Page 95 - Making PIC Microcontroller Instruments and Controllers

fllitEFo

Timers can have prescalars and/or postscalaN associared with rhem that can be used
to multiply the timer setting by a limited number of integer counts. The scalitg ability
is not adequate to allow all exact time intervals to be created but is adequate for all prac-
tical purposes. To the inability to create perfectly timed intenupts, we have to add the
uncertainty in the fiequency of the oscillator crystal, which is usually rct exacrly what
it is stated to be (and which is affected by the ambient temperature as the circuirry warms
up). Though fairly accurate timings can be achieved with the hardware as rec€ived, addi
tional software adjustments may have to be added if exrremely accumte results are
desired. The software can be designed to make a correction to the timing every so often
to make it more accurate. We will also need an extemal sourc€ that is at least as accu-
rate as we want our timer to be, so we can veriry the acclrmcy of the device we create.

TimerO

l-€t's write a simple program to see how Time works. We will use the LED bargraph
to show us what is going on inside the miqocontroller.
As always, the bargraph is connected to the eight lines of PORTD o{ the LAB Xl.
First,Iet's write a program that will light the two LEDS connected to D0 and Dl alter
nately. (See Program 6.1 .) Having them light altemarely lers you know thar the program
is running, or morc accurately, it lets you know that the segment of the program that
conlains this pan of the code is working. These two LEDS will be used to repr€sent the
foreground task in our program. There is no timer process in this program at this srage.
There are no interrupts. The program just blinks the LEDS.

lF!i0E!]!ffiS: Foreground program blinks two LEDS alternately (No timer is being
used in this program atthis lime)
CIEJAR
DEFINE OSC {
TRISD . t11110000 make D0 lo D3 outputs
turn off bi! D0
PORTD.I - 1 turn on bit D1
IIIIPHA 1IAR WORD

MAIAILOOP !
I F P O R I F D . 1 = 0 X H E N the next tines of code turn the LEDS ON
PORTD.I = 1 if lhey are oFF
PORTD.O = O
EIJSE
PORTD.I = O and OFF if they
PORTD.o - 1
EI1IDII'
F O R . N T P S A = 1 T O 3 0 0 this loop replaces a lonq pause comand
PAUSTOS 100 uith short pauses thal are essentially
NEJI(T II,PIIA r ' 1 1 6 . | ^ . | , ' c ' . | ' ' '
GOTO MAIIILOOP do it all forever
END all prosrds need to end with END