Electronic – PWM generation PIC16F628A

On the bottom of page 350 of the microcontroller datasheet, it mentions that writing a small value to the timer value register during the overflow interrupt might cause the next interrupt to be triggered only on the next pwm iteration, since the timer continues to count while the interrupt routine is being executed.

An unsynchronized write to the TIM channel registers to change a pulse width value could cause incorrect operation for up to two PWM periods. For example, writing a new value before the counter reaches the old value but after the counter reaches the new value prevents any compare during that PWM period. Also, using a TIM overflow interrupt routine to write a new, smaller pulse width value may cause the compare to be missed. The TIM may pass the new value before it is written.

This is confirmed by the fact that the pwm value is held high for one entire pwm clock period + what looks like the timer length (based on the surrounding lengths). The value being written to the timer length register is probably close to 0 at the time of error, so it is quite viable that the counter has passed the smaller value during the interrupt, and would only trigger on the following cycle.

This could be fixed by increasing the sinusoid minimum level to a level higher than the time it takes to execute the ISR, or changing the mechanism by which the new level is set. The top of page 351 details how this may be done.

Your code is good, but the Timer0 input on Microchip parts is the instruction cycle time, which is the oscillator frequency/4. So with your present setup, you are running with an input frequency of 1 Mhz instead of 4Mhz. After going through your prescaler, you are down to 125 KHz, and since you are counting up from 250, you are getting an interrupt at a rate of 500 Hz. Since you are turning the LED on and off every 20 interrupts, you are down to 25 Hz, which is what you can see. Try a prescaler of one. This will speed up the pwm to 200 Hz, but will also reduce the time to fade out. You can increase the fadeCounter >= number to 192 (24*8) to compensate and make it fade at the same rate. If it's still fading too fast, change fadeCounter from an unsigned char to a 16-bit integer so you can use a larger number than 255. Have fun!