So what you're saying is that you enable the PLL, wait for it to lock, and then - once you select the clock-source as PLL it crashes / resets itself.
Have you tried this on other stm32 (in other words is it specific to stm32f302 and not on others)? It's not a problem if you haven't, it just helps us to narrow down the problem. I don't see any pll-related errata for that device.
One possibility is that you're telling the PLL to go to a higher frequency than HSI or HSE. The FLASH memory can only work at a certain rate, and you need to tell the memory subsystem to insert a certain minimum number of wait-states in FLASH reads before you switch to the PLL source. (That number depends on the processor-family and the selected frequency; see the data-sheet and reference manual).
Have you tried running the PLL at 8 MHz or 16 MHz to rule this out?
Another possibility is that you're not properly checking that the PLL has locked. Although I guess not as you've tried both HAL and LL.
A third possibility - if this is your own pcb rather than a commercial demo-board - is that the power-supply is not sufficiently clean / stable for the PLL to be reliable. I've never had this problem myself, but don't rule it out.
A fourth possibility is that the PLL does not lock sufficiently-fast for your surrounding code. Maybe you have a watchdog-reset that fires while you're waiting.
Hope this helps,
Danish
