Consider a PIC16 processor with Fosc=20MHz, an 8-bit timer and pre-scaler of 4. Around how many interrupts will happen per second?

now shouldn t we divide 20MHz by 4 to find the number of increments of the timer and then divide that to 2^9-1 to find the number of interrupts? because apparently it doesn't give the correct answer.

Peter H

We just don't have sufficient information here. Unless the circuit is built that way, there is no relation between the clock frequency and the occurrence of interrupts. The whole point of interrupts is that they cause a program which is running to be suspended to allow the running of an interrupt routine. The interrupt may therefore be entirely random (like the response to the push of a button), or occurring regularly (like a circuit signalling the end of a timing period).