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Is it safe to parallel PIC chip outputs

Discussion in 'Electronic Basics' started by [email protected], Apr 7, 2007.

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  1. Guest

    For more output current drive, can I parallel the outputs of a PIC
    chip port? Assume the pins are all on the same port (e.g., RC0-RC7)
    and that my firmware will never accidentally drive individual pins to
    unequal values.
  2. Chris

    Chris Guest

    Yes. It's a good idea. Remember not to exceed port and total IC
    output current limits (in the data sheets).

  3. Guest

    Thanks, Chris. It seemed like it should work, but a second opinion is
  4. Jamie

    Jamie Guest

    I think you would be better off using a unity gain amp or depending on
    what you need for example, a sink output ? just pass the output via a
    resistor to the base of a npn common emitter style config. Use the
    collector as your sink driver.. Of course, you'll need to invert the
    logic of your output in the code.
  5. Guest

    I need a source output. Basically I want to power an op amp and some
    sensors, so I can shut them down when the PIC is in sleep mode. Is
    there a drawback to running the output pins in parallel, that
    justifies the additional components for an external transistor?
  6. Randy Day

    Randy Day Guest


    Unless you're limiting yourself to SETF
    and CLRF statements, if you happen to
    leave one output 0 when all others are 1.
    say goodbye to your micro.

    How many chips are you prepared to lose
    during the debugging process?

    "Oops, that was supposed to be MOVF PORTB, W
    not MOVWF PORTB", as a wisp of smoke rises
    from your circuit...
  7. Chris

    Chris Guest

    Hi, Randy. Assuming the OP is paralleling outputs to drive a
    resistive load, he could use current limiting resistors at each output
    to help during the debugging process, like this (view in fixed font or
    M$ Notepad):

    | .------. .-------.
    | | | | | ___
    | | o--. | o-|___|---.
    | | | | | | | 270 | |
    | | | | ___ |/ | | ___ | |/
    | | o--o-|___|--| = | o-|___|---o-|
    | | PIC | | 100 |> | PIC | 270 | |>
    | | | | | | | ___ | |
    | | o--' === | o-|___|---' ===
    | | | GND | | 270 GND
    | | | | |
    | '------' '-------'
    (created by AACircuit v1.28.6 beta 04/19/05

    You have a very good point -- the OP should do everything he can to
    protect his development system from himself. ;-)

  8. Randy Day

    Randy Day Guest

    Chris wrote:

    Very true; for a one-off hobby project, that might
    work. With a bit of experimentation, he'd even be
    introducing himself to digital/analog conversion.

    But for the cost of the extra resistors, he could
    upgrade to a Darlington transistor, and only need
    one output pin.
  9. Guest

    Thanks for the helpful tips! I think that for this application, I
    should do my initial debugging with the series resistors. My program
    will be using the SETF and CLRF statements, but still, protecting
    myself from my stupid mistakes sounds prudent.
  10. Randy Day

    Randy Day Guest


    Your INITIAL debugging? Once you add the resistors,
    you're pretty much stuck with 'em. Think about it;
    how can you be sure that, in some dark corner of
    your otherwise working code, some instruction was
    only prevented from zapping the micro *because*
    the resistors were there.

    Once you take them out...

    Build with, or build without, there is no ...
    okay, enough with the Yoda impersonation. :p
    One other point to consider; do you think you
    might expand the project at a later time? Add
    some new functionality? Maybe you need more
    i/o pins that, (drat!) are used up now.

    I still don't think joining outputs is an
    optimal solution. Just my opinion.
  11. Guest

    Points well taken.
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