According to the “pin mapping table” on the outboard DFU guide here Notecard Outboard Firmware Update - Blues Wireless Developers
The aux serial interface (AUX_TX_P and AUX_RX_P) should be connected to the MCU host in order to enable DFU using the ROM bootloader.
However in the notecard datasheet Notecard Datasheet - NOTE-WBNA-500 - Blues Wireless Developers it states that for power consumption reasons the aux serial interface is disabled by default and must be enabled by pulling AUX_EN_P high. Can we get confirmation that the notecard is able to autonomously pull this pin high internally during an outboard DFU procedure?
If this is the case, I would suggest updating the outboard DFU docs to make it clear that AUX_EN_P will be pulled high internally, and it must be left NC in the design to avoid interfering with enabling that interface.