pwm basics

[luigi.malago]

Hi to everybody,
I've having the same problem here, that's to sat i need to generate a PWM for a servo motor, with period 20ms, and I cannot use the CCP since the minimun frequency is too high.

I undestand the method proposed by Mike, K8LH, where he creates a PWM wave from a series of PWM cycles. My question is:

I know that this solution is much better then a pure SW PWM where the PIC is polling waiting for chainging the output. My question is whether the Mike, K8LH's solution is better compared to a PWM wave generate using 2 timer events without the CCP.

I heard about jitter in case i use a normal pin (not PWM module) but i'd like to have your opinion too.

thanks a lot,
Luigi Malagò

 

[philba]

by jitter, I assume you mean variation in the exact period of the pulses. the amount of jitter will be pretty small and the servo won't care at all.

I agree, don't use delay loops do this but no one proposed that solution. the timer interrupt solution is quite reasonable and you really only need one timer.

 

[luigi.malago]

hi philba,
so when dealing with servo, the solution of generatin pwm wave with a timer without CCP is a good solution. Thanks for your opinion. So why using PWM? I mean, it's beacause of the relatively small frequency required by the servo or in many case i can used timers instead of CCP PWM?

I agree, don't use delay loops do this but no one proposed that solution.

Ok, i know it, I was referring to another article where different solution where proposed, icluding the polling one.. but it was clearly stated that it was inefficient.

Luigi

 

[philba]

PWM simply refers to the variable width of the pulse. It doesn't imply a specific way of generating the pulse train.