STM32F030 Timer Delay -strange jitter

[werbung]

  while (1)
  {
 GPIOA->BSRR = 0x0040; // PA6
 Delay_us(100);
 GPIOA->BRR = 0x0040; // PA6
 Delay_us(100);
  } //while(1)

Hello ST after a lot programming with STM32F103 I startet with the STM32F030 and the STDPERIPH-Lib. Just an "easy example" of using a timer delay to blink an GPIO-Output.

I noticed a strange behaviour on the oscilloscope: Sometimes the timing of the delay switches to a very small delay. First I thougt of som integer overflow effects but when I switched to 32Bit delay function the strange jitter still exists.

After initializing STM32 with 48MHZ only GPIO and TIMER 14 is initialized. I thougt of a watchdog function but this is als disabled . What on earth hits this wrong timing in between the 100us delays???

//#####  TIMER 14 Init  ######## 
void TIMER14_Init(void)
{
	TIM14->PSC =  (uint16_t) 0x47 ; // Prescale 48.000.000 / 48 = 1MHz  // Prescale 48
	TIM14->ARR = (uint16_t)0xFFFF; //Autoreload 0xFFFF
	TIM14->CR1 |= 0x0001;	//  Timer enable	
}

void Delay_us(uint16_t us) // microseconds
{
volatile uint32_t start = 65536;   // offset for not running in negative integer
start += TIM14->CNT;  //  read Counter register
while(  ( (TIM14->CNT  + 65536 ) - start)  <  us   ); // compare old with new
}

Picture: smaller delay at the timeline center

thanks in advance for your help.

Henry

[werbung]

Offcourse the timer prescaler is 47 dec and not 0x47 hex for a 1 usecond timer clock. But this doesnt change the jitter problem .

Regards Henry

[RMcCa]

To get good deterministic timing you need to use a timer and its update event interrupt. Don't use HAL callbacks, write your own ISR. Blocking delays like you are using are unnecessay & i bet if you scaled out on the scope you will find some periodicity of the short pulses, which is not jitter​.

Don't know if your processor has DMA, but if it does you can use the timer update event to trigger the DMA that could write an alternating on/off pattern to the proper GPIO. So yes, it is possible to blink an led with the core doing nothing....

Try it and let us know what you find!​

[TDK]

The overflow of the counter is causing the problem. Your offset to avoid negative numbers is not doing what you think it is doing.

void Delay_us(uint16_t us) // microseconds
{
uint32_t start = TIM14->CNT;
while ((TIM14->CNT - start) % 65536U < us);
}

Note that since CNT is volatile, you don't need to declare start as volatile as well.

Edit: changed the code. Pretty sure this will work, did not test it though.

[werbung]

Hello TDK,

thanks a lot. This works without jitter.!!. Well done !!!

There are so many examples in the web, which do not consider timer overflow. I wonder how any of these softwares run strable. Of course most of the time the delay works but the wrong delays inbetween can cause a lot of trouble when needed for pulse shaping or simple display control software.

Regards Henry

[Piranha]

> I wonder how any of these softwares run strable.

No wonders - they don't.