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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
ADC0804 Pinout and Typical Connections
As shown in the typica 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.
2.
3.
4.
Make chip select (CS) signal low.
Make write (WR) signal low.
Make chip select (CS) high.
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.
2.
3.
4.
5.
Make chip select (CS) pin low.
Make read (RD) signal low.
Read the data from port where ADC is connected.
Make read (RD) signal high.
Make chip select (CS) high.
Programming 8051 Microcontroller
8051 Assembly Programming for ADC0804
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_val equ 30H
;ADC data pins P2
;ADC read value stored here
org 0H
start:
;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 high
wait:
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
#include <REG51.H>
#define adc_port P2
sbit rd =P1^0;
sbit wr = P1^1 ;
sbit cs = P1^2;
sbit intr = P1^3;
//ADC Port
//Read signal P1.0
//Write signal P1.1
//Chip Select P1.2
//INTR signal P1.3
void conv();
void read();
//Start of conversion function
//Read ADC function
unsigned char adc_val;
void main(){
while(1){
conv();
read();
P3 = adc_val;
}
}
void conv(){
cs = 0;
wr = 0;
wr = 1;
cs = 1;
while(intr);
}
//Forever loop
//Start conversion
//Read ADC
//Send the read value to P3
//Make CS low
//Make WR low
//Make WR high
//Make CS high
//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
}