8051 – ADC0808 Interfacing

Last Updated on: June 29th, 2022

We had already seen ADC0804 Interfacing in our previous tutorial. So this Tutorial is all about ADC0808 Interfacing with 8051.

Suggestion to read

• 8051 – LCD 4-Bit Interfacing
• 8051 – ADC0804 Interfacing

Introduction

In embedded systems microcontrollers needs to take analog input. Most of the sensors & transducers such as temperature, humidity, pressure, are analog. For interfacing these sensors to microcontrollers we require to convert the analog output of these sensors to digital so that the controller can read it. Some microcontrollers have built-in Analog to Digital Convertor (ADC) so there is no need for external ADC. For microcontrollers that don’t have internal ADC external ADC is used.

Why we are using ADC for 8051 Microcontroller?

Well. 8051 doesn’t have an inbuilt ADC. So we have to use external ADC. There are many ADCs available.

• ADC 0804

• ADC 0808

ADC0808

One of the most commonly used ADC is ADC0808. ADC 0808 is a Successive approximation type with 8 channels i.e. it can directly access 8 single-ended analog signals. The difference between ADC0804 and ADC0808 is the number of analog input signals. I mean, In ADC0804 we can give only one analog input. But in ADC0808 we can give 8 analog inputs. So if you want to interface more analog sensors, please select ADC0808.

ADC0808 is an 8 bit analog to digital converter with eight input analog channels, i.e., it can take eight different analog inputs. The input which is to be converted to digital form can be selected by using three address lines. The voltage reference can be set using the Vref+ and Vref- pins. The step size is decided based on the set reference value. Step size is the change in analog input to cause a unit change in the output of ADC. The default step size is 19.53mV corresponding to 5V reference voltage. ADC0808 needs an external clock to operate, unlike ADC0804 which has an internal clock. The ADC needs some specific control signals for its operations like start conversion and brings data to output pins. When the conversion is complete the EOC pins go low to indicate the end of conversion and data ready to be picked up.

Features

• Easy interface to all microprocessors
• Operates ratio metrically or with 5 V DC or analog span adjusted voltage reference
• No zero or full-scale adjust required
• 8-channel multiplexer with address logic
• 0V to 5V input range with single 5V power supply
• Outputs meet TTL voltage level specifications
• Standard hermetic or molded 28-pin DIP package
• 28-pin molded chip carrier package

Pin Diagram

Pin Description

Pin No	Function	Name
1	Analog Input Pin 3	IN3
2	Analog Input Pin 4	IN4
3	Analog Input Pin 5	IN5
4	Analog Input Pin 6	IN6
5	Analog Input Pin 7	IN7
6	Start conversion; input pin; a low to high pulse is given	START
7	End of conversion; output pin; goes low when the conversion is over	EOC
8	Digital output bit	D3
9	Input pin; a low to high pulse brings data to output pins from the internal registers at end of conversion	Output Enable
10	Clock input; to provide external clock	Clock Input
11	Supply voltage; 5V	Vcc
12	Positive reference voltage	Vref+
13	Ground (0v)	GND
14	Digital output bit	D1
15	Digital output bit	D2
16	Negative reference voltage	Vref-
17	Digital output bit	D0
18	Digital output bit	D4
19	Digital output bit	D5
20	Digital output bit	D6
21	Digital output bit	D7
22	Address latch enable; Input pin; low to high pulse is required to latch in the address	ALE
23	Address line C	Address C
24	Address line B	Address B
25	Address line A	Address A
26	Analog Input Pin 0	IN0
27	Analog Input Pin 1	IN1
28	Analog Input Pin 2	IN2

Selection of Channel

We can select any input channel by using the Address lines ADD A, ADD B and ADD C). We can select the input line IN0 by keeping all three address lines (ADD A, ADD B, and ADD C) Low. If we want to select input channel IN4 then we need to keep ADD A, ADD B low and ADD C high. For selecting all the other input channels, have a look on the given table:

ADC Channel Name	ADD C	ADD B	ADD A
IN0	LOW	LOW	LOW
IN1	LOW	LOW	HIGH
IN2	LOW	HIGH	LOW
IN3	LOW	HIGH	HIGH
IN4	HIGH	LOW	LOW
IN5	HIGH	LOW	HIGH
IN6	HIGH	HIGH	LOW
IN7	HIGH	HIGH	HIGH

Calculating Step Size

ADC 0808 is an 8 bit ADC i.e. it divides the voltage applied at Vref+ & Vref- into 28 i.e. 256 steps.

Step Size = (Vref+ - Vref-)/256

Suppose Vref+ is connected to Vcc i.e. 5V & Vref- is connected to the Gnd then the step size will be

Step size= (5 - 0)/256= 19.53 mv

How to use the ADC0808?

