The code I'm working on does this. The CPU starts up at 180/60MHz (core peripheral).
Very early in our startup/boot we have code that ***
1 - CHECKS to see what the core is currently running at by looking at the divider,
2 - If it isn't ALREADY at 240/80MHz it then:
3 - Changes to "limp" mode,
4 - Reprograms the clock dividers,
5 - Changes back out of limp mode
6 - Waits for PLL lock (not sure this is needed).
There's sample code around that does the above, but don't ask me where.
So by now you should be asking "if the CPU runs at 180/60 out of reset, why are you checking to see if it is ALREADY at 240/80"?
Because, in spite of what the Reference Manual, CPU Reset does not always reset the PLL. Power-on reset does. The Watchdog reset doesn't. The software-initiated preset probably doesn't either. The external hardware reset probably does.
Good luck.
