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Using ADC Module of PIC Microcontroller – Hi Tech C

Ligo George

Contents

Analog to Digital Converter (ADC) is a device that converts an analog quantity (continuous voltage) to discrete digital values. This is very useful when we want to do some processing on physical quantities, which are normally analog in nature. Most of the PIC Microcontrollers have built in ADC Module. Here we are using PIC 16F877A for demonstrating the working.

ADC_Symbol

ADC of PIC Microcontrollers have 5 inputs for 28 pin devices and 8 inputs for 40/44 pin devices. It is a 10-bit ADC, ie the conversion of analog signal results in corresponding 10-bit digital number. The positive and negative reference voltage (+Vref and -Vref) of PIC ADC is software selectable, which can be VDD, VSS, voltage at RA2 or RA3. This A/D Converter module can also operate in sleep mode in which clock is derived from its internal RC oscillator. Following points may help you to understand the concept of reference voltages.

We require some hardware knowledge to program PIC ADC in Hi-Tech C. If you don’t need hardware knowledge please skip this part and go to Circuit Diagram.

ADC Module in Detail

Registers

PIC ADC has 4 registers :

 ADCON0 Register

ADCON0 Register – PIC 16F877A

ADCON1 Register

ADCON1 Register – PIC 16F877A
PIC ADC Port Configuration Bits

Note : TRIS Registers of Analog inputs must be configured as input for the proper operation.

ADC Block Diagram

ADC Module Block Diagram – PIC16F877A

Selection of A/D Conversion Clock

The time for A/D Conversion per bit is defined as TAD and it requires minimum 12TAD to generate the 10-bit result. The time TAD is determined by the A/D Conversion Clock which is software selectable to following options.

TOSC is the time period of the device clock usually provided by the crystal oscillator.
Care should be taken while selecting the A/D Conversion Clock, such that the clock should provide the minimum TAD (1.6μS) required for the correct A/D Conversion. So refer the following table before setting the ADC clock.

ADC Clock Selection Table – PIC 16F877A

Circuit Diagram

Using Internal ADC Module of PIC Microcontroller

I hope that you already go through our first tutorials, so I am not going to explain deeply about the circuit. The reference voltages of ADC is set to VDD (5V) and VSS (GND) though software (code given below). Analog input is given to Channel 0 using a potentiometer, thus we can vary the ADC input voltage from VSS to VDD. The 10-bit result of ADC is displayed using 10 LEDs, and 470Ω resistors are used to limit current through them.

Hi-Tech C Code

#include<htc.h>
#include<pic.h>

#define _XTAL_FREQ 8000000

void ADC_Init()
{
  ADCON0 = 0x41; //ADC Module Turned ON and Clock is selected
  ADCON1 = 0xC0; //All pins as Analog Input
                 //With reference voltages VDD and VSS
}

unsigned int ADC_Read(unsigned char channel)
{
  if(channel > 7) //If Invalid channel selected 
    return 0;     //Return 0

  ADCON0 &= 0xC5; //Clearing the Channel Selection Bits
  ADCON0 |= channel<<3; //Setting the required Bits
  __delay_ms(2); //Acquisition time to charge hold capacitor
  GO_nDONE = 1; //Initializes A/D Conversion
  while(GO_nDONE); //Wait for A/D Conversion to complete
  return ((ADRESH<<8)+ADRESL); //Returns Result
}

void main()
{
  unsigned int a;
  TRISB = 0x00; //PORTB as output
  TRISC = 0x00; //PORTC as output
  TRISA = 0xFF; //PORTA as input
  ADC_Init(); //Initializes ADC Module

  do
  {
    a = ADC_Read(0); //Reading Analog Channel 0
    PORTB = a; //Lower 8 bits to PORTB
    PORTC = a>>8; //Higher 2 bits to PORTC
    __delay_ms(100); //Delay
  }while(1); //Infinite Loop
}

I think the program is completely explained through its comments. If you have any doubts please comment below.

You can download Hi-Tech C and Proteus files here…

Using Internal ADC PIC Hi-Tech C

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