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Odd clock issue on STM32F0DISCOVERY

Question asked by mucker.matthew on Aug 29, 2012
Latest reply on Aug 29, 2012 by Clive One
I wanted to explore the SYSCLK on the STM32F0DISCOVERY board so I wrote a quick program to route SYSCLK to the MCO pin. I connected a logic analyzer to the pin and I see a ~200Hz clock with lots and lots of noise on it. I'm wondering if anyone can help me determine what's going on. The screenshot is here.

When I break into the debugger and set RCC_CFGR.MCO to 0b101, I get a nice square 8MHz output from the 8MHz HSI just like I expect. When I change MCO back to 0b100 (SYSCLK), I get this strange signal again.

My source code is below; it's really an only slightly modified version of the Atollic default project. I'm sure there's probably something external to the code that I pasted that affects SYSCLK, but I don't know what that would be.

Can anyone tell me what I'm seeing in the logic analyzer output? The logic analyzer is capturing at 24MHz.

int main(void)
  GPIO_InitTypeDef        GPIO_InitStructure;
  /* TODO - Add your application code here */
  SysTick_Config(4800);  /* 0.1 ms = 100us if clock frequency 12 MHz */
  ii = SystemCoreClock;    /* This is a way to read the System core clock */
  //enable the peripheral clock on GPIO Port A.
  //configure the MCO pin to output the SYSCLK. We need to set
  //bits 26:24 of RCC_CFGR to 100
  RCC->CFGR &= 0b11111000111111111111111111111111;
  RCC->CFGR |= 0b00000100000000000000000000000000;
  //Configure PA8 to alternate function MCO
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;          //alternate function mode
  GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;        //push-pull output type
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_8;
  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;      //no pullup/pulldown
  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_Level_3;   //fast
  GPIO_Init(GPIOA, &GPIO_InitStructure);                //apply the settings to PA8
  GPIO_PinAFConfig(GPIOA, GPIO_PinSource8, GPIO_AF_0);  //set alternate function 0 for PA8