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TXS0102: TXS0102 output Drooping waveform observed for clock (MDIO interface)

Louis Sankoorikal
Expert 2345 points
Part Number: TXS0102
Other Parts Discussed in Thread: DP83867IR, 2N7001T, TXU0101

Hi,

I am using the TXS0102 device on the MDIO interface between Xilinx SOC device and the TI DP83867 Phy device (Phy device located on external board connected via 1.5 feet cable). 

For the MDIO data signal, I have a 2.2K external pull up close to TI DP83867 Phy device as per device datasheet recommendation. For the MDC clock signal, I do not have external pull ups (TXS0102 has 10k internal pull ups)

The Xilinx SOC device MDIO interface has Push pull output.

I have a few questions:

  1.  I observe a drooping waveform (droop from 3.3V to 2.3V) for the MDC clock high level at the TXS0102 A output

/resized-image/__size/320x240/__key/communityserver-discussions-components-files/151/TXS0102_5F00_MDC_5F00_A_5F00_Output_5F00_DroopingWaveform.jpg.

The input waveform is clean.

/resized-image/__size/320x240/__key/communityserver-discussions-components-files/151/TXS0102_5F00_MDC_5F00_B_5F00_Input.jpg

  • Can you please help advise what is causing this droop and if it can be corrected?

For reference the MDIO output waveform is clean:

/resized-image/__size/320x240/__key/communityserver-discussions-components-files/151/TXS0102_5F00_MDIO_5F00_A_5F00_Output_5F00_GoodWaveform.jpg

2. Is there a specification for Overshoot/ Undershoot at the TXS0102 input pins?

I could not find this information in the TXS0102 datasheet. Or can I consider the section 6.1 Absolute maximum ratings of minimum -0.5V and maximum VCC +0.5V as the spec?

3. There is an overshoot of 3.63V at the TXS0102 Clock MDC:

I guess this is caused by the PMOS transistors turning on for short duration during the transition. I am considering if I should include a series resistor at TXS0102 output to damp this overshoot. Please advise if you see concerns with this.

Thanks

Louis

  • 0
    Guru 317320 points

    1. The DP83867IR's MDC pin has an internal pull-down resistor. This does not work with an open-drain translator like the TXS. Better use a fixed-direction translator like the 2N7001T or TXU0101.

    2. Yes.

    3. This is probably caused by the TXS's edge accelerators. Source termination indeed might help.

    What is the SoC voltage?