threewheeler7 wrote:
> i was told by a microchip rep that i should get something from the
> PIC18Fxx31
> family to have a one-chip solution to control 4 brushed dc motors
> with PWM.
> so i ordered myself some PIC18F2331's to get started.

While it's good to get the advice of someone that knows more about these
parts then you do, you are still ultimately responsible for your design and
no amount of external advice is a substitute for your own brain.  Ordering
parts just because someone said they would work for you application is
silly.  You should have taken the advice as a pointer, then looked in the
datasheet to see if it indeed would address your issue.

> i have done PWM
> with 16f chips via the CCP module. but crap, the PIC18Fxx31's use 22
> control
> registers!?

So?  Once you read the datasheet and understand what each bit in those
registers do, you will see that you won't need most of the features for your
application.  The vast majority of these 22 registers (taking your word
that's really the number, sounds high) will be set up once at module
initialization and then left alone.  This is really not a big deal, assuming
of course you have read the manual section on the motor control PWM module.

Note that this module only has 3 independent PWM outputs.  It was intended
for 3 phase AC motor control or up to 3 phase switching power supplies.  The
6 outputs are complimentary phases of each of the 3 PWMs.

Since you are only driving brushed DC motors, you don't need the fancy
complimentary PWM outputs.  However, you will need to use at least one of
the normal PWM (CCP module) outputs to drive 4 motors independently.  You
wont have exactly the same code driving all the motors, which isn't much
harder but does present additional opportunity for screwing up.  Have you
looked at the 30F parts?  I think some of them have more native PWM outputs
of the same type.

Do you need to drive the motors in both directions, or just variable power
in a single direction?  The latter will be considerably easier both in
firmware and in hardware.

> i am ready for a whole bunch of you to start getting mad
> at me,
> but i am writing this in ASM.

Why do you think that is bad?  It sounds like your application is a bunch of
low level bit twiddling and management of the hardware, which will be easier
in assembler.

> besides this will be pretty small
> code at first any way.

The actual PWM management won't be that big, but if you need bi-directional
drive you will have to wake up to write the code.  The larger part of the
code will be deciding the desired PWM value given inputs and a PID
controller or whatever.  This is certainly doable, but not what I would
classify as "trivial" or "pretty small".

> Besides for "just read"ing the datasheet, what
> suggestions do you have for me to learn to use the pcPWM on these
> beasts?

It sounds like you haven't even read the datasheet yet.  Until you do that,
it's pointless to suggest other things.  Once you've read the manual you can
come back and as about specific things that didn't make sense.


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