Chapter 2
A Loop in the Pattern
Designing the Main Loop and
Timing
Checklist
The following tools will be used throughout the course:


MPLAB X, Integrated Development Environment (v1.8 or later,
free)
MPLAB XC16, C compiler (v1.11 or later, free)
The following pieces of documentation will be used during this
lesson:

PIC24FJ128GA010 Datasheet –DS39747 (latest rev.)

PIC24 Family Reference Manual - Section 14. Timers
Make sure they are available and/or installed and ready to use on
your computer.
You can download them from Microchip web site at:
http://www.microchip.com/mplabx
And
http://www.microchip.com/xc16
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
A New Project

Use the New Project wizard to create a new project


Use the New File wizard to create a new source file

/*
Call it: “2-ALoopInThePattern”
Call it: Loop.c
* File:
Loop.c
* Author: your name here
*
* Created current date here
But you can also use the form:
*/
void main( void)
{
#include <xc.h>
// no return
required
int main( void)
}
{
Or simply:
return 0;
main()
}
{
}
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
“while” Loops

In C there are several ways to design a loop. The first we are
going to explore is the “while” loop:
while ( x)
{
// your code here…
}

Where ( X) is a logical expression
false is represented as the integer zero
 true is represented by any integer except zero

Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Logical Operators





||
the logic OR operator,
&&
the logic AND operator,
!
the logic NOT operator
These operators consider their operands as
logical (Boolean) values using the rule
mentioned above, and they return a logical
value.
Here are some trivial examples, assuming:





In other words, they are both “true”
a = 17
b=1
( a || b)
( a && b)
( !a)
is true,
is true
is false
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Comparison Operators

There are, then, a number of operators that compare numbers
(integers of any kind and floating point values, too) and return logic
values.
They are:
Composed of two equal signs to
== the equal-to operator,
distinguish it from the assignment
!=
the NOT-equal to operator.
operator ‘=‘ we used in the previous
>
the greater-than operator.
lesson.
>= the greater-or-equal to operator.
<
the less-than operator.
<= the less-or-equal to operator.

Here are some examples, assuming:












a = 10
( a
(-a
( a
( a
> 1)
>= 0)
== 17)
!= 3)
is true
is false
is false
is true
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Curious Cases
while ( 0)
{
// your code here…
}
Will never execute this code!
while ( 1)
{
// your code here…
}
Will execute this code for ever!
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Writing a Main Loop

It is time to add a few new lines of code to the
‘loop.c’ source file and put the while loop to
good use.
main()
{
// init control registers
TRISA = 0xff00;
// all PORTA as output
// main application loop
while( 1)
{
PORTA = 0xff;
PORTA = 0;
} // main loop
} // main
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Not so fast, please!

Run > Run Program from the main menu.

MPLAB X will now recompile the program for immediate
execution. It will be downloaded into the PIC24 flash memory
and execution will start immediately

NOTE: When in run mode, there is no animated icon and
MPLAB X seems inactive, but the target PIC24 is alive
and executing continuously the application at full speed

Warning: You will not be able to see any flashing of
the LED bar!


This is due to a limitation of our human eyes.
The PIC24 is actually turning the LEDs on and off but, assuming
our default configuration of the main oscillator (32MHz), this is
happening at the rate of several million times per second!
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Using a Timer

Timer1 Block Diagram:
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Configuring Timer1: T1CON

Use the following basic configuration:

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


Once we assemble all the bits into a single 16-bit binary value, we get:


Activate Timer1: TON = 1
Use the MCU clock as the source (Fosc/2): TCS = 0
Set the prescaler to the maximum value (1:256): TCKPS = 11
The input gating and synchronization functions are not required, since we use the
MCU internal clock directly as the timer clock: TGATE = 0, TSYNC = 0
Do not worry about the behavior in IDLE mode, for now: TSIDL = 0 (default)
T1CON = 0b1000000000110000;
Or in a more compact hexadecimal notation:

T1CON = 0x8030;
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
A Timed Loop
#include <config.h>
#define DELAY
16000
main()
{
// init control registers
TRISA = 0xff00; // all PORTA as output
T1CON = 0x8030; // TMR1 on, prescale 1:256 Tclk/2
// main application loop
while( 1)
{
//1. turn pin 0-7 on and wait for 1/4 of a second
PORTA = 0xff;
TMR1 = 0;
while ( TMR1 < DELAY)
{
}
// 2. turn all pin off and wait for a 1/4 of a second
PORTA = 0;
TMR1 = 0;
while ( TMR1 < DELAY)
{
}
} // main loop
} // main
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Notes for Assembly Experts

Logic vs. Binary operators in C:



Binary logic operators take pairs of bits from each operand and
compute the result according to the defined table of truth.
Logic operators, look at each operand (independently of the
number of bits used) as a single Boolean value.
Example on byte sized operands:
binary OR
gives
11110101
00001000
-------11111101
11110101 (true)
logical OR 00001000 (true)
-------gives 00000001 (true)
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Notes for PICmicro Experts

Difference between 8-bit PICmicros and the PIC24:
 There is no Timer0
 All timers are 16-bit wide.
 Each timer has a 16-bit period registers (PR).
 A new 32-bit mode timer-pairing mechanism is
available for Timer2/3 and Timer4/5.
 A new external clock gating feature has been added
on Timer1.
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Tips and Tricks

When designing applications that have to
operate reliably on large time scales (months,
years… ), consider providing a periodic refresh
of the most important control registers of the
essential peripherals used by the application.


Group the sequence of initialization instructions in one or
more functions.
Call the functions once at power up, before entering the
main loop, but also make sure that inside the main loop the
initialization functions are called when no other critical task
is pending and every control register is re-initialized
periodically.
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Suggested Excercises


Output a counter on the PortA pins instead of the
alternating on and off patterns.
Use a rotating pattern instead of alternating on and
off
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Recommended Readings

Ullman, L. & Liyanage, M. (2005), C Programming, Peachpit
Press, Berkeley, CA.

Adams, N. (2003), The flyers, in search of Wilbur and Orville
Wright, Three Rivers Press, New York, NY
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)
Online Resources


http://en.wikipedia.org/wiki/Control_flow#Loops
A wide perspective on programming languages and
the problems related to coding and taming loops.
Di Jasio – Programming 16-bit Microcontrollers in C (Second Edition)