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SN74LV164A: Logic forum

Part Number: SN74LV164A
Other Parts Discussed in Thread: SN74AHC244

Hi TI team

Please refer attached Function Block Diagram, we used two TI logic IC, one is octal buffer and one is shifter logic IC.

Currently the U349 pin2 of input clock waveform whithout the overshoot on rising edge, but the U349 output clock waveform all whith the overshoot on rising edge.

We trird to adjust the series resistors(R5321 to R5328) to 33 ohm, it looks the overshoot are not changed obviously, we need to adjust the series resistors(R5321 to R5328) to 127 ohm, the overshoot will be changed obviously, please refer attached waveform.

My question are:

1. Why the U349(octal buffer) input clock waveform without the overshoot on rising edge, but he U349 output clock waveform all with the overshoot on rising edge?

2. It looks the series resistors(R5321 to R5328) change to 127 ohm, the overshoot will be changed obviously, how can we make sure the series resistors(R5321 to R5328) value is reasonable?

3. The U349 buffer output is connected to U352(TI shifter logic IC SN74LV164APWR), the shifter logic IC VIH/VIL all meet the SPEC, but the clock waveform with the overshoot on rising edge, is it acceptable?




244 buffer with shifter logic_TI.pdf

  • 1. LVC outputs have a very high drive strength, so ringing can indeed happen if the trace is long enough and not properly terminated. Adding a source termination resistor is helpful; another way to reduce the mismatch between the buffer's output impedance and the trace's characteristic impedance is to use a buffer with less drive strength like the SN74AHC244‍PWR.

    2. A typical trace impedance is near 50 Ω, so 33 Ω source termination resistors should be OK. I suspect that the ringing that you are seeing with 33 Ω is not in the trace itself, but caused by the resonant circuit formed by the oscilloscope probe's ground clip. Consider using a ground spring instead; see the Making Accurate Oscilloscope Measurements for Noise, Switching, and High Frequency Signals presentation.

    3. If the ringing is just a measurement error, it does not hurt. Actual overshoots that do not exceed the absolute maximum rating of 7 V are not really a problem, but the ringing can become a problem if it goes back down below VIH (or above VIL for ringing after falling edges).

  • Hi TI team

    I put the waveform as attachedment.

    So for the U352(TI shifter logic IC SN74LV164APWR), we just need check below:

    1. the input CLK and CLR signal overshoot/undershoot was not over 7V.(please provide the document for proving)

    2. the input CLK and CLR signal ringing was not below VIH, and the ringing not above VIL

    Please help confirm these.



  • Correct.

    These waveforms are OK.

    (You have not specfied the measurement method. If you are using a ground clip, then the actual waveforms will be better.)

  • Hi Ti team

    thanks for the confirmation.

    Do you have any document to disucss the overshoot is not able to over 7V?



  • Hi TI team

    We would like to change the series resistors(R5321 to R5328) to 127 ohm, because the overshoot will be reducing obviously,

    Do you have concern with the series resistors(R5321 to R5328) change to 127 ohm with U352(TI shifter logic IC SN74LV164APWR)??



  • The absolute maximum ratings are specified in section 6.1 of the datasheet.

    Increasing the series resistors to 127 Ω would be no problem; your waveforms show that the signal is still OK.