How to create a microseconds delay function in STM32F0

Question

I was using STM32F4 and i had created a microsecond delay function using DWT register. And now, i'm using STM32F030F4P6. And this MCU has not DWT register. So, i tried to create another delay function.

void delay_ticks(uint32_t ticks)
{
    SysTick->LOAD = ticks;
    SysTick->VAL = 0;
    SysTick->CTRL = SysTick_CTRL_ENABLE_Msk;
    // COUNTFLAG is a bit that is set to 1 when counter reaches 0.
    // It's automatically cleared when read.
    while ((SysTick->CTRL & SysTick_CTRL_COUNTFLAG_Msk) == 0);
    SysTick->CTRL = 0;
}


static inline void delay_ms(uint32_t ms)
{
    delay_ticks((ms * 8000)); // number of ms*8000000 can overflow so i use 8000 instead of (ms * 8000000) / 1000)
}

when i try to set delay to 1 second, i get 8 seconds. And when i try to set delay to 500 miliseconds i get 4 seconds. There is always a 1:8 ratio

note: the VAL register is automatically loaded with the value of the LOAD register when the timer is enabled.

EDIT: I added these lines of code to choose HCLK as a source and now it works. (HCLK/8 is default value. so i need this configuration):

SysTick->CTRL &= ~SYSTICK_CLKSOURCE_HCLK_DIV8;

SysTick->CTRL |= SYSTICK_CLKSOURCE_HCLK;

Answer 1

System Clock is clocked by AHB bus divided by 8. So simply divide the result by 8. Or better yet, when you currently multiply by 8000, multiply only by 1000.