Software PWM

[vsmguy]

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

Cheers

Vimal

[vsmguy]

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.

[SimpleAsPossible]

I recently rejected a software PWM on a project. Not that my needs match yours....

Random thoughts:

For true 8 bit resolution, don't you need ( 500 Hz * 256 counts ) = 128 kHz timer ticks? Are you sure you don't want a hardware solution? It's so very convenient to set a timer compare value and just let the interrupt happen when it happens. Of course, I'm a brute force kind of programmer. No elegance from me.

If you want 1% steps, you're still looking at ( 500 Hz * 100 counts ) = 50 kHz. It's doable but not fun to code.

Mmmm....donuts..... (hey, I said they were random!)

Could you live with a "sidecar" 8-pin PIC with built-in PWM? Or even one of those little 6-pin SOT-23 devices? I guess that wouldn't work for very many outputs.

What about the motor-specific PICs? Or (pardon the heresy) one of the other brands of microcontrollers that have devices specifically intended for motor PWM?

Mmmm....chocolate.....

I think you can have pretty much any number of outputs. It should just cost you some RAM and a little bit of processing time for each one.

[vsmguy]

I believe I can use 250Hz.. infact I want to use from 25Hz to 250Hz

I will devote an entire PIC for the MCS. This PIC will do :

1. Dual channel PWM
2. Current sensing (ADC)
3. Quadrature decoding
3. Interface to the ControlUnit via SPI/I2C

Smaller PICs have single channel PWM. Using 3 PICs to control 2 motors is not only wasteful financially but board space too besides consuming more power.

I am sure a software PWM is the way to go UNLESS someone shows me it's not...