The debug launcher can be configured to start execution at a custom address, different from the reset vector, so it's possible to execute some function instead of (or before) the reset handler when debugging, but still jumping to the reset handler when the device is reset.
The custom startup address entry field can be found under Run / Debug Configurations / Startup / Set program counter at (hex). It requires a fixed address, so you have to arrange a fixed address for the custom startup function. You can put it right after the vector table, as it is at a fixed address, and its length is not likely to change.
Open the linker script (STM32L4*_FLASH.ld), find the .isr_vector section, duplicate and rename the copy both inside and outside. You should get something like this
.isr_vector :
{
. = ALIGN(4);
KEEP(*(.isr_vector)) /* Startup code */
. = ALIGN(4);
} >FLASH
.debug_in_sleep :
{
. = ALIGN(4);
KEEP(*(.debug_in_sleep))
. = ALIGN(4);
} >FLASH
/* The program code and other data into "FLASH" Rom type memory */
.text :
Now write the function to set DBGMCU
void __attribute__((used, section(".debug_in_sleep"))) debug_in_sleep(void) {
DBGMCU->CR |= DBGMCU_CR_DBG_SLEEP;
asm volatile("ldr pc, [%[vectors],4]"
:
:[vectors]"r"(0x08000000)
);
}
The used attribute tells the linker not to drop this function even if it isn't referenced anywhere in the code. The section attribute puts it in the linker section created above. The inline asm code jumps to the real reset handler. (Simply calling Reset_Handler() instead would work, but it makes the compiler save some registers on the stack before. Having a different value in the stack pointer while debugging would make debugging a stack corruption issue more challenging as it is already.)
Build the project, and open the .map file generated by the linker. (No .map file? Enable it in Project / Properties / C/C++ Build / Settings / Linker / General.) Find the address of the function in the .map file, and copy it into the Set program counter field in the debug launcher configuration dialog. Hitting the debug button now should start execution at debug_in_sleep().
