Page 115 - Making PIC Microcontroller Instruments and Controllers

couItEhg t05

I Decide which counter (timer) to use.
I Tell the counter where the signal is coming from.
t Tell it whether to count on a rising or falling edge.
t Decide what target count we are looking for.
I Tell the counler wheie to start countirg because rhe interrupt will occur when the
counter gets full.
I Decide whether we will need to scale the count by sedng the scalar(s).

Once we start a counter, the counting contin es until we deactivate it. There is no other
way to stop it, nor any reason for doing so. It will reset if the MCU is r€set and it can
be reset by writirg to it. The rest has to do with knowing what bits to ser in the control
registers to get the counlers fo operate in the way we want them to.

USIIIG TIIIIERO AS A COUNTER

Note Though often called TIMER0, a d referred to as TIMER' herc andi the
datasheet, the real designation of this timer address i TMRj.
The thr€e registers r€lated to the control ofthe TIMER0 module are TMRo,INTCON,
and OPIION_REG. INTCON is the intenxpt control register. See the datasheet for mol€
information.
This counter has the following properties:

I 8-bit timer/counter
r Readable and writable
r 8-bit software programmable prescalar
: Internal or extemal clock select
: Interupt on overllow ftom FFh to 00h
I Edge select for extemal clock

Counter mode is selected by setting OPIION_REG.5= l.

I The extemal input for the timer will come in or PORTA.4, which is pin 6 on the PIC.
I The edge dir€ction is selected in OPION_RXG.4 (l=Rising edge).
I The prescalar is assigned with bits 0 to 3 of the OPTION REc register. (See page
49 oftbe datasheet.)

Note again that the watchdog timer.d,rror use the prescalar when the prescalar is
being used by Time .
Since this is an 8-bit counter, it is suited to the counting of small number of counts,
but longer counts can be accomrnodated by using a murine ro k€ep t{ack of the ifterrupts.
Let's use the counter to count the pulses received from a 20,s1ot encoder mounted
on a small DC motor. This same sowce will be used later lor the Timerl experiment
for comparison between countets,
We will set up to use the LCD display so we calt display ce(ain rcgisters during the oper-
ation of Progmm 65. We will also set up to rcad the potentiometers so we can use th€ir
values to modily the pmgram as it runs. ODly POT0 and POT1 are used in the program.