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Original question:

SN74LVC1G08-Q1: SN74LVC1G08DCKRQ1 to beat 74LVC1G08GW-Q100

SN74LVC1G08-Q1: the state of input/output pin when VCC is under voltage

Zehui Li
Intellectual 440 points
Part Number: SN74LVC1G08-Q1
Other Parts Discussed in Thread: SN74LVC1G99-Q1

Tool/software:

Hi,

     What is the state of input/output pin when VCC is under voltage?

  • 0
    Guru 262920 points

    Inputs pins always have a high impedance.

    Below 1.65 V, correct operation is not guaranteed, and the output might try to drive high or low independently from the input.

    To guarantee that the output is disabled during power up, you need to use a device with /OE input (e.g., SN74LVC1G99-Q1) and pull /OE high during power up.

  • 0 in reply to Clemens Ladisch
    Intellectual 440 points

    Hi Clemens,

         1.What about input pins? Let's only consider itself. Pull-up,pull-down or high-Z?

         2.Output driving high means output equals to VCC? Will output be exactly high(=VCC) or low(=0), or any level between them. It cannot be above VCC, I guess.

         3.Driving strengh is strong enough so pull-down/up resistor is useless during power up?

  • +1 in reply to Zehui Li
    Guru 262920 points

    1. Input pins should have a valid voltage at all times (when powered). But with the supply below 1.65 V, its state might not have the desired effect.

    2. The device itself will try to drive to a power rail, but the actual voltage depends on the load.

    3. Correct.