I have a board that has a PIC16F18855 running at 8MHz from an external crystal. Most of the time it works fine, but if it gets rapidly power cycled it will restart at 4MHz. The main symptoms of this are things such as the UART baud rate and the PWM frequency dropping by half.
More specifically, if I flick the power switch on and off in less than ~2 seconds, it will run at the lower frequency. Longer and it works properly. When I program the board, it restarts at 4MHz, meaning I have to physically turn it off and on before it will work. If I short the MCLR pin to ground, the behavior is inconsistent - sometimes it will start at 4MHz and but usually it will start at 8MHz.
I have also had one instance where it was taking substantially longer, where turning it on within 10 minutes of turning it off made it start at 4MHz. I'm not sure if the problem is related, but I'm not able to reproduce that error at the moment.
From probing the OSC2/CLKOUT pin, I can see that the crystal itself is still running at 8MHz. With the probes I have I can't probe the OSC1 pin without it resetting the micro.
When the board is running at 8MHz, its OSCCONx registers are:
OSCCON1: 0x70 OSCCON2: 0x70 OSCCON3: 0x10
and when its running at 4MHz they are:
OSCCON1: 0x70 OSCCON2: 0x60 OSCCON3: 0x00
So it's running off the HF internal oscillator. Will look at this a little closer. Knowing this I may be able to come up with a work around, but I'd rather figure out the cause of the problem.
OSCFRQ = 0x02, which is the setting for 4MHz, so this is definitely whats happening. Terrible workaround would be to set the
OSCFRQ register to the 8MHz setting, so it works the same either way, but then I lose the precision of the crystal. That may not ruin everything, but I have some timing sensitive things so I prefer to use the crystal if I can.