STM32H750 - Timer preload has no effect - Interrupt

[Kisen]

Hi.
I am hoping someone can solve this problem im having.

I have Timer3 running and trying to get a 1 second interrupt. However i am getting strange behaviour that i simply dont understand.

With the below code my interrupt appears to be firing but its too fast. When i scope the LED pin its toggling at 33.3Khz. This suggests to me that my clock prescaler was wrong.
This is where it gets odd for me. Changing the TIM3->ARR i get a change on the scope that is sensible, i.e increase the value and get a slower interrupt.
However, changing the prescaler does nothing at all. it doesnt matter what value i enter nothing changes.

I have been trying for a couple of days to find the fix for this when i came across a post that said the order of the prescaler and reload was important. I cant see how but i thought i would try it. So i loaded ARR value then PSC value.
This resulted in me getting a fixed 4 second interrupt. Changing either the PSC or ARR has no effect.

So my question.
Is there a reason why my code as below seems to ignore the PSC value entirely?

Further info.
My clock is startup default. So 4Mhz.
I have tried polling instead of interrupt, and i seem to get a reasonable 1 second blink. PSC = 3, ARR = 9999.
I have tried a different timer (Timer4) and get the same result as i do on Timer3.

#include "stm32h7xx.h"

static volatile uint8_t led_on = 0;

void TIM3_IRQHandler(void)
{
  /* if UIF flag is set */
  if(TIM3->SR & (1<<0))
  {
    if( led_on == 0)
    {
            led_on = 1;
    }
    else
    {
            led_on = 0;
    }
        
        /* Clear the Interrupt Status */
        TIM3->SR &= ~(1 << 0);
  }
}

int main(void)
{
    // GPIO SETUP CODE
    
    RCC->AHB4ENR |= (1<<4);                            // Enable GPIOE clock
    GPIOE->MODER &= ~((1<<7) | (1<<6));        // Reset PE3
    GPIOE->MODER |= (0<<7) | (1<<6);        // Pin PE3(bits 5:4) as Output (01)
    
    GPIOE->OTYPER &= ~(1<<3);            // Set PE3 as Push-Pull
    
    // Speed register
    
    // pull up / pull down
    
    
    // TIMER 3 SETUP CODE
    
    RCC->APB1LENR |= (1 << 1);        // Enable the APB clock FOR TIM3
    
    TIM3->PSC = 3999;    // Setup prescaler - Req Freq = (CLK / (PSC + 1)) 1000 = 4000000 / 4000
    TIM3->ARR = 999;
    
    //TIM3->CR1 &= ~(1<<1);        // Clear UDIS bit - MADE NO CHANGE
    TIM3->CR1 |= (1<<2);        // Set URS bit - MADE NO CHANGE
    TIM3->EGR |= (1<<0);         // Update prescaler
    //TIM3->CR1 &= ~(1<<4);        // Clear DIR bit - MADE NO CHANGE
    
    /* Enable the Interrupt */
  TIM3->DIER |= (1 << 0);
    
    /* Clear the Interrupt Status */
  TIM3->SR &= ~(1 << 0);
    
    __NVIC_SetPriority(TIM3_IRQn, 2);   // Set TIM3 interrupt vector priority to 0
 
  __NVIC_EnableIRQ(TIM3_IRQn);    /* Enable NVIC Interrupt for Timer 3 */
    
    /* Enable TIM3 module */
    TIM3->CR1 = (1 << 0);
    
    
    while(1)
    {
        
        if(led_on)
    {
            GPIOE->ODR |= (1<<3);     // Set PE3 to ON
    }
    else
    {
            GPIOE->ODR &= ~(1<<3);     // Set PE3 to OFF
    }
    }
}

 

[rjenkinsgb]

Have a look at the code in this library - it works fine:

STM32_TimerInterrupt - Arduino Reference

The Arduino programming language Reference, organized into Functions, Variable and Constant, and Structure keywords.

www.arduino.cc

 

[Kisen]

Thanks,
I have been looking further at this. I have taken a closer look at the registers while in debug. I noticed that if i breakpoint at TIM3->PSC then it started working as it should. Where with no breakpoint it didnt work.
I have found that the register was even being written unless it was in a breakpoint.

So as a quick fix i wrote that same register twice and it now works. Yay isnt coding fun haha.

Is there any reason why a register wont write? Do they need time to process or some other random reason?

 

[rjenkinsgb]

Is there any reason why a register wont write?

Odd.. I don't know. Possibly buffering or a timing glitch with it reloading??

 

[aliarifat794]

Did you ask this question to the STM community? What did they say?