Library does not work with Feather M4 Express and Adafruit_SleepyDog-master

[Casey10110]

I was having trouble localizing a problem before with the SPIFlash library and finally pinned it down to its interaction with the SleepyDog library. I created a minimal example which basically causes SPIFlash writes to corrupt the file system (see attached). This example works when you comment out the Watchdog.sleep(1000); ... to reproduce the error, follow these steps, first with that line commented out:

1. format the M4 Express using the SdFat_Format example
2. run the minimal example attached
3. run the SdFat_OpenNext example to see what was written

Then repeat that with the Watchdog.sleep(1000); line and watch the file system become corrupted. Any ideas what in the SleepyDog library is causing this?

Flash_Memory_Problem_Minimal_Example.txt

Update: this same crash happens when using CircuitPython to create the file system and see what is written.

[Casey10110]

Adding

OSCCTRL->DFLLVAL.reg = OSCCTRL->DFLLVAL.reg;
while( OSCCTRL->DFLLSYNC.bit.DFLLVAL );

after sleeping seems to fix the problem. Basically, some of the configuration done in startup.c is apparently lost when the SAMD51 goes to standby. The only reason I thought to look at this is because I noticed that sleeping also changed the pitch of a physical oscillator (a change of about 230 cents) I have connected to the device ... so I assumed it was changing the timers' configuration. Looking at wiring.c and startup.c, I localized an area of code that appeared to re-configure it properly after sleep. In all honesty, I have no idea what it does, but it seems to fix both problems. Any clues as to why this works?