i just discovered this site and i'm wondering if anyone can help me with PIC timing. i'm using a PIC16F872 clocked at 20 MHz. ideally i'd like to use timer1 (16 bit) at a frequency of 625 kHz, so in CSS i iniated it with:

setup_timer_1(T1_DIV_BY_8);

i expected the following:
20 MHz freq of oscilation / 4 cycles per clock tick = 5 MHz internal clocking
5 MHz / 8 = 625 kHz timer1 frequency

when i read from the timer at an interval that should be within its range at this frequency, i am getting negative values. i need to measure intervals ranging from 100 uSec to 100 mSec. any help would be greatly appreciated.