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Bootloader Framing Errors - STM32L052

Question asked by Steve Krattiger on Apr 28, 2017
Latest reply on Apr 29, 2017 by Steve Krattiger



I'm writing an application on an STM32L052 that uses the UART (115200-N81), and I need to use the Bootloader for code upgrades.   


If run the Bootloader using the BOOT0 pin, the UART initializes fine and I can upload code at any baud.   However if I jump to the Bootloader code from my application, the Bootloader isn't initializing the UART correctly, and I get framing-errors transmitted out from the CPU.   


From the scope traces (good and bad are attached), I can see that at any baud rate (115200-E81 shown), the Tx signal is switched to '0' between each byte instead of staying high, this causes a framing error.   It almost seems as though the stop-bit is missing or inverted.   


I'm guessing that in my application's use of the serial port, something is left in a register somewhere that doesn't get initialized in the Bootloader code.   Since I cannot modify the Bootloader code, I need to find the errant register that causes a missed stop-bit, or causes this framing error.   


Anyone run into this before???    


- SteveK