但是在GPIO章的11.7.1节复用功能输入(Alternate function input)提到: “Alternate function inputs are not selected automatically you select them by writing to a control bit in the registers of the corresponding peripheral.”, 即是说复用功能的输入模式需要在对应外设寄存器的控制位进行操作才能使用;(For Alternate Function input you should select floating or pull-up input configuration in the DDR and CR1 registers. 该节提到的另一句,就是说要将PD5的GPIO配置为floating或者pull-up)
而在11.8.1节复用功能输出(Alternate function output)则说: “Alternate function outputs provide a direct path from a peripheral to an output or to an I/O pad taking precedence over the port bit in the data output latch register (Px_ODR) and forcing the Px_DDR corresponding bit to 1.” 即是说,复用功能的输出优先级比ODR高,直接强迫DDR设为1。