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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

To make the process easy I have created three interactive script modules written in TCL (Tool Command Language).

Installing TCL

This is a very easy bush button process, and once done you can double click the .tcl file to run it.

It is at this location

Activestate.com

...Install the free runtime from ActiveState

From the page in the above link, click Languages, then select “Active TCL”. Then click the "Active TCL" link under the title:

“Download the Free Community Edition:”

Then finally, download the TCL environment for your machine - probably windows installer (x86).

After installation just double click the TCL file to run it (that you wil download below).

TCL Scripts

Each of these scripts is geared towards the most typical use of each timer and lets you change prescaler, postscaler or register value using slider controls.  This makes it easy to experiment with different values as the result is immediately displayed in the web page (frequency and period are displayed from each part of the timer e.g. after the prescaler, after the register, after the postscaler).

So you can do what-if type operations (all without a calculator in sight) e.g. I need a 15ms repeat rate.

From Timer 2 the closest I could get is 15.136 (took 1 minutes to test) - perhaps I'll try timer 1 - Ah that gives an exact 15ms (took 30 seconds to test). Just check with Timer 0 - this gives 15.040ms (took ~30 seconds to test).

Of course you can also set the main clock frequency (internal or external crystal) as well.

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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