Cortex M4 systick details

Cortex M4 Provides SysTick function ,SysTick It's a 24bits Timer , A cycle interrupt can be provided by simple programming , Commonly used as an operating system ” heartbeat “.

One 、 register

1、STK_CTRL

2、STK_LOAD

3、STK_VAL

  Two 、 Clock source

SysTick Two clock sources can be selected , Through registers STK_CTRL Of CLKSOURCE To choose ,‘0’ Indicates that the clock source is AHB（HCLK） Of 1/8,'1' Indicates that the clock source is AHB（HCLK）.

3、 ... and 、 Reload and interrupt

Through registers STK_CTRL Of ENABLE Set up ‘1' You can start SysTick. After starting SysTick Value is subtracted by one per clock cycle , The duty of ‘0’ Will register STK_CTRL Of COUNTFLAG Set up . If enabled at this time SysTick Interrupts trigger interrupts . You can set the register STK_CTRL Of TICKINT Location ‘1’ Can make SysTick interrupt .SysTick After counting to 0 The next clock cycle of will start from STK_LOAD Load value in register .

Four 、 Cycle interruption

Can make SysTick After the interrupt is turned on, a cycle interrupt will be generated , Cycle frequency and clock source and STK_LOAD The value of , The formula is as follows ：

F = FSRC / (STK_LOAD + 1)

The code is as follows ：

uint32_t SysTick_Config(uint32_t ticks)
{
  if ((ticks - 1UL) > SysTick_LOAD_RELOAD_Msk)
  {
    return (1UL);                  /*   Reload whether the value is valid （24 position ）*/
  }

  SysTick->LOAD  = (uint32_t)(ticks - 1UL);  /*   Set reload value  */
  NVIC_SetPriority (SysTick_IRQn, (1UL << __NVIC_PRIO_BITS) - 1UL); /*  Set up  SysTick  Interrupt priority  */
  SysTick->VAL   = 0UL;                                            /*  Clear the counter               */
  SysTick->CTRL  = SysTick_CTRL_CLKSOURCE_Msk |   /*  Set up  SysTick  From  AHB（HCLK）  */
                   SysTick_CTRL_TICKINT_Msk   |   /*  Can make  SysTick  interrupt               */
                   SysTick_CTRL_ENABLE_Msk;       /*  start-up  SysTick                  */

  return (0UL); 
}