you should look into the datasheet for your device.it typically specify the maximum current through the clamping diodes. Usually that's 20 - 25ma: anything greater than that can cause permanent damage to the clamping diodes.
so at 5v, the minimum resistance allowed is 5v/20ma = ...
If you are doing adc on that pin,you will look into the maximum resistance specification.
if you are applying an ac signal to it, you need to look into the input capacitance figure to make sure that the resistance isn't too large for your signal (as the resistance forms a low-pass filter with the pin's input capacitance).
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so at 5v, the minimum resistance allowed is 5v/20ma = ...
If you are doing adc on that pin,you will look into the maximum resistance specification.
if you are applying an ac signal to it, you need to look into the input capacitance figure to make sure that the resistance isn't too large for your signal (as the resistance forms a low-pass filter with the pin's input capacitance).
...
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