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SN74ALVC244: Logic forum

Wu Elaine
Prodigy 20 points
Part Number: SN74ALVC244

Schematic is as below, buffer is used for MDC (12MHz/6MHz) signals, crosstalk issue is happened on MDC signals. Now I think there is crosstalk of the buffer when multiple channels are open to use.  Pleae help verify.

MDC Waveform (Yellow):

1. Test pointed at TP1, crosstalk issue as below.

2. Test pointed at TP2, crosstalk issue as below.

3.Test pointed at TP3: Waveform is ok, no crosstalk.

4.Remove series resistor R791 which is placed closed to buffer, test pointed at TP4, Waveform still has crosstalk issue.

5.Remove series resistor R724 which is placed closed to driver, test pointed at TP4, Waveform is ok, no crosstalk.

  • 0
    Prodigy 20 points

    Part Number: SN74ALVC244

    Schematic is as below, buffer is used for MDC (12MHz/6MHz) signals, crosstalk issue is happened on MDC signals. Now I think there is crosstalk of the buffer when multiple channels are open to use.  Pleae help verify.

    MDC Waveform (Yellow):

    1. Test pointed at TP1, crosstalk issue as below.

    2. Test pointed at TP2, crosstalk issue as below.

    3.Test pointed at TP3: Waveform is ok, no crosstalk.

    4.Remove series resistor R791 which is placed closed to buffer, test pointed at TP4, Waveform still has crosstalk issue.

    5.Remove series resistor R724 which is placed closed to driver, test pointed at TP4, Waveform is ok, no crosstalk.

  • 0
    Guru 267110 points

    The schematic does not show any power supply decoupling. Please show how this is implemented on the board.

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Capacitor C844(0.1uF) is placed close to the buffer U42 for 3.3V decoupling.

  • 0 in reply to Wu Elaine
    TI__Genius 14300 points

    Hi Wu,

    Can you take a scope shot at TP1/TP2 simulatnousely on a ns scale? 

    This device doesn't have noise characteristics in the datasheet, however comparing to other devices in the family, the cross talk noise you are seeing is within spec. ALVC devices will have crosstalk when switching multiple channels. The SN74LV244A will have better noise immunity although I don't know if you need the drive strength of the ALVC 24mA vs 16mA.

    Regards,

    Sebastian