Anyone happened to know the command/register to select/lower internal oscillator frequency?
also Anyone know the frequency of crystal oscillator for pic16f877?
Have you looked at page 61 and 62 of the PiC16 datasheet, looks like a pretty comprehensive guide for what you are trying to do, frequency is selected by writing to the PR2 register and PS register it seems, haven't done this myself though, don't take my word for it.
PR2 doesnt handle the crystal oscillator frequency, it acts as a stop--count-mark for counter of PWM
check IRCF<0:2> in OSCCON (for pic18)
If you are talking about the actual frequency of the oscillator, I'm pretty sure that's a physical property of the crystal oscillator, and can't be changed, PR2 changes PWM period, which is equivalent to changing frequency, I guess that's not what you want?
and PR2 cant change to anything lower than 244Hz