Setting up timers reference?

[wmeade]

I am using a 18F242 and need to setup a internal timer to interrupt every 16ms. I am having trouble figuring out how to configure a timer to do a 16ms interrupt. Is there a good reference I can use that will show the math on how to set up the Timers on a PIC? Thanks in advance.

[Neutone]

First you need to understand what a prescaler and post scaler are as well as the overflow value.

There is one instruction cycle for every 4 cycles of the oscilator or clock.

A prescaler value of 1 means that one instruction cycle increments the timer once. (1:1 prescaler)
A prescaler value of 4 means that four instruction cycles increments the timer once. (4:1 prescaler)
A prescaler value of 16 means that sixteen instruction cycles increments the timer once. (16:1 prescaler)

In the CCS manual prescaler is refered to as mode.

A postscaler of 1 means that one overflow will trigger an interupt.

An overflow occurs when incrementing past the period of the counter. Timer 1 has a period of 2^16(65536)
Timer 2 has an adjustable period of from 1 to 255.

To solve your problem of achieving a 16mS interupt you first must know how many instruction cycles occur in 16mS.

This should be enough information for you to setup a spreadsheet using the available options. Once you have seen your options you can simply pick the best one.

[wmeade]

I created a spread sheet for timer 1. If I got it right the following will be true:

With 20MHz Clock

setup_timer_2(T2_DIV_BY_16,255,16) will interrupt every 13.056mSec

[Neutone]