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On our board we use 74AC244-SO buffer to isolate microcontroller pins from outer world, buffer is shown on the picture below. Just for the purpose of test I've simple SPST switch attached between one of the inputs - AUTO_NAV and 3V3. When switch is closed I see 3V3 on corresponding output pin MCU_AUTO_NAV, but when switch is open I see MCU_AUTO_NAV voltage jumping low and high. When I try to attach scope to the input AUTO_NAV - problem dissapears. What can be the problem? Can it be that this buffer is very sensitive, so wires to the switch pick up EMI? MCU_AUTO_NAV is connected to the microcontroller (STM32F4) input pin with 40K internal pulldown.

buffer

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You have a floating input pin, which is picking up noise and causing erratic outputs. The added impedance of the scope probe on the input is enough to stabilize the input. You need to add a pullup/pulldown resistor if you are just using an SPST switch.

Also, note that if the actual device driving the buffer has an output that can drive both high and low, you will not need a pullup/pulldown. If, on the other hand, it's an open-collector output or similar, you will need a pullup.

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