Microcontroller – Do Microcontrollers Have a Big Resistance?


Well, it maybe seems a very basic question, but I have this question because I'm trying to understand the 0 weak and 1 weak on pull-ups and pull-dows, for example:

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Here, when the button is not pressed, we have a weak 1 on the input pin, and when the button is pressed we have a strong 0 on the input pin (or, at least, this is what our professor taught us). But this strong 0 is because the MCU has a BIG resistance so all the current will go directly to the left GND instead of going through the MCU. Am I right?

Can you give also typical values of pull-ups resistors and values of MCU resistance?

Best Answer

That is correct, in the sense that it can be thought like that for the sake of simplicity, but in reality there is no resistance at all, as it's a CMOS input which has an extremely high input impedance. It can be thought as resistor so large in value that it's not even there, and theory no current flows in or out that pin.

However, nothing is perfect, and there can be leakage current flowing in or out of that pin, but in worst case it is in the order of 1 microampere at 5V voltage, so it can be thought of as resistance larger than 5 Megaohms.

Typical values for pull-ups is a more broad question, as there is no single answer. Depends on supply voltage, switch current rating, level of coupled noise etc. So from 1k to 100k maybe.