Chapter 2 A Loop in the Pattern

1
Chapter 2 A Loop in the Pattern

• Designing the Main Loop and Timing

2
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

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A New Project

• Use the New Project wizard to create a new
  project
• Call it 2-ALoopInThePattern
• Use the New File wizard to create a new source
  file
• Call it Loop.c
• / File Loop.c Author your name here
  Created current date here /
• include ltxc.hgt
• int main( void)
• return 0

But you can also use the form void main(
void) // no return required Or
simply main()
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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

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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
• a 17
• b 1
• ( a b) is true,
• ( a b) is true
• ( !a) is false

In other words, they are both true
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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
• the equal-to operator,
• ! the NOT-equal to operator.
• gt the greater-than operator.
• gt the greater-or-equal to operator.
• lt the less-than operator.
• lt the less-or-equal to operator.
• Here are some examples, assuming
• a 10
• ( a gt 1) is true
• (-a gt 0) is false
• ( a 17) is false
• ( a ! 3) is true

Composed of two equal signs to distinguish it
from the assignment operator we used in the
previous lesson.
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Curious Cases

• while ( 0)
• // your code here
• while ( 1)
• // your code here

Will never execute this code!
Will execute this code for ever!
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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

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Animation

• NOTE The Animation feature has been removed from
  MPLAB X!

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Not so fast, please!

• Run gt 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!

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Using a Timer

• Timer1 Block Diagram

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Configuring Timer1 T1CON

• Use the following basic configuration
• Activate Timer1 TON 1
• Use the MCU clock as the source (Fosc/2) TCS 0
  
• Set the prescaler to the maximum value (1256)
  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)
• Once we assemble all the bits into a single
  16-bit binary value, we get
• T1CON 0b1000000000110000
• Or in a more compact hexadecimal notation
• T1CON 0x8030

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A Timed Loop

• include ltconfig.hgt
• define DELAY 16000
•  
• main()
• // init control registers
• TRISA 0xff00 // all PORTA as output
• T1CON 0x8030 // TMR1 on, prescale 1256
  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 lt DELAY)
•  

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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
• 11110101 11110101
  (true)
• binary OR 00001000 logical OR 00001000
  (true)
• -------- --------
• gives 11111101 gives 00000001
  (true)

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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.

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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.

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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

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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

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Online Resources

• http//en.wikipedia.org/wiki/Control_flowLoops
• A wide perspective on programming languages and
  the problems related to coding and taming loops.