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I am using a SN74HCS09 Schmitt-Trigger AND gate that has the output pins stuck at low. Despite trying different combinations of high and low channel inputs, the outputs remain stuck at 0V.

I am supplying 3.3V for VCC and have a capacitor at the supply. I have tried 10k pull-down resistors on the input signals and output signals without any success. Even connecting 3.3V directly to both inputs still leaves the output of the AND gate stuck at 0V. Should I be using 5V instead of 3.3V for the supply and signals? The datasheet says that 3.3V should be fine but there doesn't seem to be anything else that could be going wrong (https://www.ti.com/product/SN74HCS09)

Any help would be greatly appreciated.

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  • \$\begingroup\$ UPDATE: placing a pullup resistor (10k but value shouldn't matter) to 3.3V VCC at the output worked. Thank you for the answers! \$\endgroup\$
    – queryit
    May 3, 2023 at 22:39

3 Answers 3

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These gates have open-drain outputs, meaning that they will only either actively drive the output low, or let it float. Add a pullup resistor that pulls the output up to your supply voltage. Both 3.3 V and 5 V are within the allowable supply voltage range listed on the product brief.

While this seems annoying/tedious, open-drain outputs are actually useful in some scenarios - for example, you can easily AND the output of multiple of these chips together with a single pullup resistor and shared output line; if any of the outputs are 0 they will drive the shared output low, and if all outputs are floating the line will be pulled high.

Some chips with open-drain outputs also have modified output networks that allow you to use a higher voltage on the output as compared to input/supply - however that's not safe to do with this chip because of the clamping diodes on its output (which are used to protect against electrostatic damage to the chip)

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  • \$\begingroup\$ How would adding a pullup resistor to the supply voltage help? Should it be a pullup resistor at the output instead of the supply to the chip? \$\endgroup\$
    – queryit
    May 3, 2023 at 21:48
  • \$\begingroup\$ @queryit that should say a pullup resistor that pulls the output up to the supply voltage (I.e. a resistor between those two nodes). Edited to make the post more clear. \$\endgroup\$
    – nanofarad
    May 3, 2023 at 21:49
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3.3V for Vcc should be fine.

This is a open drain device. You need to provide a pullup resistor to Vcc on the output. The value is not critical for your setup.

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You mentioned 10k pull-down resistors on the input but not pull-up resistors at the outputs:

enter image description here

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  • \$\begingroup\$ I mentioned adding a 10k pull-down resistor on the output. Should I be adding a pull-up resistor instead? \$\endgroup\$
    – queryit
    May 3, 2023 at 21:47
  • \$\begingroup\$ Yes, you should provide a current path from the open drain to the Vcc. A few mA will work. Check this answer for more information on this type of output: electronics.stackexchange.com/a/150663/194393 \$\endgroup\$
    – devnull
    May 3, 2023 at 21:52
  • \$\begingroup\$ Thanks! Since the pullup resistor worked, do I also need the pulldown resistors on the inputs? The output seems to be stable without any pulldown resistors on the inputs. \$\endgroup\$
    – queryit
    May 3, 2023 at 22:42

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