| 我用STM32F105RB UART1 ISP 可以下载  另外: If USART1 is used to connect to the bootloader: the USART2_RX (PD6),
 CAN2_RX (PB5), OTG_FS_DM (PA11) and OTG_FS_DP (PA12) pins have to be
 kept at a high or low level and must not be left floating during the detection phase
 
 我悬空这些引脚也是可以下载的。没有问题
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