What is happening when digitalWrite(pinname, LOW) is invoked after pinMode(pinname, INPUT_PULLUP)?

Question

I read the Arduino official docs on digitalWrite and am confused about what is happening from an electrical perspective when digitalWrite is invoked on INPUT_PULLUP pins.

Say my code says this:

pinMode(4, INPUT_PULLUP);
digitalWrite(4, LOW);

The docs say:

If the pin is configured as an INPUT, digitalWrite() will enable (HIGH) or disable (LOW) the internal pullup on the input pin. It is recommended to set the pinMode() to INPUT_PULLUP to enable the internal pull-up resistor. See the Digital Pins tutorial for more information.

Does this mean pin 4 is floating or is it at ground? Are those two lines of code equivalent to pinMode(4, OUTPUT); digitalWrite(LOW); ?

Answer 1

It's pretty explicitly stated there in your quote: if you write a LOW when in INPUT mode it disables the pullup resistor. That's all it does.

pinMode(4, INPUT);
digitalWrite(4, HIGH);

is exactly the same as:

pinMode(4, INPUT_PULLUP);

And

pinMode(4, INPUT_PULLUP);
digitalWrite(4, LOW);

is exactly the same as

pinMode(4, INPUT);

Of course this exploits a peculiarity of the AVR's IO block, but any decent core will emulate that functionality for compatibility with older code.