4 output modes of STM32 pins

1. Normal push-pull output (GPIO_Mode_Out_PP):

Application: Generally used in 0V and 3.3V applications. The circuit passes through two P_MOS and N_MOS tubes, which are responsible for pulling up and pulling down current.

Usage: Direct use

Output level: The low level of push-pull output is 0V and the high level is 3.3V.

2. Normal open-drain output (GPIO_Mode_Out_OD):

Application occasions: Generally used in situations where the levels do not match, such as when a high level of 5V needs to be output.

How to use: You need to connect an external pull-up resistor, the power supply is 5V, set the GPIO to open-drain mode, and when the output is high, the pull-up resistor and the power supply output a 5V voltage.

Output level: In open-drain output mode, if the output is 0, low level, the N_MOS is turned on and the output is grounded. If the control output is 1 (cannot directly output high level), neither high level nor low level is output, which is a high configuration. For normal use, a pull-up resistor must be connected externally.

Features: It has a "wired-and" feature, that is, when many open-drain mode pins are connected together, the pull-up resistor provides a high level only when all pins output a high impedance state. The voltage of this high level is the voltage of the power supply connected to the external pull-up resistor. If one of the pins is at a low level, the line is equivalent to a short circuit to ground, making the entire line at a low level, 0 volts.

3. Multiplexed push-pull output (GPIO_Mode_AF_PP): used as serial port output.

4. Multiplexed open-drain output (GPIO_Mode_AF_OD): used in IIC.