1. Start
2. Select the channel.
3. A Low – High transition on ALE to latch in the address.
4. A Low – High transition on Start to reset the ADC’s SAR.
5. A High – Low transition on ALE.
6. A High – Low transition on start to start the conversion.
7. Wait for the End of cycle (EOC) pin to become high.
8. Make Output Enable pin High.
9. Take Data from the ADC’s output
10. Make Output Enable pin Low.
11. Stop

ADC0808 interfacing with 8051

Circuit Diagram

In this tutorial, I’m going to connect the PotentioMeter (Variable Resistor) instead of the real sensor. You can connect the real sensor to Hardware.

LCD:

• RS  – P1.0
• RW  – GND
• EN  – P1.1
• LCD Data Lines – P1 LSB 4 bits

ADC:

• Output Enable    –  P2.0
• EOC                      –  P2.1
• Start                      –  P2.2
• Addr A                  –  P2.4
• Addr B                  –  P2.5
• Addr C                  –  P2.6
• ALE                       –  P2.7
• ADC Data lines   – Po

Code

You can download the entire project here.

#include<reg51.h>

#define adc_data P0
#define lcd    P1

unsigned char temp, gas;

sbit rs=P1^0;
sbit en=P1^1;

sbit OE     =P2^0;
sbit EOC    =P2^1;
sbit START  =P2^2;
sbit AA     =P2^4;
sbit BB     =P2^5;
sbit CC     =P2^6;
sbit ALE    =P2^7;

unsigned char adc_val;

void lcd_init();
void cmd(unsigned char a);
void dat(unsigned char b);
void show(unsigned char *s);
void lcd_delay();

unsigned char adc(unsigned int ch);
void ch_sel(unsigned int sel);

void  main()
{
    lcd_init();
    show("TEMP: ");
    cmd(0xc0);
    show("GAS : ");
    while(1) {
        cmd(0x86);
        temp=adc(0);                 //Reading Value from ADC Channel 0
        dat((temp/100)%10 +0x30);
        dat((temp/10)%10 +0x30);
        dat(temp%10 +0x30);
        
        cmd(0xc6);
        gas=adc(1);                 //Reading Value from ADC Channel 1
        dat((gas/100)%10 +0x30);
        dat((gas/10)%10 +0x30);
        dat(gas%10 +0x30);
    }
}

void lcd_init()
{
    cmd(0x02);
    cmd(0x28);
    cmd(0x0e);
    cmd(0x06);
    cmd(0x80);
}

void cmd(unsigned char a)
{
    rs=0; 
    lcd&=0x0F;
    P1|=(a&0xf0);
    en=1;
    lcd_delay();
    en=0;
    lcd_delay();
    lcd&=0x0f;
    lcd|=(a<<4&0xf0);
    en=1;
    lcd_delay();
    en=0;
    lcd_delay();
}

void dat(unsigned char b)
{
    rs=1; 
    lcd&=0x0F;
    lcd|=(b&0xf0);
    en=1;
    lcd_delay();
    en=0;
    lcd_delay();
    lcd&=0x0f;
    lcd|=(b<<4&0xf0);
    en=1;
    lcd_delay();
    en=0;
    lcd_delay();
}

void show(unsigned char *s)
{
    while(*s) {
        dat(*s++);
    }
}

void lcd_delay()
{
    unsigned int lcd_delay;
    for(lcd_delay=0;lcd_delay<=1000;lcd_delay++);
}

unsigned char adc(unsigned int ch)
{
    adc_data=0xff;

    ALE=START=OE=AA=BB=CC=0;
    EOC=1;

    ch_sel(ch);

    ALE=1;
    START=1;
    ALE=0;
    START=0;
    while(EOC==1);
    while(EOC==0);
    OE=1;
    adc_val=adc_data;
    OE=0;
    return adc_val;
}

void ch_sel(unsigned int sel)
{
    switch(sel) {
        case 0: CC=0;BB=0;AA=0; break;          //000
        case 1: CC=0;BB=0;AA=1; break;          //001
        case 2: CC=0;BB=1;AA=0; break;          //010
        case 3: CC=0;BB=1;AA=1; break;          //011
        case 4: CC=1;BB=0;AA=0; break;          //100
        case 5: CC=1;BB=0;AA=1; break;          //101
        case 6: CC=1;BB=1;AA=0; break;          //110
        case 7: CC=1;BB=1;AA=1; break;          //111
    }
}

Output – ADC0808 interfacing with 8051

[ Please find the output image Here ]

 

In our next tutorial, we will see an Ultrasonic sensor interfacing with the 8051.

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