# Is it generally OK to ignore the current in and out of a MCU pin when analyzing circuit external to the MCU?

From here, I see below illustration about the pull-up resistor.

And the article says:

With a pull-up resistor, the input pin will read a high state when the button is not pressed. In other words, a small amount of current is flowing between VCC and the input pin (not to ground), thus the input pin reads close to VCC.

It gives me an impression that the current in and out of a MCU is so small that it can be ignored when analyzing the behavior of the circuit external to the MCU.

Such assumption may simplify things a bit. But is it OK?

• An input pin usually has fairly high impedance. But it depends on how it is configured, too. If it is simply a digital input pin, it's likely that the input impedance is on the order of $100\:\text{k}\Omega$ or better (to both rails.)
– jonk
Commented Oct 25, 2020 at 4:08
• @jonk It seems for a digital input pin, the higher the input impedance is the better. Commented Oct 25, 2020 at 4:09
• The datasheet for the MCU will tell you a lot more detail. It's not hard to read one. And if you were asking (I cannot tell), then yes higher values (such as $1\:\text{M}\Omega$) is better.
– jonk
Commented Oct 25, 2020 at 4:10
• @jonk Yes, I believe a datasheet can give me a specific value. But I just want to know why high input impedance is better. Is it because high input impedance can make the disturbance of connecting a MCU insignificant? Just more like nothing is connected. Commented Oct 25, 2020 at 4:12
• Yeah. Infinite impedance would have zero interference. So big numbers are better.
– jonk
Commented Oct 25, 2020 at 4:16