12F675 comparator test circuts and code.

The PIC comparator is the least used peripheral but it is just as fast as the standard LM311...

Note: This information is  applicable to any PIC Microcontroller that has a built in comparator.

There is one comparator in the 12F675 which you can set up in many different ways and you can even supply a comparison voltage from an internal reference. This voltage can be set to any of 32 steps.

This is just a simple project designed to show simple operation so that you can use it.  It shows all you need to do to operate the comparator.

Response time

The comparator is high speed with a specification of between 150ns to 400ns (150ns typ , 400ns max).  

The typical response time is better than the standard response time of an LM311 which is 200ns - but you may get up to 400ns for the PIC comparator - thats still very good.

Note This is one of the strengths of the comparator module - the fact that it operates independently from the processor and has an extremely fast response time.

One "Gotcha"

One piece of information that is only implied by the documentation is that the comparator inputs must be set to analogue inputs - otherwise the comparator does nothing!


So you need to control ANSEL.

Links

Circuit.

Download.

Code.

Comparator Modes.

Registers associated with the comparator.

CMCON Comparator control register.

VRCON Comparator voltage reference control.


Comparator Modes

The following table shows the eight different ways  of configuring the comparator:


12F675 comparator modes

Registers

These are the registers for controlling the Comparator and Voltage reference:

The main controls are:

  • CMCON for the Comparator and
  • VRCON for the Voltage reference.

...the rest are for interrupt control and I/O Direction (don't forget ANSEL not in table!)

12F675 comparator and voltage reference


CMCON:

12F675 Comparator control CMCON

VRCON:

12F675 Comparator voltage reference VRCON

Simple Circuit to test the comparator

12F675 comparator test circuit

Download

Download code here : Click to download.

C Code for the comparator

The following code sets up the comparator as a fully external device i.e. the internal voltage reference is not used:

///////////////////////////////////////////////////////
//
// File: 16F675_comparator.c
//
// Author: J F Main.
//
// Description:
//
//   Use comparator GP0,GP1,GP2
//
// Compiler : mikroC, mikroElektronika C compiler
//            for Microchip PIC microcontrollers
//            Version: 6.2.0.0
//
// Note Testing:
//
//   Tested on 12F675
//
// Requirements:
//
//   Target : 12F675
//
// Notes :
//
//   Uses internal oscillator.
//
// Version:
// 1.00 Initial release.
//
// Copyright : Copyright © John Main
//   https://www.best-microcontroller-projects.com
//   Free for non commercial use as long as this entire
//   copyright notices included in source code
//   and any other documentation.
//
///////////////////////////////////////////////////////

///////////////////////////////////////////////////////
// Definitions
#define COMPARATOR_OP 2
#define LED 4

///////////////////////////////////////////////////////
void init_ports(void) {
   //GP0 & GP1 are inputs
   TRISIO = 0 | (1<<GP0) | (1<<GP1); // 0 - op, 1 - ip

   ANSEL = (1<<GP0) | (1<<GP1); ; // Ana. ip on GP0 GP1
}

///////////////////////////////////////////////////////
void init_comparator(void) {
   // Comparator with external input and output.
   // Cout = 0 (comparator output), Cinv =0 (inversion)
   CMCON = 0x01;
}

///////////////////////////////////////////////////////
// Start here
void main() {
int i;

   init_ports();

   // Show device is active on power up.
   for (i=0;i<5;i++) {

      GPIO |= (1<<COMPARATOR_OP);
      delay_ms(100);

      GPIO &= ~(1<<COMPARATOR_OP);
      delay_ms(100);
   }

   init_comparator();

   while(1) {;
      GPIO |= (1<<LED);
      delay_ms(100);

      GPIO &= ~(1<<LED);
      delay_ms(100);
   }
}
Note: To run the above code you may need to remove the ICSP connections after programming as the comparator pins are on the PGD and PGC lines.

The code simply flashes the comparator output LED 5 times at start up and after this the comparator is turned on.  Then the second LED is flashed continuously.

By changing the POT setting you can see the comparator output turn on and off.



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