PIC18F4520: Comparator

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Yet another function that the PIC18F4520 is capable of is that of an analog comparator. It is capable of running two separate analog comparators.

Contents

Available Pins

The PIC18F4520 allows A0-A3 (pins 2-5) to be used as analog comparator inputs, and A4-A5 (pins 6-7) to be used as comparator outputs. The red pins in the diagram below represent all pins available for use with an analog comparator. The grey pins are most commonly used for communication or power.

4520pindiagramcomp.jpg

Comparator Example

Sample Code

Program to implement an analog comparator.

First include header file with definitions for specific PIC. Set fuses. HS is type of external clock, low voltage programming (LVP) is off, and the watchdog timer (WDT) is off. External clock frequency of 20 MHz is specified.

  #include <18f4520.h>
  #fuses HS,NOWDT,NOPROTECT,NOLVP
  #use delay(clock=20000000)

Begin main body of program.

  void main(void) {

Setup the comparators C1 and C2. A0 is C1- and A3 is C1+. If C1+ > C1-, C1 will output high (C1OUT = 1 = 5V). If C1- > C1+ C1 will output low. A second comparator (C2) has been set up, but is not being utilized. See the header file 18F4520.h for all possible constants used in the setup_comparator() function.

     setup_comparator(A0_A3_A1_A2);

Setup an infinite loop using a while statement.

     while(TRUE) {

Check the output of the comparator and light an LED accordingly.

        if(C1OUT){
           output_low(PIN_D0);}
        else{
           output_high(PIN_D0);}
     }
  }

Circuit Diagram

Comparatorckt.jpg

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