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Clarification of usage of USB DFU bootloader on the STM32F405 micro

Question asked by halherta on Apr 18, 2012
Hello I had a hardware design question on the usage of the USB DFU bootloader on the STM32F4 microcontroller. 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."

This means that each of these pins will need to be attached to an external pull-up or pull-down resistor. I'd imagine that pull-ups would work better. This will of course limit the functionality of these 4 pins. I was wondering if there's anyway by which we sacrifice (by putting external pull ups on them)  less  or no pins and still get DFU bootloader functionality on the STM32F4.

I'd appreciate any insight on how to go about a DFU friendly but efficient board design.