Programming in C Using the A D converter module

This blog post describes the ADC part of PIC16F877 microcontroller and addresses the least CCS C inbuild function required to activate the ADC, to read the analog signal and display the converted digital signal on a LCD. This tutorial uses Proteus, MPLAB and CCS C compiler.

A PIC microcontroller has Analog to Digital converter or ADC module build inside in it. The function of this part is to convert the external circuitry analog signal into n=8, 10.. bits digital signal. For example PIC16F877 is a 10 bit converter while PIC16F73 is an 8 bit converter. The number of levels of quantization is n to the power of 2. For n=8 the level is 2^8=256 levels and for n=10 its 2^10=1024 levels. The voltage resolution is then Vref/L, where Vref is voltage reference and L is the level. If Vref=5V and L=256 then voltage resolution is 5V/256=0.0195V.

The functioning of ADC module inside the PIC16F877 microcontroller is shown below-

Setting up ADC and reading values using C-language

The process of reading analog and using the ADC module to convert to 8bit digital value requires to set up the ADC device properly. Few of the things that must be specified are the clock source for the ADC, port settings and which port will be used to read the analog signal. The C functions to do these are pointed out below-

• Specify the ADC clock for sampling( setup_adc(ADC_CLOCK_INTERNAL)
• Set up the ports type(setup_adc_ports(ALL_ANALOG)
• Specify which port is used(setup_adc_channel(0))

 Once the A/D port is configured properly the analog signals can be read. The command to read the value is read_adc( ). This value can be strored as an integer as follows-

• x = read_adc( )

Then the value x(digital value) can be manipulated and displayed onto the screen.

This value x is stored in the ADRESH and ADRESL registers which are 8 bit registers. The value x can be arranged into these two register from left and right. This selection is controlled by the bits value in the ADFM register.

The following code illustrates reading reading analog signal and displaying it on a LCD.

#include "16F877.h"
#device ADC=8
#use delay(clock=4M)
#use rs232(baud=9600, xmit=PIN_D0, rcv=PIN_D1)

void main()                      
    {
        int x;
        setup_adc(ADC_CLOCK_INTERNAL);
        setup_adc_ports(ALL_ANALOG);
        set_adc_channel(0);

        for(;;)
    {
            delay_ms(500);
            x = read_adc();
            x = (x/32)+0x30;

            putc(254);
            putc(1);
            delay_ms(10);
            printf("Level=");
            putc(x);
    }

}

The Proteus Simulation circuit is below-

 Scroll down and up the variable resistor and the corresponding level is displayed in the LCD display.

See How to write C-program to display character on LCD interactively tutorial and other microcontroller tutorials with proteus.

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