TIM3 timer period

[asecen89]

Posted on May 21, 2014 at 15:46

Hi,

I use STM32F427 chip. In my application, I am generating a 25.6 kHz signal using an external signal with 9.984 MHz. In order to do so, the calculation for timer period should be as follows; (9.984MHz / 25.6kHz) - 1 = 389. However, in the application the microcontroller generates the 25.6 kHz when I set the timer period to 390. I am confused by that, shouldn't I subtract 1 by 390 to obtain the timer period? 

Thanks

[Tesla DeLorean]

Posted on May 21, 2014 at 16:11

Yes, 390-1 should be the number, you'd want to consider how you are measuring the respective signals, and if the resynchronization of the input put might impact it.

[asecen89]

Posted on May 21, 2014 at 16:32

I have a counter in my code to count the number of pulses that the timer generates. Also a PPS signal is connected to one of microcontroller pins which triggers an interrupt at each pulse. It turned out that when I check the number of pulses that counted between two PPS signals, it is exactly 25600 when period is 390, and it is 25666 when period is 389.

[asecen89]

Posted on May 21, 2014 at 16:33

I have a counter in my code to count the number of pulses that the timer generates. Also a PPS signal is connected to one of microcontroller pins which triggers an interrupt at each pulse. It turned out that when I check the number of pulses that counted between two PPS signals, it is exactly 25600 when period is 390, and it is 25666 when period is 389.

[waclawek.jan]

Posted on May 21, 2014 at 16:58

Do you set directly the timer ARR register, or do you use some sort of ''library call'', which performs the decrement?

JW

[asecen89]

Posted on May 22, 2014 at 12:45

The piece of code that sets the timer period is below;

  TIM_TimeBaseStructure.TIM_Prescaler = 0;

  TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;

  TIM_TimeBaseStructure.TIM_Period = 390;

  TIM_TimeBaseStructure.TIM_ClockDivision = 0;

  TIM_TimeBaseInit(TIM3, &TIM_TimeBaseStructure);

[asecen89]

Posted on May 22, 2014 at 16:23

In reference manual it was written that the counter starts from 0 and counts to ARR value. I use PWM1 mode to generate the pulse, can this cause to counter to count from 1 to ARR value? It doesn't really make sense, but could this happen?

[waclawek.jan]

Posted on May 22, 2014 at 16:49

We would need to see all the relevant portions of your code to judge.

Can you measure the input and output frequency by some independent method, e.g. using a frequency counter? Or, alternatively, you can try two smaller ARR values to clearly see the difference - say, 10 and 11 - and have a look at the resulting waveform with an oscilloscope?

JW

[asecen89]

Posted on May 23, 2014 at 09:25

I am posting the whole timer configuration code below;

  TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;

  TIM_OCInitTypeDef  TIM_OCInitStructure;

  GPIO_InitTypeDef GPIO_InitStructure;

  TIM_ICInitTypeDef  TIM_ICInitStructure;

  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_8;

  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;

  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;

  GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;

  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP ;

  GPIO_Init(GPIOC, &GPIO_InitStructure);

  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_5;

  GPIO_Init(GPIOB, &GPIO_InitStructure);

  GPIO_PinAFConfig(GPIOB, GPIO_PinSource5, GPIO_AF_TIM3);

  GPIO_PinAFConfig(GPIOC, GPIO_PinSource8, GPIO_AF_TIM3); 

  TIM_TimeBaseStructure.TIM_Prescaler = 0;

  TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;

  TIM_TimeBaseStructure.TIM_Period = 390;

  TIM_TimeBaseStructure.TIM_ClockDivision = 0;

  TIM_ETRClockMode1Config(TIM3, TIM_ExtTRGPSC_OFF,       TIM_ExtTRGPolarity_NonInverted, 0);

 

  TIM_TIxExternalClockConfig(TIM3, TIM_TIxExternalCLK1Source_TI1,          TIM_ICPolarity_Rising, 0);

  TIM_SelectSlaveMode(TIM3, TIM_SlaveMode_External1);

  TIM_TimeBaseInit(TIM3, &TIM_TimeBaseStructure);

  TIM_ICInitStructure.TIM_Channel = TIM_Channel_3;

  TIM_ICInitStructure.TIM_ICPolarity = TIM_ICPolarity_Rising;

  TIM_ICInitStructure.TIM_ICSelection = TIM_ICSelection_DirectTI;

  TIM_ICInitStructure.TIM_ICFilter = 0;

  TIM_SelectHallSensor(TIM3, ENABLE);  

  TIM_ICInit(TIM3, &TIM_ICInitStructure);

  TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_PWM1;

  TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;

  TIM_OCInitStructure.TIM_OutputNState = TIM_OutputNState_Disable;

  TIM_OCInitStructure.TIM_Pulse = 15;

  TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High;

  TIM_OCInitStructure.TIM_OCIdleState = TIM_OCIdleState_Reset;

  TIM_OC2Init(TIM3, &TIM_OCInitStructure);