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PICs and Vss/Vdd

Discussion in 'Electronic Basics' started by Randy Day, Nov 7, 2007.

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  1. Randy Day

    Randy Day Guest

    Browsing the datasheets, I notice some
    varieties of PICs have multiple Vss and/or
    Vdd pins; must they *all* be connected to
    power, or are they redundant to provide
    PCB routing flexibility?

    I'm referring to straight Vss/Vdd, as
    opposed to special stuff like Avss/Avdd.

    No, I'm not planning to power a whole section
    of circuits via my micro, but I am wondering
    what the intent is for those pins.

    TIA
     
  2. Not only should they all be connected, you should add bypass
    capacitors between all close pairs of Vss Vdd pins. Chips
    with lots of possible outputs can pull large currents
    through the Vss and Vdd pins, so having more than 1 pair
    reduces the IR and inductive drop of the pins and wire
    bonds. This is especially important if you plan on using
    any of the chip analog functions without having supply
    voltage change errors and noise added into their function.
     
  3. john jardine

    john jardine Guest

    Well, I've just pulled off a Vss on a PIC 16F873 on the protoboard and it
    carried on working.
    I wouldn't intentionally leave 'em off though, as they'll be needed to
    maintain good noise margins under poor working conditions.
     
  4. Jamie

    Jamie Guest

    I don't know which Pic you're looking at how ever, If it's
    anything like other chips that do that. If you look at the
    Vdd description. It maybe labeled as Vdd 2 or something in that
    order.
    To give you an example of why this would be..
    Lets assume the PIC's internal logic is only 5 volts and you
    have a bunch of outputs.
    Now, if you decided to operate these outputs in SOURCE mode,
    you could then select a different output voltage other than the
    one the CPU area is using..
    for example, the output SOURCE voltage could be 12 volts..

    The other options are, internally isolated outputs. this would
    require a different Vdd and Vss to make it truly isolated output or,
    if you decide that you don't need the isolation, you then tied them
    to the same rail.

    Also, they some times offer multiple Rail lines for construction and
    internal power stability..
     
  5. Eeyore

    Eeyore Guest

    More random STUPIDITY from the Janie TROLL.

    Graham
     
  6. Jamie

    Jamie Guest


    Come back for more "Arm chair bull shit artist" antics

    Your comments display lack of colour with your responses
    which is parallel to your intelligence.

    Ever hear the terms?

    "You were nothing but a little squirt"

    or,

    "I scraped you off the sheets" ?

    something you might have heard when you were a kid?

    So go take a hike pecker head.
     
  7. At least he's trying to be helpful. OTOH, you're just being a
    dick........AGAIN!

    Now to get nearer to the topic at hand, we're all still waiting to see some
    modest demonstration of your expert coding ability. I"ve been playing with
    one of these for a couple of weeks in my spare time, using some icky old C,
    it's pretty neat IMO:
    http://www.sparkfun.com/commerce/product_info.php?products_id=8280
    Get one and we'll have a contest to see who can come up with the neatest
    application using just the onboard hardware.
     
  8. Eeyore

    Eeyore Guest

    The problem with Janie is that she gives VERY BAD 'advice'. For her she sees it
    as a competition to be the stupidest poster in the thread and usually wins the
    stupidest award by far.

    Graham
     
  9. Jamie

    Jamie Guest

    She? Should I drop my trousers and stick what I have hanging there some
    where for you? Or should I just wave it around in front of you since you
    obviously don't have one.

    Typical ignorant social disordered psychopath.

    P.S.
    In case you wasn't aware, we're out of the dark ages. It is no longer
    a standard practice to test voltage with wet fingers. I just thought
    I would remind you of that because you seem to be a few cells short of a
    picnic.
     
  10. ian field

    ian field Guest

    Why discourage him from voltage testing with a wet finger?!
     
  11. How are your PIC adventures going?
     
  12. ian field

    ian field Guest

    Still trawling through back issues of EPE, that's what I have the most
    downloaded sample code for. One problem is that the TK3 and SEND programmes
    have an odd convention with the ORG instruction, using ORG 4 & ORG 5 rather
    than ORG 0, this could be an unwanted hassle as EPE has standardised on LPT
    port programmers and my Velleman is RS232 port.

    The PIC in Practice book is a bit vague on configuration and header file,
    I've jumped to chapter 6 which is supposed to explain this as I don't see
    much point in trying to understand the code examples while I haven't a clue
    how to set up the configuration that obviously has to come first.
     
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