DocID017068 Rev 4

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AN3156

Bootloader code sequence

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Bootloader code sequence

There is no difference in terms of protocol (requests and commands) between different

Bootloader DFU versions. For detailed difference list please refer to Section 6.

Once the system memory boot mode is entered and the STM32 device has been configured

(for more details refer to AN2606), the bootloader code configures the USB and its

interrupts, and waits for the "enumeration done" interrupt.

The USB enumeration is performed as soon as the USB cable is plugged (or immediately if

the cable is already plugged). If you do not want the STM32 to enter the USB DFU

bootloader application, the USB cable has to be unplugged before reset.

The bootloader version is returned in the device descriptor in the MSB of the bcd Device

field (example: 0x2000 = Version 2.0).

For Connectivity line USB DFU bootloader, the device first tries the 25 MHz configuration,

then, if it fails, the 14.7456 MHz configuration, and finally, if it fails, the 8 MHz configuration.

If it still fails, this operation is repeated with a large timeout value (the three configurations

are tested again). If the second trial also fails, a system reset is generated.