Posted on July 07, 2011 at 01:09I'm not too familiar with the STM32W.
What I can tell you is the 0x7F pattern is not checked by the boot loader on the STM32, it is used as a timing pattern, and based on the perceived timing it then sets a baud rate, and responds. What you see at the host side will depend on the accuracy or disparity in the baud rates at either end.
Now it's only checking for 0x7F timing pattern at initial start up, after it has the baud rate, it will most likely respond to subsequent 0x7F's with a NACK. Try sending some other identification commands. The pattern also assumes 8-bit, Even Parity, 1 Stop bit. As best I can tell the STM32W uses the same serial protocol with the BOOTx straps as the other members of the family. I'm looking for a clear cite right now.
On the application side, you could pretty much do what you want in terms of protocols. Using something like XMODEM, HTTP or TFTP. The one key consideration is however that running from, and writing to the flash memory will cause significant stalling of the processor. To the point where peripherals will over/under run their buffers. Ideally running something from ROM or RAM to perform the update to avoid this problem.
