A (midrange) PIC Timer guide

Do you get stuck with PIC timer calculations juggling between datasheet, calculator and your design requirements?

If so then you need some tools to automate the process.

Calculations

Typically you will need to focus on the end result which is usually to make the timer generate a frequency or create a repeating interrupt time of the correct period.  This is fairly difficult when you first have to find the information in the datasheet and then account for prescalers and postscalers etc.

If you do this a lot you have quite a few calculations and decisions to make as the hardware in each timer is different and you'll also have to do battle with interrupts.

TCL Scripts for PIC Timer calculation

PIC Timer 0

This has an 8 bit prescaler and an 8 bit timer and can be driven from an external clock.

Prescaler : 8 bit
Timer register : 8 bit

Link to PIC-Timer 0 calculator.

PIC Timer 1

This has a 4 bit prescaler and an 16 bit timer and can be driven from an external clock. It can also be driven from a slow speed crystal e.g. 32kHz.

Prescaler : 4 bit
Timer register: 16 bit

Link to PIC-Timer 1 calculator.

PIC Timer 2

This has a 4 bit prescaler and an 8 bit timer and an 8 bit period register and is only driven from the internal clock (Fosc/4)

Note: the Period register lets you create an output frequency with no further processing i.e. you don't have to update the timer register value in an interrupt routine as you do with Timer 0 or Timer 1 - this makes it easy to use (although it's tricky to get right - unless you use the calculator scripts).

Prescaler : 2 bit (1:1, 1:4, 1:16)
Timer register: 8 bit
Period register : 8 bit
Postscaler : 4 bit (1:1 to 1:16 inclusive)

Link to PIC-Timer 2 calculator.

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