All about Occillators? Frequency/PLL?

[wdmagic]

ok I think I got that part now as it relates to the USB section, what about if your not messing with USB at all though and you want to run your chip at a higher frequency than the crystal your using, say I have a 4mhz but I want my chip to run at 20mhz. is that possible, or am I supposed to be using CPU DIV for that and get 16mhz? or would I have to just start with a large freq crystal and just downgrade its freq. thats where im getting confused. I have many 4mhz &8mhz crystals and a single 20mhz, and a couple of oddballs that are not suitable for pics. I can order 48mhz ones though if I need too.

[HenrikOlsson]

Hi,
The only way to increase the frequency (on this chip) is to derive the CPU clock from the 96MHz PLL by setting the OSC CONFIG to PLL mode. For this to work you must ensure that the x-tal frequency you're using is able the be divided down to 4MHz by any of the available PLLDIV ratios, 4MHz is fine. You then have an internal 96MHz clock available and possible dividing ratios of 2, 3, 4 & 6 giving a CPU clock of 48, 32, 24 or 16MHz respectively - no way to get 20MHz that way.

If you want to use your 8MHz x-tal, same as above but now you must set PLLDIV to divide by 2 in order to feed 4MHz to the 96MHz PLL. The rest would be exactly the same.

If you "must" run the CPU at 20MHz then use a 20MHz x-tal and use normal HS mode instead of PLL mode for the OSC CONFIG.

/Henrik.