Testing a board with STM32U5A5.
VDD is 3.3V
I have configured one GPIO as input with an external pullup to 3.3V through a 470k ohm resistor.
The input reads high, but when I measure the input with a scope, I read 2.2V on the input, which is below 0.7VDD which is guaranteed as V_IH, but above 0.5VDD+0.2 which is specified by design as V_IH.
Is 470k ohm too high of a pullup resistance?
Best answer by Maxime_M
It sounds like you are using a x1 oscilloscope probe.
These have 1 Megohm input impedance, so 1 MΩ to ground.
Combined with 470k to +3.3V, I would expect about 2.2V on the pin.
Remove the probe and the voltage will go back up to 3.3V (but you won't see it!)
Many 'scope probes are switchable x1 / x10. If you switch it to x10 you should see perhaps 3.0 V (but might have to tell the 'scope that you're using x10, or scale what you see on the screen).
Imho 470k is a little high for comfort, particularly if the track to the pin is fairly long, as it might pick up electrical interference. Where I have had to use high value pull-up resistors for extremely low-power operation where e.g. a toggle-switch might pull the pin low (perhaps 10 MΩ) I might add a 1nF or 10nF capacitor to ground to kill any electrical interference. It killed the speed of response but was acceptable for that job.
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