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Why the strange pinouts?

Discussion in 'Electronic Basics' started by Glenn Ashmore, Mar 25, 2005.

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  1. Why is it that some chips just don't seem to have any logic to their
    pinouts. For example, why do they split up ports on microcontrollers? Or
    on a BCD 7 segment display driver, why are a through e in order but f & g
    are on the other side of a?

    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at:
    Shameless Commercial Division:
  2. Lord Garth

    Lord Garth Guest

    The typical reason is for the convenience of the die layout. Another reason
    would be for economics, nobody would use a more expensive package that
    is also larger and therefore used more PCB real estate. There are also
    power dissipation reasons as well as electrical noise minimization. Much
    depends on the function of the IC.
  3. John Fields

    John Fields Guest

    Because you've got to bring all the inputs and outputs to the edges of
    the silicon to connect to the lead frame, and most most times
    (always?) you can't get all the I/O's to come out perfectly ordered
    just the way you want them to. It's like laying out a single-sided
    PCB. (At least it was back when I was doing that kind of stuff. I
    don't know how many "layers" are available now though...)
    Same story, I suspect.

    In truth, it doesn't matter much since you're going to have to do a
    PCB layout anyway, and even if all the I/O's came out of the chip
    ordered perfectly, the layout would vary from PCB to PCB.
  4. Ah, sweet mysteries of life.

    I've always been annoyed by the fact that PIC microcontroller chips have
    their power pins in the wrong places. For the PIC12F675, for example,
    Vdd is pin 1, whereas Vss is pin 8. For an 8 pin PDIP package, ground is
    usually pin 4, and Vdd pin 8.

    Some chips use pin placement in an attempt to deal with heat. For
    example, the TI SN754410 quad 1/2 H driver chip has 4 ground pins in the
    center, which double as heatsinks.

    I've also heard that some of the AC chips have unique placement of power
    and ground in order to mitigate ground bounce problems, which can occur
    when all the outputs go up and down simultaneously. Since they are so
    fast, they can induce an oscillation on the ground plane that can affect
    other chips in the vicinity.

    I'm sure most of the placement of pins for logic families is historical.
    I believe certain chips (some earlier cmos 4000 series) have fallen out
    of favor because of pin placement, and been replaced with chips that
    have more 'standard' placement.

    I had problems with the f,g thing on a recent project. It would have
    been so nice to have a single sided PC...

    I'm suprised there aren't CPLDs targeted towards prototyping/hobbyist
    use. Seems like using this technology, you could replace nearly any
    logic chip with a faster version that has somewhat more flexible
    pinouts. However, atmel and the other cpld mfgrs have proprietary
    programming protocols that forces one to buy expensive programmers. That
    makes it hard for people to do on the cheap in their garage.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  5. Rich Grise

    Rich Grise Guest

    You could have:

  6. Rich Grise

    Rich Grise Guest

    It's because of the way the ins and outs landed on the chip when they
    designed it - you don't design a chip the way you design a circuit, and
    stringing a conductor all the way around the die to come out at a certain
    pin is a LOT different from doing it with a PCB trace. Rearranging inputs
    and outputs would have made the chips more expensive.

    Lately, they've been doing it since designs are so complicated that one
    jumper or so here or there doesn't add that much to the cost.

    Hope This Helps!
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