Writing Systick tick timer and delay function for STM32

1. What is the Systick tick timer?

The Systick timer is a system tick timer, a 24-bit countdown timer that automatically reloads the timing initial value from the RELOAD register when it counts to 0. As long as the enable bit in the SysTick control and status register is not cleared, it will never stop and can work even in sleep mode.

A timer is used to execute an action after a specified time.

 Systick timer is often used for delay or heartbeat clock of real-time system. This can save MCU resources and avoid wasting a timer. For example, in UCOS, time-division multiplexing requires a minimum timestamp. Generally, in STM32+UCOS system, Systick is used as UCOS heartbeat clock.

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2. Systick is generally used for delay and precise delay.

There are 4 Systick registers in total

    CTRL SysTick Control and Status Register 

    LOAD SysTick automatically reloads the initial value register 

    VAL SysTick current value register 

    CALIB SysTick Calibration Value Register

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3. Systick timer principle

This timer sets an initial value, and then decrements the initial value to 0, which means the timing is completed. After completion, an interrupt can be generated or not.

Bit 0 is the enable bit, which can enable or disable the timer

The first bit is the interrupt enable bit, which determines whether an interrupt is generated after the subtraction is completed.

The second bit is the clock selection bit, which can select external or internal clock as the clock source.

The 16th bit is the flag bit. After the number is subtracted to 0, this bit is set to 1 and is automatically cleared after being read.

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4. uSysTick reload value register-LOAD

When the current value register is decremented to 0, the value of RELOAD will be automatically assigned to the current value register.

if(VAL == 0)

    VAL = RELOAD

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5. SysTick Current Value Register - VAL

VAL is counted down from the initial value (RELOAD) by one, and when it reaches 0, the initial value (RELOAD) is reassigned to VAL.

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6. Implementation of tick timer

For STM32, the external clock source is 1/8 of HCLK (AHB bus clock) and the core clock is HCLK clock

1. Select the clock source

    SysTick_CLKSourceConfig(); //Select the clock source in the misc.c file

    SysTick_Config(uint32_t ticks) //Initialize systick, set the clock to HCLK, and enable interrupt in core_cm4.h file

2. Write a delay function

3. Set interrupt priority grouping

    NVIC_PriorityGroupConfig( NVIC_PriorityGroup_2);

    Generally, when using interrupts, we need to group our priorities at the beginning, and then set the priorities after the grouping.

    Try not to change groups frequently in the same program. Frequent switching of groups may cause unexpected errors in the program.

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7. Use interrupts to implement delay

Systick interrupt service function: void SysTick_Handler(void);

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Notice:

  In uCortex-M system, Systick code can be used universally.

  If you find that the delay is inconsistent during use, the problem is usually caused by different core clocks. You can modify the ticks value.

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The reference code is as follows:

void init_delay(void)

{

    /*Our external crystal oscillator is 8MHz, and then multiplied to 168M, then the Systick clock is 21M, which is the Systick counter

    Every time VAL decreases by 1, it means 1/21us has passed*/

    //1. Select the clock source (select external clock)

    //External clock needs to be /8, internal clock is never used

    SysTick_CLKSourceConfig(SysTick_CLKSource_HCLK_Div8); 

}

//2. Write a delay function

//The maximum delay does not exceed 798.915ms, which is (2^24)/21/1000

void delay_us(u32 nus)

{

    u32 temp = 0;

    //1. Realize a delay of 1us*nus

    //21 is the external clock frequency, if you choose the internal clock it is 168M/8

    SysTick->LOAD = 21*nus; //Set the automatic load value to 21 (1us)

    SysTick->VAL = 0x00; //Set the current initial value to 0

    SysTick->CTRL |= SysTick_CTRL_ENABLE_Msk; // Turn on (enable) counting: enable SysTick timer

    do

    {

        //Read the control register

        temp = SysTick->CTRL;

    }while(!(temp & (1<<16)));//Wait for the count time to arrive (bit 16)

    SysTick->CTRL &= ~SysTick_CTRL_ENABLE_Msk; // Disable count

    SysTick->VAL = 0x00; //Set the current initial value to 0 and reset VAL 

}

void delay_ms(u32 nms)

{

    u32 temp = 0;

    //1. Realize a delay of 1us*nus

    SysTick->LOAD = 21000*nms; //Set the automatic load value to 21 (1us)

    SysTick->VAL = 0x00; //Set the current initial value to 0

    SysTick->CTRL |= SysTick_CTRL_ENABLE_Msk; // Turn on (enable) counting

    do

    {

        //Read the control register

        temp = SysTick->CTRL;

    }while(!(temp & (1<<16)));

    SysTick->CTRL &= ~SysTick_CTRL_ENABLE_Msk; // Disable count

    SysTick->VAL = 0x00; //set the current initial value to 0 

}

void delay_s(u32 s)

{

    while(s--)

    {

        delay_ms(500);

        delay_ms(500);

    }

}