Nuts & Volts

GETTING STARTED WITH PICs
through the major sections of code.
The top section states: *** Define
Program Variables *****. This is where
the variable gets defined. A single variable
is established and given the simple
nickname of “x.” The program will use
that later as a general storage space for
the FOR-NEXT loop command counter.
Program variables need to have
their size defined. In this example, “x”
does not need to be larger than 255,
so a byte size will do.
‘ *** Program Variables ***
x var byte
After the variable definition, the
main program loop is entered. The
description line ‘*** Main program
loop *** begins this section and it’s
quickly followed by the main: label.
‘ *** Main program loop ***
main:
This label defines a location within
the program where the main section of
code begins. Under this label is where
the LEDs are sequenced. The HIGH and
LOW commands are used to turn the
LED on (HIGH) and off (LOW). Because
this program has to repeat the same
function for each LED, the program
could have a bunch of HIGH and LOW
commands. That would work, but it
would also take up more than 31 lines.
To simplify the program, the FOR-
NEXT command will be used. The
FOR-NEXT command creates a small
loop where everything between the
line that starts with FOR and the line
that starts with NEXT is rerun a specified
number of times. For example,
look at the code section below.
for x = 8 to 15
high x
pause 10
low x
pause 10
next
The section of code starts with
the FOR command followed by a
simple little math type statement, x =
8 to 15. What this means is that every
time this list of commands is executed,
increment the variable x by one
starting with 8 and ending at 15. This
is known as a FOR-NEXT loop.
The value of the variable x is
actually tested at the NEXT command
line. If x equals 15, the program leaves
the FOR-NEXT loop and moves on to
the commands following the NEXT
command line. If x does not equal 15,
the program control jumps back to
the FOR line and x is increased by 1.
This allows you to write a chunk of
repeating commands without having
to write them over and over again.
In this loop of code, notice how
the HIGH and LOW commands share
the variable x. Each time the program
runs through the FOR-NEXT loop, a
different LED should be lit. By using
the variable x after the HIGH and
LOW commands, a different LED is
turned on and then off.
The value of x must match up to
the pins connected to the LEDs. In
this case, because the LEDs are connected
to Port C, we can use 8 through
15 as the FOR-NEXT counter values
while also using them as the pin variable
in the HIGH or LOW commands.
This project uses a very short
delay time between the HIGH and
LISTING 1
LOW commands. The intent is to make
the light scroll forward quickly. Notice
that I only said forward. That’s because
this FOR-NEXT loop only lights LEDs 8
to 15. It does not reverse the direction
and light 15 through 8. That requires a
separate FOR-NEXT loop. The next
section of code is very similar and is
another FOR-NEXT loop. In this
section, though, the FOR-NEXT loop
counts down instead of counting up.
for x = 15 to 8 step -1
high x
pause 10
low x
pause 10
next
That change of counting direction
is done with the step –1 added to the
FOR command line. The step –1 directs
the FOR-NEXT command to add a -1
(negative 1) to the value of x each time
through the loop. The FOR command
line starts at 15 and goes to 8. It’s the
opposite of the first FOR-NEXT loop.
The purpose of counting the opposite
direction is to make the LED light
‘******************************************************************
‘* Name : chap6code.BAS *
‘* Author : Chuck Hellebuyck *
‘* Notice : Copyright (c) 2005 Electronic Products *
‘* : All Rights Reserved *
‘* Date : 3/31/05 *
‘* Version : 1.0 *
‘* Notes : Scroll 8 LEDs *
‘* : *
‘******************************************************************
‘ *** Program Variables ***
x var byte ‘ FOR-NEXT loop variable defined
‘ *** Main program loop ***
Main:
for x = 8 to 15
high x
pause 10
low x
pause 10
next
‘ Main loop label
‘ Loop 8 times thru code
‘ Turn on next LED
‘ Delay 10 milliseconds
‘ Turn off LED
‘ Delay 10 milliseconds
‘ Is x = 7 yet?
for x = 15 to 8 step -1 ‘ Loop 8 times in negative direction
high x
‘ Turn next LED on
pause 10
‘ Pause 10 milliseconds
low x
‘ Turn off LED
pause 10
‘ Pause 10 milliseconds
next
‘ Is x = 0 yet?
goto main
end
‘ Jump to main and do it again
‘ Stop if the program gets here
April 2006 77