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Dev kit for lots of PWM outputs?

Discussion in 'Electronic Design' started by [email protected], Feb 13, 2007.

  1. Guest

    Hello,

    I'm considering an unusual embedded application which requires 250
    simultaneous PWM outputs, all independent. I will poll digital
    feedback values from each of the 250 devices (heaters) and use this
    information to adjust the output duty cycles on a PID loop algorithm.
    The chip would be dedicated to essentially this task only, with only a
    handful of discrete I/O (start/stop signal & a couple of indicator
    LED's).

    Is there a single microcontroller that can do this? I looked at
    Atmel, but there are so many possibilities and I'm afraid I'll waste
    money on a development kit for a specific processor and then find out
    that there was something better, cheaper, easier to learn, or more
    appropriate.

    Thanks for any leads you can provide.

    Sid
     
  2. Guest

    Sounds like a job for an FPGA. You can probably make each pin its own
    PWM channel. And you get get devices with up to several hundred I/O
    pins.

    You have several choices for the smarts:

    1) hardwired computational structure in the FPGA (ugh!)
    2) external processor controlling the FPGA (think an addressable
    register for each PWM channel)
    3) processor core packaged inside the FPGA package
    4) processor implemented in the FPGA logic fabric

    Basic Xilinx FPGA kit is about $100 from Xilinx or Digilent which
    makes it for them. Their processor cores can't be implemented in that
    chip but other people's can. Digilent also has NEXSYS with a much
    larger chip for $120 - could be useful as you might run out of program
    memory trying to squeeze things into the little chip (FPGAs don't
    offer much memory compared to microncontrollers)
     
  3. Tim Wescott

    Tim Wescott Guest

    Use an FPGA as a PWM peripheral, all managed with one busy processor, or
    find a modest processor that has a high PWM output to pin ratio, and use
    as many as you need to get the job done.

    I think I'd evaluate both solutions; I honestly don't know which one
    would end up looking better.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  4. Genome

    Genome Guest

    If I were to assume that your heaters are of a reasonable size then the time
    constants involved are going to be in the order of minutes to tens of
    minutes. As a result the bandwidth of the loop you use to control them can
    be as slow as a slug with it's foot torn off.

    Even with 250 of them on one processor you are not going to need
    blisteringly fast speed. Not knowing much modern stuff I would suggest you
    could dangle them all on an RS-485 cable running at 9600Baud.

    Read back data for temperature then send a 4bit adjustment back to each one
    for local processing with some shitty analog controller stuff.

    One PIC for the central controller should do and another one for local
    processing at the heater.

    I could be wrong though.

    Of course..... you could use an FPGA......?

    DNA
     
  5. Anirban

    Anirban Guest


    I agree with sylvester. No modern microcontroller will have 250
    separate input/output pins.
    I believe that you need to use an FPGA in tandem with a
    microcontroller. The FPGA handles the I/O and the PWM waveforms while
    the microcontroller can do the maths stuff.
     
  6. DJ Delorie

    DJ Delorie Guest

    Heaters don't need that fast a response, and you don't actually need
    PWM. You just need the current to be on some specified percent of the
    time, right?

    If you can find a CPU with enough general purpose I/O pins, try this:

    For each pin, keep an 8 bit counter and an 8 bit "setting" value. At
    regular intervals, add the setting to the counter, and if there's a
    carry, turn the I/O pin on, else turn it off.

    With this technique, you can do the math much less often (as slow as
    one loop per second for heaters) and it "spreads out" the "on" pulses
    (instead of lumping them together like PWM does). This is the same
    technique used to draw angled lines, and similar to dithering and
    sigma-delta.
     
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  8. FPGA, perfect for this:)
    As much IO lines as you like.
     
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