Newsgroups: comp.robotics
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From: sasrer@unx.sas.com (Rodney Radford)
Subject: Re: PWM controller chip
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Date: Wed, 20 Jul 1994 23:13:51 GMT
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akr@philabs.philips.com (Arvind K. Ramadorai) writes:
>I am looking for a PWM controller chip. The functionality I am looking for
>is to input a digital word ( 4 to 8 bits) to the chip
>and a PWM (TTL compatible) waveform comes out, which can drive 
>a H- or T- bridge amplifier. Are ther any chips available?
>If so so I'd like to know the names/vendors/cost.

Don't know of any PWM controller chip that does exactly what you need, but
the same functionaility can be obtained in several simple ways:

   - feed the digital inputs to a simple R-2R ladder network for a simple
     A/D and connect it to one input of a comparator. Connect the other
     input of the comparator to a triangle waveform generator. In fact
     you could probably do this with a 555 and use the builtin op-amp
     on the 555...
   - feed the digital signals to a digital comparator with the other input
     of the digital comparator coming from a free running counter. This is
     basically the digital equivalent of the previous solution.
   - use two oscillators of an 8753 counter chip - the first counter
     sets the period of the PWM, 
   - get one of the relatively high priced PWM control chips like the 
     LM628/LM629. These are nice but a bit expensive (for me) at $70+ each.
   - dedicate a small microcontroller to the task. This is the solution
     I used on a robot I built last year. I used a PIC16c54 and passed it
     two 8bit numbers (with a strobe and ack) and the PIC generated two
     PWM outputs - 8bit accuracy, 4Khz max (I ran it at 1khz).
   - use a smarter microprocessor with builtin hardware PWM. This is the
     method I am using in a robot now (design based on the 87c752 with
     commands sent via the I2C bus - I am even using the interrupts for
     wheel incremental encoders, and the A/D to sense motor voltage/current).
     The microcontroller code is still under development....

I'm sure there are may other simple solutions - anyone want to add their
favorite methods to the list above?

--
---
Rodney Radford,     Senior Systems Developer,  Image Technology Group
sasrer@unx.sas.com  SAS Institute, Inc, Cary, NC 27513  (919) 677-8000 x7703

