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SN74HCU04A: waveform generated at the second stage is abnormal.

Kevin Huang8
Genius 3555 points
Part Number: SN74HCU04A
Other Parts Discussed in Thread: SN74LVC1GX04

Hi Team,

I use SN74HCU04APWR as an oscillator, but the waveform generated at the second stage is abnormal. Do you have any suggestions?
The related circuit used, the waveforms of 1Y and 2Y are as shown in the figure,
The wave amplitude on the 2Y side seems to be unable to reach VOL.

  • 0
    Guru 272770 points

    Are you sure that the waveforms are labelled correctly? The amplitude should not become smaller.

    The second-stage amplifier should be a buffered one to get a proper digital output signal. Or at least add more unbuffered stages.

    I do not know if HC is actually fast enough for 18 MHz at 3.3 V. The SN74LVC1GX04 might be a better choice.

  • 0
    TI__Guru* 96712 points

    One thing I noticed is that R297 is 0 ohms -- usually you'd want something in the 1kohm range to reduce the power dissipation in the crystal. It may be OK for your system (HC drivers are fairly weak), but that would be my first thing to adjust. I'd start big (maybe 2.2k) and move down if necessary for stability.

    I'd also double check the measurements as Clemens suggested, and try another IC to make sure nothing is wrong with the one you are testing -- just in case channel 2 had taken some damage.