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
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
...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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