DocID17066 Rev 6

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AN3155

USART bootloader code sequence

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USART bootloader code sequence

Figure 1. Bootloader for STM32 with USART

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

configured (for more details refer to application note AN2606 “STM32 system memory boot

mode”) the bootloader code begins to scan the USARTx_RX line pin, waiting to receive the

0x7F data frame: one start bit, 0x7F data bits, even parity bit and one stop bit.

The duration of this data frame is measured using the Systick timer. The count value of the

timer is then used to calculate the corresponding baud rate factor with respect to the current

system clock.

Next, the code initializes the serial interface accordingly. Using this calculated baud rate, an

acknowledge byte (0x79) is returned to the host, which signals that the STM32 is ready to

receive commands.