multiplexed open drain output GPIO multiplexed into other peripherals, the output data register GPIOx_ODR is invalid; the high and low levels of the output come from other peripherals, the Schmitt trigger is turned on, the input is available, and the I/can be obtained through the input data register O The actual state is the same as the open-drain output function except that the source of the output signal changes; on-chip peripheral functions (TX1, MOSI, MISO.SCK.SS)