GPIO_Mode_Out_PP Push-Pull Output Normal Digital Output
GPIO_Mode_Out_OD Open-Drain Output Level Conversion, Bus
GPIO_Mode_AF_PP Alternate Function Push-Pull SPI, USART, etc.
GPIO_Mode_AF_OD Alternate Function Open-Drain I2C, Multi-Master Bus
Alternate Functionality
· The pin is no longer used as a regular GPIO
· Instead, it is connected to specific peripherals inside the chip (such as USART, SPI, I2C, TIM, etc.)
· The function of the pin is controlled by the corresponding peripheral
Open-Drain Output
· The output circuit only has a pull-down MOSFET, with no pull-up MOSFET
· Output characteristics:
· Can actively pull the level low (output 0)
· Cannot actively output a high level, requires an external pull-up resistor
· In high level, it presents a high-impedance state