What is the problem with this delay function?

[ahmetgunduz]

Hi,

i'm trying to create a delay function for microsecond delays. But it doesn't work. I couldn't understand where the problem is. Could you please let me know?

(i'm using stmf030f4)

edit: To understand where the problem is i'm trying to create a milisecond delay function. And i set delay to 1000 miliseconds. But i see 8 seconds as a result. (and if set delay to 500 i see 4 seconds delay )

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)
 
}

 

[berendi]

The subexpression (us * 8000000) would overflow whenever us > 536 (2^32 / 8000000), because us is a 32 bit integer. Try

delay_ticks(us * 8);

instead.

[ahmetgunduz]

Thanks, i edited post. Can you take a look again please?

[turboscrew]

Note that systick is 24-bit. And with 8 MHz systick clock, 8000 = 1 ms. One second (0x7A1200) _should_ fit, though.

You should cast the 64-bit uint to 32-bit before writing it into the reload register.

[ahmetgunduz]

i changed uint64_t to uint32_t. But still i see the same thing. When i change

delay_ticks((ms * 8000))

this line with

delay_ticks((ms * 8000))

this, it works properly. But i don't understand why there is a 1/8 ratio

[turboscrew]

Maybe you should check from the generated code, that the clocks ate, indeed, set up the way the picture shows. Especially check the systick clock divisor selection bit.

I recall that bit defines whether the systick runs in the bus clock speed or if the clock is divided by 8. At least Atollic debugger shows the HW registers.

[ahmetgunduz · Accepted solution]

thank you now it works fine. i added these.

SysTick->CTRL &= ~SYSTICK_CLKSOURCE_HCLK_DIV8;
SysTick->CTRL |= SYSTICK_CLKSOURCE_HCLK;