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SN74HC138: Y5/Y6Y7 output level behavior

Henry Wang3
Genius 3345 points
Part Number: SN74HC138

Tool/software:

Hi Sir,

My customer found that the SN74HC138 Y5/Y6/Y7 output level does not follow the function table change level when C=1, B=0 to 1; A=1 to 0.

Could you review the related waveform on attachment and provide comments to fix this issue? Thanks!

SN74HC138 measure waveform.docx

  • +1
    TI__Genius 12871 points

    Hi Henry,

    What appears to be happening is the device is detecting the input C=H,B=H,A=H, as the signals are still moving before it detects the final state of the pins.

    You could try to speed up the falling edge of A to pass the VIL spec before the B passes the VIH spec. I will say it's going to be very hard to hit exact state transitions like this where two signals are switching at the same time, and avoiding accidentally hitting the HHH state or the HLL state.

    The better option would be to add a small RC filter on the outputs so any fast transitions of the outputs would be filtered out allowing you a wider window of transitioning both inputs.

    Regards,

    Owen