Decoupling Capacitors

Discussion in 'General Electronics Chat' started by KansaiRobot, Oct 30, 2015.

  1. KansaiRobot

    Thread Starter Active Member

    Jan 15, 2010
    Hello and thanks always.

    I have a very basic question. I would appreciate some help.

    Regarding decoupling capacitors,(the ones that go between VCC and ground), so far I have used the PIC18F2550 so I just had one pin for each so I put one.

    Now I am using PIC18F45K20 which has two VCC pins and Two Ground pins. Should I put only one capacitor between one pair of those pins , or do I need to put two capacitors?? I am guessing only one but would like to confirm.

    Sorry for the basic question .
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
    Use two capacitors, one for each Vdd/Vss pair of pins, as close to the PIC as possible.
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    The rule of thumb is to put a cap on every power input to every chip. So use at least two, especially if the pins are not next to each other.

    Bypass caps are part of the art side of design. True you could calculate out these things, but you are trading 40 hours of engineering time for 2cents worth of parts.

    Generally sprinkle bypass caps ecerywhere. If the design ever goes to market you can spend one late night taking caps out one by one till your thing falls apart.
    PeterCoxSmith, cmartinez and nsaspook like this.
  4. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    That chip has split power pins for a reason internally as do most K (K represents a silicon process) series designs. Bypass all power connections at the chip and have a high quality energy smoothing capacitor near the chip on all power supply lines to reduce the effects of trace inductance during quickly changing chip power levels.

    A little more about K
  5. WBahn


    Mar 31, 2012
    I haven't looked at that particular part, but many designs that have multiple power/ground pins have split them out for a reason and you want to bypass them separately -- and that means decoupling a particular power pin to a particular ground pin. Usually the pin pairs are placed obviously close to each other or are named in a paired way. If all else fails, check the documentation.