Bootloader hardware requirement：The hardware required to put the STM32 into System memory boot mode consists of any circuitry, switch or jumper, capable of holding the BOOT0 pin high and the BOOT1 pin low during reset.To connect to the STM32 during System memory boot mode, an RS232 serial interface (example, ST3232 RS232 transceiver) has to be directly linked to the USART1_RX (PA10) and USART1_TX (PA9) pins.
Note:USART1_CK, USART1_CTS and USART1_RTS pins are not used, therefore user can use these pins for other peripherals or GPIOs.For more details about hardware recommendations, refer to application note AN2586: “STM32 hardware development: getting started”, available from the STMicroelectronics website: www.st.com”
原文：http://my.oschina.net/u/2252538/blog/466733