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SN74HC244: hysteresis levels & timing

Steve Widener
Genius 10125 points
Part Number: SN74HC244
Other Parts Discussed in Thread: SN74LV244A

How much input hysteresis does the SN74HC244 have? Is it more or less than the SN74LV244A device?

We have a situation where the HC244 devices are powered by 5V and there are non-monotonic inputs that ring close to the Vcc/2 level - amplitude is ~ 650mVpp and the period of the ring (single ring) - 8 nsec - see attached waveform. Note the input comes form a bus switch so Voh is about 4V.

There is concern that there will be double clocking in this legacy system and we'd like to know if the LV244A would perform better than the HC244 in this regard - esp considering the hysteresis levels & timing.

  • 0
    TI__Guru* 96802 points

    Hi Steve,

    The SN74HC244 and the SN74LV244A do not have Schmitt-trigger inputs, and thus do not have any input hysteresis.  You would need to switch to a device with Schmitt-trigger inputs to have this, for example the SN74LV8151. 

  • 0 in reply to Emrys Maier
    TI__Genius 10125 points

    Emrys,

    Thank you for the response. For the LV device we have found the following guidance regarding hysteresis:

    http://www.ti.com/pdfs/logic/lvabrochure.pdf

    So it appears that there is some hysteresis but it is not specified on the datasheet as you point out - please verify.

    For the 'HC244, we were wondering if the input stage had some similar (but perhaps less) unspecified hysteresis. The found that the 'AHC244 also has the same sort of hysteresis as the LV244A part (though a bit less) - we can't find guidance for the HC244 device.

    One aspect of the hysteresis that is not clear for the AHC244 and LV244A is the sensitivity to dv/dt of the input & how that impacts the observed hysteresis - any guidance you can provide would be appreciated.

  • 0 in reply to Steve Widener
    TI__Guru* 96802 points
    Any hysteresis on the input of the device is not guaranteed or specified. All I can guarantee is what is in the datasheet.