This will definitely be clock frequency dependent (note maximum frequency of 33 MHz). If I am reading the data sheet correctly, I suspect that you could easily get the type of behavior you are looking for using auto-reload mode at clock frequencies at or above 12 MHz. The shortest time between interrupts would then result when the reload value is 0xFFFF, thus causing overflow every machine cycle (clock/12). Thus, if you were using the maximum allowed oscillator frequency, you could be getting timer interrupts about every 3.6 x 10^-7 seconds. But, what would you do with all those interrupts? You wouldn't have time to process them if you got one every machine cycle.

--Sasha Jevtic