Load from Decoupling Cap on PIC Output

Discussion in 'Embedded Systems and Microcontrollers' started by CraigHB, Dec 19, 2011.

  1. CraigHB

    Thread Starter Member

    Aug 12, 2011
    I'm doing a project where a PIC output pin supplies power to an on-board peripheral chip. When the device is turned on by the user, power is supplied to to the peripheral chip. When the device is turned off by the user, power is removed. The peripheral chip only draws about 5uA so there's no issue with steady current demand.

    My concern is the .1uF decoupling capacitor required on the peripheral chip's Vdd pin. Will the the charging and discharging of that decoupling capacitor through the PIC's output pin eventually damage the PIC?

    I've actually been working with the device for some time with no issue. I'm just wondering if I really should change that. I'd rather not if I don't have to.
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    Personally, I would not hang a cap directly off an I/O pin that I expect to be changing. Not that the PIC could be damaged by such, but I would be concerned about an inadvertent reset or brown-out condition. The cap looks like a short circuit at t0 when the pin switches from low to high. This could bring the Vdd low enough for a long enough time period to trigger an internal reset, or, possibly some other random perturbation.

    Use either peripheral ICs that have either a hardware or software enable, or build your own discrete switching networks using transistors (bipolar or FETs).

    In this specific case, you can put a 100 ohm resistor in series with your 5uA peripheral IC, with the cap on the far side. This should prevent any inadvertent consequences wrt your PIC.
  3. CraigHB

    Thread Starter Member

    Aug 12, 2011
    Thanks man, that sounds like the thing to do.

    Most simple to put a resistor in-line. Though, I haven't experienced any spurious resets, but regardless, I don't like the idea of loading one of the PIC's outputs like that.

    I don't really want to add the parts required to isolate switching for the peripheral IC. The device is battery powered so I need minimal draw and the peripheral IC can be either on or off in sleep mode depending on user settings. I can't figure out how to simply design a non-inverting switch with no power overhead for both states. I could probably do it with a couple complimentary pairs, but that's too many parts.