STM32 Cortex system timer (SysTick)

SysTick clock, commonly known as "tick timer", can generate an interrupt at a set time. Functions such as delay_ms() can be seen everywhere in control engineering code. But its internal mechanism has never been clear. I spent some time to study it today.

First, let’s take a look at the configuration of the SysTick register in the data sheet, as shown in the figure: 

Register Description

 
 
When the clock source of stm32 
is selected as an external clock source, the Systick clock is HCLK /8. 
When the core clock source is selected, the Systick clock is HCLK

Delay Programming Principle 
Systick timer is a 24-bit down counter. After setting the initial value and enabling it, it will reduce the counter by 1 for each system clock cycle. 
When the count reaches 0, it will automatically reload the timer initial value from the RELOAD register. As long as the enable bit in the SysTick control and status register is not cleared, it will never stop.

Delay programming steps 
1. Calculate how many clock cycles fac_us are needed to generate 1us 
2. Calculate the value of the RELOAD register 
, which is the number of clock cycles required to generate the corresponding delay 
RELOAD = fac_us * nus 
3. Start counting 
4. Loop to detect the flag bit when the count reaches 0 
5. Clear the counter and turn off the timer

SysTick exception configuration steps 
1. Configure the three registers CTRL/LOAD/VAL 
2. Initialize the clock used by SysTick 
3. Clear the current value of the system and load the reload value 
4. Enable SysTick so that SysTick can respond to interrupts

====================== 
The library function version 
uses the ST function library to use systick in strict accordance with the following order:

1. Call SysTick_CounterCmd() – Enable SysTick counter 
2. Call SysTick_ITConfig () – Enable SysTick interrupt 
3. Call SysTick_CLKSourceConfig() – Set SysTick clock source. 
4. Call SysTick_SetReload() – Set SysTick reload value. 
5. Call SysTick_ITConfig () – Enable SysTick interrupt 
6. Call SysTick_CounterCmd() – Start SysTick counter

Systick interrupt service function 
void SysTick_Handler(void);

The register version 
uses an external 8M clock, the frequency from the phase-locked loop is 72M, and the AHB pre-divided frequency is 72M. 
Systick fixes 1/8 of the HCLK clock, that is, 9M, so the delay of 1us is 9 clocks.

void delay_init(u8 SYSCLK) //The system clock is 72MHz, SYSCLK=72{
    SysTick->CTRL &= 0xfffffffb ; //bit2 is cleared to 0, that is, the configuration selects the external clock
    fac_us=SYSCLK/8; //Hardware frequency division by 8, the value obtained by fac_us is used for the following clock function
    fac_ms =(u16)fac_us*1000;

}void delay_us(u32 nus) //nus if 10us{
    u32 temp;
    SysTick->LOAD = nus*fac_us; // If the delay is 10us, it is 10*9=90, which is loaded into the load register
    SysTick->VAL=0x00; //The counter is cleared to 0, because when the currrent field is manually cleared, load will automatically reload into VAL
    SysTick->CTRL = 0x01; //Configure the exception to take effect, that is, an exception notification will be issued when the counter counts down to 0
    do
    {
       temp = SysTick->CTRL; //After the time is up, this bit will be set to 1 by hardware, but will be automatically cleared to 0 after being queried
    }    while(temp & 0x01 && !(tmep &(1<<16))); //查询
    SysTick->CTRL = 0x00; // Turn off the counter
    SysTick->VAL = 0x00; //clear val}12345678910111213141516171819202122

The last while loop determines if Systick is still in the Enable state and the counter has not reached 0, 
then it will keep looping to write the current SysTick->CTRL register value into the variable temp and continue the next judgment. 
When Systick is Disabled or the counter reaches 0, the loop stops.

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