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In Cortex-M3, there are eight types of configuration for GPIO and four input and output modes.

Output mode:

1.GPIO_Mode_Out_OD open-leak output

2.GPIO_Mode_Out_PP push-pull output

3.GPIO_Mode_AF_OD multiplexing open-drain output

4.GPIO_Mode_AF_PP multiplexing push-pull output

Input mode:

1.GPIO_Mode_AIN analog input

2.GPIO_Mode_IN_FLOATING floating input

3.GPIO_Mode_IPD drop-down input

4.GPIO_Mode_IPU pull-up input

Open-drain output (Open Collector Output): the "Drain" level refers to the leakage level of the FET, which is equivalent to the collector (Collector) of the transistor. Then the meaning of "open-drain output" is obvious: using the collector of the transistor or the leakage level of the field effect transistor MOSFET as the output end, the output high voltage usually needs to increase the pull-up resistance in the collector to increase the output potential (at this time the transistor is working in the amplified state).

From the current amplification characteristics of the transistor (small current controls large current), we can know that this output mode is suitable for current mode drive, and the ability to extract current from the load is relatively strong (about 20mA).

Push-pull output (Push-pull Output): push-pull structure generally means that two transistors are respectively controlled by two complementary signals, the emission stage of one transistor and the collector connection of the other transistor are used as the output, always when one transistor is turned on and the other is cut off, which can output high and low levels.

Push-pull output can not only inject current into the load, but also extract current from the load. And when the circuit works, only one of the two symmetrical transistors is turned on at a time, so the conduction loss is low and the efficiency is high. The push-pull output improves the ability to carry load as well as the switching speed.

The circuit diagrams of the two output modes are as follows:

As for multiplexing open-leak output and multiplexing push-pull output, the second function is generally used as GPIO.

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