I recently bought a Fez cerb40 and had a question related to the hardware design of the board. In ST’s AN2606 bootloader appnote it says:
“If DFU (USB FS Device) is used to connect to the bootloader: the USART1_RX (PA10), USART3_RX (PC11 and PB11) and CAN2_RX (PB05) pins have to be kept at a high or low level and must not be left floating during the detection phase.”
Looking at the schematic of the cerb40 I noticed that both pins PA10 and PB05 are connected to VDD via pull up resistors to keep these pins from floating. I imagine this was done to keep these pins at a known state when not in use to allow for proper DFU functionality as per AN2606.
If that is indeed the case, how come the USART3_RX pin is not connected to VDD via pull up resistor as well ?