Analog to Digital Converter Interfacing with Microcontrollers

►ADC interfacing with Microcontrollers Tutorial Index

Introduction to ADCIntroduction to Analog to digital convertorsconversion methods and commonly used ADCs.

ADC0804 InterfacingComplete information of interfacing 8-bitADC0804 with you microcontrollers.

Programming AVR for ADC0804Programming example in both C as well as assemblyto program AVR microcontroller.

Programming 8051 for ADC0804Programming example in both C as well as assemblyto program 8051 microcontroller.

ADC interfacing with Microcontrollers: Introduction

►Introduction

In our daily life, anything we deal like sound, prassure, voltage or any measurable quantity, are usually in analog form So what if we want to interface any analog sensor with our digital controllers? There must be something that translate the analog inputs to digital output, and so Analog to digital convertors come to play. Usually we call them ADC (Analog to digital convertor). Before going to learn how to interface an ADC with a controller we first take a look at basic methods of analog to digital conversion.

This is a sample of the large number of analog-to-digital conversion methods. The basic principle of operation is to use the comparator principle to determine whether or not to turn on a particular bit of the binary number output. It is typical for an ADC to use a digital-to-analog converter (DAC) to determine one of the inputs to the comparator.

Following are the most used converion methods:

Digital-Ramp ADC Successive Approximation ADC Flash ADC

►Digital-Ramp ADC

Conversion from analog to digital form inherently involves comparator action where the value of the analog voltage at some point in time is compared with some standard. A common way to do that is to apply the analog voltage to one terminal of a comparator and trigger a binary counter which drives a DAC. The output of the DAC is applied to the other terminal of the comparator. Since the output of the DAC is increasing with the counter, it will trigger the comparator at some point when its voltage exceeds the analog input. The transition of the comparator stops the binary counter, which at that point holds the digital value corresponding to the analog voltage.

►Successive Approximation ADC

Illustration of 4-bit SAC with 1 volt step size

The successive approximation ADC is much faster than the digital ramp ADC because it uses digital logic to converge on the value closest to the input voltage. A comparator and a DAC are used in the process. A flowchart explaning the working is shown in the figure below.

►Flash ADC

Illustrated is a 3-bit flash ADC with resolution 1 volt (after Tocci). The resistor net and comparators provide an input to the combinational logic circuit, so the conversion time is just the propagation delay through the network - it is not limited by the clock rate or some convergence sequence. It is the fastest type of ADC available, but requires a comparator for each value of output (63 for 6-bit, 255 for 8-bit, etc.) Such ADCs are available in IC form up to 8-bit and 10-bit flash ADCs (1023 comparators) are planned. The encoder logic executes a truth table to convert the ladder of inputs to the binary number output.

Now we lets take a look at the various Analog to Digital convertors that are most commonly used with our controllers

Name DescriptionADC0800 8-bit ADCADC0801 8-bit ADC 100us 0.25 LSBADC0802 8-bit ADC 100us 0.5 LSBADC0804 8-bit ADC 100us 1.0 LSBADC0808 8-bit 8 channel 100us ADCADC0809 8-Bit 8 channel ADC (=~ADC0808)

AD57110-Bit, A/D Converter, Complete with Reference and Clock

MAX12045V, 8-Channel, Serial, 10-Bit ADC with 3V Digital Interface

MAX12025V, 8-Channel, Serial, 12-Bit ADCs with 3V Digital Interface

MAX19516-Bit, Self-Calibrating, 10us Sampling ADC

More information on how to interface the above listed ADC can be obtained from the datasheets of respective ICs. In the next part of tutorial we will look into the interfacing and programming of a simple 8-bit ADC (ADC0804).

ADC interfacing with Microcontrollers: Interfacing ADC0804

►ADC0804 Pinout and Typical Connections

As shown in the typical circuit, ADC0804 can be interfaced with any microcontroller. You need a minimum of 11 pins to interface ADC0804, eight for data pins and 3 for control pins. As shown in the typical circuit the chip select pin can be made low if you are not using the microcontroller port for any other peripheral (multiplexing).

