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Power on Reset troubleshooting

  1. Nov 28, 2012 #1
    Need some ideas on how to debug a POR issue -

    I have 2 major ICs on my board - MCU(atmel) and Ethernet switch(DM8603).

    The reset(active low) comes from the PGOOD pin of a DCDC converter(sic403). DCDC output is 3.3v, input is 5V. Pgood pullup resistor is 10K.

    When the reset is given from the DCDC, the MCU works fine(booting etc), but the ethernet switch does not work. By does not work, I mean it doesn't switch packets between 2 ethernet links.
    It works intermittently. Usually it works when its powered up first time and then it doesnt.

    When reset is given from RC circuit (R=100k, C=10uF), everything works.

    When reset is given from a reset IC (TLV803), everything works.

    Here's what I have done so far -

    1. Checked rise time of reset IC (~2.5uS), reset output. Got the same rise time with DCDC converter pgood output.

    2. Reset IC delay is 200mS. Got the same delay with dcdc converter.

    3. Delayed reset from DCDC converter to about a second.

    4. Added 10nF cap at the output of Pgood to filter noise.

    5. Tried different pull up resistor values(1K,10K,100K) at pgood pin.

    None of the above made any difference.
    The 3.3V ramps up slowly depending on the softstart capacitor. Does slow Vcc ramp cause problems?
    In one case, with softstart cap of 1uF, Vcc ramp time was 1sec and PGOOD(reset) delay was 1sec. Pgood went high 1sec after Vcc was 90% of 3.3V.

    Any ideas on how to troubleshoot?
  2. jcsd
  3. Nov 28, 2012 #2
    Use the reset IC!!!
  4. Nov 28, 2012 #3


    User Avatar
    Science Advisor
    Gold Member

    Any unused gates on your board?
  5. Nov 29, 2012 #4
    Look up the reset function of the DC/DC converter. At what Vcc is the reset signal come on and at what Vcc when the reset start going high. From the experiment you did using long RC reset, I don't think rise of fall time is your problem.

    Read the reset signal requirement of the Ethernet chip, the Vcc threshold, make sure the requirement is met by the DC/DC converter. Make sure you read the timing requirement also.
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