STM32 bootloader issue

[mikael2]

Posted on October 28, 2015 at 13:17

Hi!

At our company we are using STM32F205 in as a USB module. Connecting the USART3 to an FTDI converter. The MCU is then regulated to 3v and the FTDI is powered direcly from USB 3.3v. BOOT pins are set to stay in bootloader on startup.But the problem is we sometime can not communicate with the bootloader and get stuck. The only way to make it work is to powercycle our module. I have  checked withan Logic Analyser and when we get this problem it seems stm respons with an, uknown to me, byte.AFAIK when stm32 bootloader get a syncbyte it should answer with 0x79 ACK or 0x1F on NACK. But the thing is it response with 0x9D. If I try second time it will not respond. A third try I get 0xF9 and repeat from there seems to always return 0xF9We have used 115200 and 230400 and the signals seems good.

[Tesla DeLorean]

Posted on October 28, 2015 at 14:07

This is why it's important to have access to NRST for programming.

The Boot Loader is not looking for a data pattern, but rather is timing aspects of the signal in order to measure and compute the baud rate. This is a one-shot deal and is sensitive to glitching. Once measured it is simply responding to command bytes it's is seeing, at whatever baud it selected.