• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 21 subscribers
  • Views 716 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN74LVC244A: Input Signal step influence

Johnny Guo
Expert 4910 points
Part Number: SN74LVC244A


Hi, team,

My customer met one issue on SN74LVC244A.

On their board, the 244A input signal is as below, there is one step during falling edge with about 5.9ns. They tried to eliminate the signal step but no obvious improvement. If putting 244A device on board, there will be a voltage step.

My questions are listed as below.

1. Will the input signal step bring risk to the output?

2. Is there any hysteresis for input voltage detection to eliminate the influence brought by step?

3. How does 244A deal with the input signal step? We found the output signal is good without any step.

Thanks.

Johnny

  • 0
    TI__Guru* 97066 points

    Hi Johnny,

    1. Will the input signal step bring risk to the output?

    2. Is there any hysteresis for input voltage detection to eliminate the influence brought by step?

    3. How does 244A deal with the input signal step?

    I read all three of these questions as the same question: "How does the SN74LVC244A work?"

    The SN74LVC244A is a CMOS buffer with output enable.  It does not have hysteresis on the inputs; it does not have any type of brain or inbuilt error processing.  It is one of the simplest devices in TI's portfolio.

    The CMOS input is a pair of analog MOSFETs (aka a CMOS inverter).  When the input signal is transitioning in ~10ns, a 1-2ns 'pause' on the way down won't cause any problems at all because it's going to take time for the output to change, and it probably won't really start to change until the signal is past the halfway mark anyways.

    Speaking of the output -- the input stage is an inverter, as I mentioned, which means there must be at least one more stage of inversion prior to the output (probably several). It takes time for each of those to switch, and additionally, each of those adds gain to the output and cleans up the edge.

    You should see a clean output signal from the shown input signal above.

    If you were see an increase for a few ns around 1/2Vcc, then I might think there would be a problem, but the signal appears monotonic and should be fine.