I bought an stm32f103ret6. When I saw the reference manual, I saw that it has several I/O modes.

For input: floating, pull-up, pull-down
For output: analog, open-drain, push-pull

I have already worked with AVR MCUs which have tri-state and pull-up. So I can understand pull-up and pull-down, but, I can't understand the modes floating, analog, open-drain, push-pull. What do these modes mean and when do I use which?


1 Answer 1


Floating: neither pull-up nor pull-down. Your input goes effectively straight to the gate of a FET somewhere. Should be used with external driver or pullup/down only; don't leave it entirely floating to pick up ESD.

Analog (output): not quite clear from the datasheet, but I'd assume this was the output of a DAC somewhere and can therefore take a range of voltage values.

Open-drain: if set to "0", a FET will be activated to connect this to ground. If set to "1", it will be left floating. Used for I2C and similar buses where there are multiple open-drain drivers and a single pullup resistor somewhere.

Push-pull: contains both high and low output transistors. Fast, capable of actually driving things which require a positive voltage, but must not be connected to other drivers. If set to "1", connects the positive supply rail to the output via a FET.

  • 2
    \$\begingroup\$ The normal difference between "floating" and "analog" is that the former will route the pin into a logic-level input buffer, allowing code (or other logic) to see its state, but also causing extra current to be drawn if it doesn't represent a valid high or low; the latter will disconnect the pin from the logic-level input buffer, thus allowing the pin's voltage to vary anywhere between the supply rails without causing excess current draw. \$\endgroup\$
    – supercat
    Sep 12, 2013 at 19:23

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