HAL_Delay(1); takes 2ms

[PNova.2]

Hello

I just found out that the HAL_Delay() delay actually takes 1ms longer than the desired value. Why?

STM CUBE IDE 1.9.0

STM32F072RBT6

[waclawek.jan · Accepted solution]

Because it's an "at-least-N-ms" delay with 1ms granularity.

JW

[Tesla DeLorean]

If you want something that can resolve better, use a TIM that clocks faster, and read the TIMx->CNT

Code may also be interrupted

[S.Ma]

To measure or wait a precise duration, you may need a dedicated stopwatch, sharing a always running wallclock won't be the same...

[PNova.2]

waclavek.jan,Tesla DeLorean, S.Ma :

But I'm not interested in accuracy. The time is always accurate, just always +1ms, because as I found out, the function automatically increments the variable at the beginning.

__weak void HAL_Delay(uint32_t Delay)
{
  uint32_t tickstart = HAL_GetTick();
  uint32_t wait = Delay;
  
  /* Add a freq to guarantee minimum wait */
  if (wait < HAL_MAX_DELAY)// allways add  uwTickFreq (1)
  {
    wait += (uint32_t)(uwTickFreq);
  }
  
  while((HAL_GetTick() - tickstart) < wait)
  {
  }
}

For values in hundreds of milliseconds it is of course not important, but for 1ms it does 100%. And that caused me a problem that I only discovered by using a logic analyzer.

Of course, it is a weak function that I can override according to my needs, but I was more interested in the reason why it is so.

Sure, the comment says that it should guarantee at least a minimal delay, but it can certainly be better:

  /* Add a freq to guarantee minimum wait */
  if (wait < 1)
  {
    wait += (uint32_t)(uwTickFreq);
  }

Or am i wrong?

[waclawek.jan]

I tried to answer ecactly this in the article I linked to above.

JW

[Tesla DeLorean]

Depends when it is called.

A fraction of a second prior to the systick will result in a very short delay.

The solution is really to have a time base that resolves in much finer resolution, say 1 or 2 orders of magnitude.

[PNova.2]

Yes, that's what I wanted to know. I just didn't notice that "Because" was a link.

Thanks