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SN74AHC595: My customer meet some the OE waveform of SN74AHC595

Frank Xiao
Genius 12055 points
Part Number: SN74AHC595
Other Parts Discussed in Thread: SN74LVC1G17

Hi team,

My customer meet some the OE waveform of SN74AHC595  has a as below. Whether it will cause some risks here? Thanks.

  • 0
    Guru 316920 points

    That falling edge is about 200 ns/V, which exceeds the recommended operating conditions. (And that plateau in the middle makes it extra likely that there will be oscillatins about the switching threshold.)

    This signal needs to be cleaned up with a Schmitt-trigger buffer (or comparator).

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 45815 points

    Hi Frank,

    Clemens has explained the issues that can arise. My suggestion is to use a device like the SN74LVC1G17 to clean up the signal at the input of the SN74AHC595.

  • 0 in reply to Dylan Hubbard
    TI__Genius 12055 points

    Hi team,

    That falling edge is about 200 ns/V, which exceeds the recommended operating conditions. (And that plateau in the middle makes it extra likely that there will be oscillatins about the switching threshold.)

    My customer has modified the slew rate to be ok here as below. We also want to check what's influence of the plateau, whether it will trigger the latch or somethings else. If not the customer can accept it.

    They also mention NXP 74AHC595 not has the risks, i am not sure whether it's correct or not.

    Thanks.

  • 0 in reply to Frank Xiao
    Guru 316920 points

    The 74AHC595's recommended operating conditions impose exactly the same limit of 100 ns/V on the rise/fall time.

    It's possible that both devices happen to work with 200 ns/V, but neither manufacturer will guarantee it.

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 45815 points

    Hi Frank,

    Based on the spec of the NXP device and the fact it also has CMOS inputs, their device will face the same issues so I don't believe that is accurate.

    The signal will pass through the threshold hold region causing a switch in states. This means they will be driving the OE pin low to enable the device and during the plateau event its possible the outputs to be disabled again briefly before being enabled once again as the signal continues to fall to GND. This may only result in a small glitch at the output that wouldn't cause any issues, but there is no guarantee. I would monitor one of the Q outputs during this event.