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stm32l432 boot sequence (specifically from previously empty flash)

Question asked by brown.geoffrey.001 on Mar 27, 2018

The stm32l432 reference manual has the following cryptic remark about the boot sequence


"A Flash empty check mechanism is implemented to force the boot from system Flash if the first Flash memory location is not programmed (0xFFFF FFFF) and if the boot selection was configured to boot from the main Flash."


But does not specify when this check occurs.  I've found the following anomaly that may relate to this.  


1) Start with an erased device

2) Program the part

3) Without power cycling, reset


In this case, the part will jump to the system boot loader.  However, the mysterious thing is that  SYSCFG->MEMRMP is 0 (indicating that the memory is not remapped).   I don't see how it is possible to boot to the programmed code without a power cycle -- even through the SWD interface, because it appears that the "Flash empty check" is persistent.   What am I missing ?    I know how to use the SWD interface to access all of the stm32 resources, but I don't see what visible state is affecting the boot sequence.