Generate a square wave in output compare mode

Posted on March 10, 2015 at 14:34

Hi,

I'm using the STM32F411RE Nucleo board and I would like to generate

a 25 MHz square wave by using the output compare of timer3 in toggle mode.

Timer is configure as follow :

  TimHandle.Instance = TIM3;

  TimHandle.Init.ClockDivision = TIM_CLOCKDIVISION_DIV1;

  TimHandle.Init.CounterMode = TIM_COUNTERMODE_UP;

  TimHandle.Init.Period = 65535;

  TimHandle.Init.Prescaler = 0;

  Config.OCMode = TIM_OCMODE_TOGGLE;

  Config.OCPolarity = TIM_OCPOLARITY_HIGH;

  Config.Pulse = 1;

  if(HAL_TIM_OC_ConfigChannel(&TimHandle,&Config,TIM_CHANNEL_1) != HAL_OK)

  {

    Error_Handler();

  }

  if(HAL_TIM_OC_Start_IT(&TimHandle,TIM_CHANNEL_1) != HAL_OK)

  {

    Error_Handler();

  }

The pulse value is updated in the callback function after two period of timer clock (cnt = capture compare), so that generate a 25 MHz which is half timer clock frequency.

But when i'm watching the output pin on oscilloscope, the signal appears to have a half period of 1.3 ms, which is the value of 65535 clock cycles, while i'm expecting to have a capture compare value incremented by one and changing the state of the pin every 20 ns.

Thanks in advance,

Regards

Christophe