is it possible to see system clock from GPIO?

[vicki19880929]

Posted on April 08, 2014 at 13:28

Hi all, 

I'm working on stm32l MCU and wonder if it is possible to get the system clock waveform from GPIO output pin?  If yes, where could I find it from the datasheet?

Thanks a lot,

Shan

[Tesla DeLorean]

Posted on April 08, 2014 at 15:23

The MCO pin (PA.8) should allow fractional internal clocks to be output. Some abstracted, but synchronous, clocks can be generated by timers.

[vicki19880929]

Posted on April 09, 2014 at 05:13

Thanks clive!  That's a good start.  

I gave it a try and found the PA8 waveform frequency isn't the SYSCLK freq 32MHz but only around 2.17MHz.  The frequency did change if I changed the divider value.  Could anyone point out  if I did anything wrong here?  Many thanks!  :)

   

int main(void)

{

  GPIO_InitTypeDef GPIO_InitStructure;

  RCC_AHBPeriphClockCmd(RCC_AHBPeriph_GPIOA, ENABLE);

  GPIO_PinAFConfig(GPIOA, GPIO_PinSource8, GPIO_AF_MCO);

  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_8;

  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;

  GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;

  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_40MHz;

  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;

  GPIO_Init(GPIOA, &GPIO_InitStructure);

  RCC_MCOConfig(RCC_MCOSource_SYSCLK, RCC_MCODiv_1);

    while(1) {

        delay();

    }

}

 

[waclawek.jan]

Posted on April 09, 2014 at 09:53

Looks good.

Your SYSCLK probably is actually around 2MHz - I guess it never changed from the default nominally 2.097MHz MSI clock.

JW