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SN74HC595: /OE Pin Pull-up Resistor

PY (Pengyu) Song
Expert 5860 points
Part Number: SN74HC595

Hi team,

What is the recommended pull-up resistor at /OE pin to pull it up to VCC during power-up to avoid potential glitch? How to calculate/estimate pull-up resistor? 

Use case:

  • VCC = 3.3V
  • 4x SN74HC595 sharing the same /OE bus, pulling-up by the same resistor to VCC. 
  • Currently using 10kohm and observed output glitch during VCC power up. 

I believe this is due to during power-up, /OE is slightly slower than VCC, and momentarily /OE is < 0.7*VCC, causing /OE is logic '0' for a short period time during power, resulting in unexpected output observed. 

  • +1
    Guru 243120 points

    In general, the rise time of the power supply is slow enough so that this does not happen. My first recommendation would be to use a larger decoupling capacitor on VCC.

    What is the supply voltage when this happens? HC devices are not guaranteed to work correctly below 2 V.