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.