Additional PWM control

ferrumvir · Joined: 01 Feb 2006 Posts: 64 Location: England

Hi,

I'm using a CAN enabled PIC, one of the PIC18Fxx8(x) series chips to interface with a CAN network (think I've just stated the obvious)

The 28 pin chips only have one CCP module and the 40 pin chips have two. Unfortunately for my application I require 1 CCP and 2 PWM outputs (which are on the CCP pins).

I was wondering what the options for expanding the PWM channels were and which were best/simplest?

These are the options I think I have.

1. To use DS1050-xxx, which are 5bit PWM outputs controlled through a two wire interface (I2C effectively), however these power up to 50% duty cycle and therefore I would require a relay to prevent power flow during this period. This is the only digital interface PWM chip I could find.

2. To use a second PIC (something like the PIC16F636, 14pins 2 CCP modules) and program it as I2C slave, giving me the 2 additonal PWM outputs I require.

3. Bit Bang a standard pin (a software PWM), which I suspect will be less than accurate and certainly will not be an elegant solution.

Are there any options I'm over looking? Question

Are there other chips similar to the DS1050, which my search has missed? Question

Thanks for your help

Cheers Scott

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

ferrumvir · Joined: 01 Feb 2006 Posts: 64 Location: England

Hi,

The frequency of the manual controller we're using is 25KHz, and I was intending to duplicate this, though I suspect a slower PWM would probably not cause the motors a problem.

The PWM frequency accuracy is an not an issue for this applicaton, power flow controller to a motor.

Idealy I'd like 1% resolution, the minimum we'd accept would be 10% (4bit).

I'll have a look into the ISR Timer, am I correct in thinking that this just using the Timer0 interupt, and then setting the timer with a number corresponding to delay to the next switch (up or down)?

Cheers Scott