There is a universal rule to find out how to use an IC. All you need is the datasheet of the IC you are working with and take a look at the timing diagram of the IC which

shows how to send the data, which signal to assert and at what time the signal should be made high or low etc.

Note: Keep this in mind that whenever you are working with an IC and you want to know how to communicate with that IC, then simply look into the timing diagram of that IC from its datasheet. It gives you complete information that you need regarding the communication of IC.

The above timing diagrams are from ADC0804 datasheet.

The first diagram (FIGURE 10A) shows how to start a conversion. Also you can see which signals are to be asserted and at what time to start a conversion. So looking into the timing diagram FIGURE 10A. We note down the steps or say the order in which signals are to be asserted to start a conversion of ADC. As we have decided to make Chip select pin as low so we need not to bother about the CS signal in the timing diagram. Below steps are for starting an ADC conversion. I am also including CS signal to give you a clear picture. While programming we will not use this signal.

1. Make chip select (CS) signal low.2. Make write (WR) signal low.3. Make chip select (CS) high.4. Wait for INTR pin to go low (means conversion

ends).

Once the conversion in ADC is done, the data is available in the output latch of the ADC. Looking at the FIGURE 10B which shows the timing diagram of how to read the converted value from the output latch of the ADC. Data of the new conversion is only avalable for reading after ADC0804 made INTR pin low or say when the conversion is over. Below are the stepts to read output from the ADC0804.

1. Make chip select (CS) pin low.2. Make read (RD) signal low.3. Read the data from port where ADC is connected.4. Make read (RD) signal high.5. Make chip select (CS) high.

In the next section of this tutorial we will follow the above mentioned steps to program the ADC.

ADC interfacing with Microcontrollers: Programming for ADC0804

►Programming 8051 Microcontroller

►8051 Assembly Programming for ADC0804

CODE:        rd equ P1.0           ;Read signal P1.0        wr equ P1.1           ;Write signal P1.1        cs equ P1.2           ;Chip Select P1.2        intr equ P1.3         ;INTR signal P1.3

        adc_port equ P2       ;ADC data pins P2        adc_val equ 30H       ;ADC read value stored here

        org 0Hstart:                    ;Start of Program        acall conv            ;Start ADC conversion        acall read            ;Read

converted value        mov P3,adc_val        ;Move the value to Port 3        sjmp start            ;Do it again

conv:                     ;Start of Conversion        clr cs                ;Make CS low        clr wr                ;Make WR Low        nop        setb wr               ;Make WR High        setb cs               ;Make CS highwait:        jb intr,wait          ;Wait for INTR signal        ret                   ;Conversion done

read:                     ;Read ADC value        clr cs                ;Make CS Low        clr rd                ;Make RD

Low        mov a,adc_port        ;Read the converted value        mov adc_val,a         ;Store it in local variable        setb rd               ;Make RD High        setb cs               ;Make CS High        ret                   ;Reading done 

►Programming 8051 in C for ADC0804

CODE:#include <REGX51.H>#define adc_port P2 //ADC Port#define rd P1_0                  //Read signal P1.0#define wr P1_1                 //Write signal P1.1#define cs P1_2                  //Chip Select P1.2#define intr P1_3                //INTR signal P1.3

void conv();                     //Start of conversion functionvoid read();                     //Read ADC function

unsigned char adc_val;

void main(){        while(1){                   //Forever loop                conv();                 //Start conversion                read();                 //Read ADC                P3 = adc_val;          

//Send the read value to P3        }}

void conv(){        cs = 0;                     //Make CS low        wr = 0;                     //Make WR low        wr = 1;                     //Make WR high        cs = 1;                     //Make CS high        while(intr);                 //Wait for INTR to go low}

void read(){        cs = 0;                     //Make CS low        rd = 0;                     //Make RD low        adc_val = adc_port;         //Read ADC port        rd = 1;                     //Make RD high        cs = 1;                     //Make CS high

}