Frequency/Pre-scale Registers

The Timer Pre-Load register R21 specifies the minimum number of signal cycles to count and ranges from 1 to 256 (0). The Frequency Pre-scale register R22 multiplies the cycle count according to:

R22	Pre-scale factor
0	2^0 (1)
1	2^1 (2)
:	:
8	2^8 (256)

The number of cycles "CC" over which to measure the frequency is programmed as:

CC = 2 * R21 * 2^R22 * GPS

where the factor of 2 accounts for the fact that Spock divides all frequencies it sees by 2 before passing them on to the PIC, regardless of the input selected.

GPS is 1 for all inputs except the 1 GHz Prescaler in which case it is 64.