Why Doesn't My Ethernet Switch Work on Power On Reset?

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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?
 
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Use the reset IC!
 
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.
 



I would first suggest checking the datasheets of the MCU and Ethernet switch to ensure compatibility with the DCDC converter and reset IC. It's possible that the slow Vcc ramp time is causing issues with the components. Additionally, I would recommend checking the power requirements and voltage tolerances of the MCU and Ethernet switch to see if they are being met during the reset process.

I would also suggest checking for any potential noise or interference in the circuit that could be causing the intermittent issue with the Ethernet switch. This could be done by using an oscilloscope to monitor the signals during the reset process.

Another approach would be to simulate the reset process using a power supply and see if the same issue occurs, as this could help isolate the problem to the DCDC converter or the reset IC.

Furthermore, I would recommend checking the hardware design and layout of the board to ensure that there are no issues with signal integrity or power distribution. It's possible that there could be a problem with the routing of the signals or the placement of components that is causing the issue.

In summary, troubleshooting a power on reset issue can be a complex process, but by systematically checking the components, power requirements, and circuit design, it is possible to identify and resolve the issue.