ST STM32F105 Series Application Note page 10

Bootloader description
interfaces can be used to communicate with the bootloader and download the application
code to the internal Flash.
The embedded bootloader firmware is able to auto-detect the peripheral interface to be
used. In an infinite loop, it detects any communication on the supported bootloader
interfaces.
To use the USART bootloader on USART1 or USART2, connect the serial cable to the
desired interface. Once the bootloader detects the data byte 0x7F on this interface, the
bootloader firmware executes the autobaudrate sequence and then enters a loop, waiting
for any USART bootloader command.
To use the CAN2 interface, connect the CAN cable to CAN2. Once the bootloader detects a
falling edge on the
to check the external clock frequency value, if the HSE is 8 MHz, 14.7456 MHz or 25 MHz
CAN bootloader firmware enters an infinite loop and waits until it receives a message,
otherwise a system reset is generated.
If a USB cable is plugged into the microcontroller's USB interface at any time during the
bootloader firmware selection sequence, the bootloader then enters the DFU bootloader
loop waiting for any DFU bootloader command.
To use the USART or the CAN bootloader, it is mandatory that no USB cable is connected to
the USB peripheral during the selection phase. Once the USART or CAN bootloader is
selected, the user can plug a USB cable without impacting the selected bootloader
execution except commands which generate a system reset.
Once one interface is selected for the bootloader, all other interfaces are disabled.
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, the bootloader firmware enters a CAN loop and starts
CAN2_RX pin (PB5)
Doc ID 14156 Rev 1
AN2662