The range and performance of software PWM

Guys

I am on the lookout for an all software PWM with perhaps 8 bit resolution (256 states).

The frequency is ofcourse sub khz - definitely around 500Hz.

What I wanted to know is what is the range of PWM freq available using software methods ? Will it change with clock frequency.

Also, how will the performance of the peripherals - like USART, ADC, CCP, etc be affected with such a technique. And how many channels of PWM I can have this way ?

I am led to believe that the output created using software PWM is as good as that of the hardware PWM - atleast for 8 bit resolution.

How true is it ?

Please note that I am not as interested in the code rightaway (I located LOTS of soft PWM code here AND google) - I wanted to hear what you guys have to say about this.

My needs are to generate sub khz (around 100 to 500Hz - not all at same time) WITH adjustable duty cycle (minimum 10% to 90%) to drive a DC Motor control system. (I am NOT on the lookout for DDS !)

If possible I should be able to leave the TIMER free because this same IC will be used for quadrature decoding.

I will heavily prefer a technique that does not use 18F parts and will work nicely on a 16F like the 628a/877a.

4/8 channels of PWM will be nice because I will be able to implement locked anti phase control instead of signed magnitude.

Keep the suggestions flowing in ! Oh - ofcourse they must be in CCS C

I read 00654a.pdf titled "PWM, a Software Solution for the PIC16CXXX".

It's definitely an interesting read.

Anyone coded that already in C ? The code need not necessarily be tested, or even compiling for that matter.

Cheers

soft pwm ccs

If I understand what I've read so far, you will need a timed interrupt and no other interrupts.

First update the PWM values, then poll the USART and anything else that needs to be serviced.

A few lines of code will be needed for each channel of PWM, and it will be limited to the low speeds. Let us assume you want 500Hz and 8 bit resolution. You will need to service it every 7.8uS (2mS/255values). With a 20MHz crystal you will need to do everything in a 39 cycle instruction loop.

I hope this helps.

software pwm

After poking around with the motors, I see I do well with 250Hz...

Anyone got some code to do that ?

Software PWM

Guys,

will it be possible to design hardware using 74xx logic to divide frequency by a known factor, BUT maintaining the duty cycle.

For example, such a circuit would divide a input of y Hz @ x % duty to (y/k) Hz at x % duty (we have divided by k)