"Running Small Motors with PIC Microcontrollers" says, on page108

"A pre-scaler is applied to the system clock and affects the timer by slowing down the system clock as it applies to the timer. Normally the timer is fed by a fourth of the basic clock frequency, which is called Fosc/4. In a system running a 4 MHz the timer sees a clock running at 1 MHz. If the pre-scaler is set for 1:8, the clock will be slowed down by another eight times, and the timer will see a clock at 125kHz."

"The post-scaler setting determines how may overflows will go by before an interrupt is triggered."

If I pre-scale TMRO 1:256 I will get overflows at about 4K per second. If I add a post-scaler of 1:64 I will end up with about 62 interrupts per second.

Close enough for government work?

Ken