STM32F407VGT7 I use system memory boot mode by USART1 (PA10, PA9). i have to use external pull-up/pull-down resistors to USART3 (PB10 / PB11, PC10 / PC11), CAN2 (PB5), USB (PA11, PA12)?

VDil.1 · ‎2021-12-02

STM32F407VGT7

I use system memory boot mode by USART1 (PA10, PA9). i have to use external pull-up/pull-down resistors to USART3 (PB10 / PB11, PC10 / PC11), CAN2 (PB5), USB (PA11, PA12)?

PA10, PA9 connect to external pull-up resistors 7.5Kohms.

If don't connect resistors, will there be programming problems?

My project use pin PA11 - exit. PA11 connect to bus transceiver SN74LVC2T45. no external pull-up/pull-down resistors.

Bob S · ‎2021-12-02

According to AN2606, any of those pins that are not intended to be used for the boot loader function need to be "non-toggling" when the bootloader is running. That usually means an external pull-up or pull-down resistor, or driving the pins from some external device that you can guarantee will keep a steady logic voltage. If you leave the pins floating, while the odds of noise making them look like a valid bootloader connect sequence is quite low, it is not zero (except possibly the USB interface). I wouldn't risk that on any product I build. That said, on the UART ports, you do not need pull-up/down resistors on the TX lines, as those are configured as outputs by the bootloader. Only the RX lines need the resistors.

Your PA11 and 2T45 transceiver - is the 2T45 driving PA11 when the bootloader is running? If so, what it driving the other side of the 2T45? It needs to be non-toggling.