Systick->LOAD = 9

written in the CMSIS standard

we write 9 although we want 10 because the counter starts counting from 0

Solution

System clock = 16MHz = 16 000 000 cycles/second

If 1 second executes 16 000 000 cycles,

how many seconds execute 1 cycle?

⇒ 1 / 16 000 000 = 62.5ns = 62.5 x 10⁻⁹ s

Then 10 cycles ⇒ 10 x 62.5 x 10⁻⁹ s = 625 x 10⁻⁹ s = 625ns

#include "stm32f4xx.h"

#define SYSTICK_LOAD_VAL   16000
#define CTRL_ENABLE        (1U<<0)
#define CTRL_CLKSR         (1U<<2)
#define CTRL_COUNTFLAG     (1U<<16)

void systickDelayMs(int delay)
{
    /*Reload with number of clocks per millisecond*/
    SysTick->LOAD = SYSTICK_LOAD_VAL;

    /*Clear systick current value register */
    SysTick->VAL = 0;

    /*Enable systick and select internal clk src*/
    SysTick->CTRL = CTRL_ENABLE | CTRL_CLKSR;

    for(int i=0; i<delay; i++)
    {
        /*Wait until the COUNTFLAG is set*/
        while((SysTick->CTRL & CTRL_COUNTFLAG) == 0){}
    }

    SysTick->CTRL = 0;
}

Systic Intr

// ...existing code...
#define CTRL_TICKINT   (1U << 1)
#define ONE_SEC_LOAD   16000000

void systick_1hz_interrupt(void)
{
    /*Reload with number of clocks per second*/
    SysTick->LOAD = ONE_SEC_LOAD - 1;

    /*Clear systick current value register */
    SysTick->VAL = 0;

    /*Enable systick and select internal clk src*/
    SysTick->CTRL = CTRL_ENABLE | CTRL_CLKSRC;

    /*Enable systick interrupt*/
    SysTick->CTRL |= CTRL_TICKINT;
    
    /***don't need NVIC_ENABLE()*/
}

void SysTick_Handler(void)
{
}