PIC32 Ethernet Start Kit - Decoupling Caps

Discussion in 'Embedded Systems and Microcontrollers' started by Hurdy, Mar 26, 2014.

  1. Hurdy

    Thread Starter Senior Member

    Feb 27, 2006
    Hi everyone,

    Thank you for taking the time out to read this post.
    My questions is about the decoupling capacitors between Vdd and various chip pins on the PIC32 Ethernet Starter Kit

    I understand that you need decoupling capacitors between Vdd and various pins to help smooth out any noise on the Vdd rail.
    I'm trying to create my very first design based on the Ethernet Starter Kit, with a few modifications (it's a good learning curve).

    If you look at the PIC32 Ethernet Starter Kit Guide here, and scroll to page 22.
    You will notice on the Texas Instruments PHY DP83848C (U8) chip that pin 39 is pulled up to Vdd via a 2.2K resistor.

    Should Pin 39 on U8 also have a 0.1uF capacitor tied between it and GND?

    Is the rule that any chip pin connected to Vdd, even with a pull up resistors or other components should have a decoupling capacitor?

    Many thanks,

  2. MrChips


    Oct 2, 2009
    The purpose of decoupling capacitors is to remove switching noise on the power supply lines. The capacitor is placed between Vcc and GND, or VDD and VSS (ground or DGND) as close as possible to the corresponding pins of each IC. SMD ceramic 0.1μF capacitors are highly recommended.

    Pin-39 of U8 DP83848C is a control pin, not a supply pin.
    You do not use decoupling capacitors in this case.

    There are special cases where filter capacitors are used on signal/control lines, for example, RESET, NRST, MCLR or PFOUT.

    When the component manufacturer provides application notes and circuit schematics, follow their recommendations.
  3. t06afre

    AAC Fanatic!

    May 11, 2009
    Hi If not done take a look here http://forum.allaboutcircuits.com/showthread.php?t=45583. But to answer your question, yes it is good rule of thumb to put decoupling capacitor(s) on any voltage supply pins. But pin 39 U8 is a IO pin that is pulled high via a resistor. So no decoupling caps will be needed on the latter pin.
  4. Hurdy

    Thread Starter Senior Member

    Feb 27, 2006
    Brilliant, thank you both very much.
    I just wanted to be sure that the decoupling caps are for Vdd supply rails only (with a few exceptions as you mentioned).

    Really appreciate the feedback.

  5. takao21203

    AAC Fanatic!

    Apr 28, 2012
    When the pins measure a short I dont see a point using capacitors for each one.


    1) Be generous with caps for PIC32, two tantalum or 4.7uF etc. suitable ceramic caps

    2) AVdd must be connected or the PIC can not even be read out

    3) Some applications may require the caps on all pins like instructed in the datasheet

    So you need at least 2 capacitors. 100nF is not good enough in my opinion. One across Vcc/Vdd, and the other from the interal regulator to GND.