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Getting PWM caounter value

Question asked by malik.lior on May 27, 2015


In my application I using 16MHz of system clock and l create a PWM (in this case PWM_D) of 200Hz generated by 4MHz clock. Therefor my timer's counter value is (4M/200=) 20,000.
Also, l have another PWM (in this case PWM_Input) which consider as input interrupt. 
l would like to get the PWM_D counter each time l get an interrupt caused by PWM_Input.

At the interrupt handler l wrote:
#pragma vector = 6
__interrupt void EXTI_PORTB_IRQHandler(void)
     prevTmpTim2Counter = tmpTim2Counter;
     tmpTim2Counter = TIM2_GetCounter()  ;
     if (GPIO_ReadInputPin(GPIOB, GPIO_PIN_5) == 0)
          delta = tmpTim2Counter - prevTmpTim2Counter;

this code gives us the delta Toff of the PWM_Input. 
So, if PWM_Input is 200Hz and 50% duty cycle, l suppose to get counter value of 10,000. (Because the counter value of 200Hz is 20,000 as we calculate before).
However, I got delta of 9900 +/- 250 i.e. sometimes l get values above 10,000 but mostly under 10,000.

In addition, if we take in consider the identification of the MCU to alternate between "1" to "0" in different voltage (depend on the hysteresis voltage), yet we should get the same delta, Not perfect 10000 but same delta each time. 

please your advice soon as possible.
Thanks, Lior