March 2014

6
March 2014
ADC
Objectives
To be familiar with analog to digital converter module in PIC18F4550.
Introduction
Analog-to-digital conversion (ADC) is necessary because, while embedded
systems deal with digital values (as we have dealt with keypads and
switches). Analog signals such as, temperature, speed and pressure are
generated by peripheral devices such as microphones, analog cameras,
sensors, and etc. They all need to be converted into digital data before being
processed by the microcontroller. At this lab, we will see how to read an
external analog signal using a microcontroller, and display the conversion
output (a digital number) on a LCD. The input analog signals will be a varying
voltage between 0-5V.
Analog to digital conversion
Signals in the real world are analog: light, sound, etc. So, real-world signals
must be converted into digital, using a circuit called ADC (Analog-to-Digital
Converter), before they can be manipulated by digital equipment such as
microcontroller. Let's say you have a sound wave, and you wish to sample it
with an ADC. Here is a typical wave:
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Embedded Systems Lab
When you sample the wave with an analog-to-digital converter, you have
control over three variables:

The sampling rate: Controls how many samples are taken per second.

The sampling precision (resolution): Controls how many different
gradations (quantization levels) are possible when taking the sample.

The reference voltages The VREF + represents the maximum analog value
that can be converted by the Analog to Digital converter. The VREF represents the minimum analog value that can be converted by the
Analog to Digital converter.
You can see that as the sampling rate and precision (resolution) increase, the
similarity between the original wave and the ADC's output improves.
The figure below shows an analog signal and quantized versions for several
different number of quantization levels. With L levels, we need N=log2 L bits
to represent the different levels or conversely, with N bits we can represent L
= 2N levels.
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Embedded Systems Lab
PIC Internal Analog to Digital Converter
A number of microcontrollers have built in Analog to Digital Converter (ADC).
Commonly, these AD converters have 8-bit or 10-bit resolution allowing
them voltage sensitivity of 19.5mV or 4.8mV, respectively.
The ADC for PIC 18F4550 module has the following characteristics
• Number of analog channels
-
The Analog-to-Digital (A/D) Converter module has 13 inputs for
PIC18F4550 (AN0 – AN12) which corresponds to (RA0-RA5 & RE0-RE2
&RB0-RB4) as indicated on PIC18F4550 pins.
-
The analog channels can be configured as digital input/output pins by
configuring ADCON1.
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Embedded Systems Lab
• Resolution (precision)
-
The conversion of an analog input signal results in a corresponding 10-bit
digital number this means the conversion result has 1024 levels.
-
The lower 8 bits of the result are stored at ADRESL register; the upper 2bits are stored at ADRESH register.
• VRef+ / Vref- Configured by ADCON1 register such that
-
Vref + is determined by RA3 voltage level pin or by VDD
-
Vref- is determined by RA2 voltage level or by VSS
Pic18f4550 divides input to 1024 level (0-1023) with references VDD (5 V) and
VSS(0 V)
Level 0 => 0V, level 1023:5 V
If we have a level number between 0-1023, we can get the voltage by the
following equation:
Level
voltage
1023
5
X
?
=> voltage =x*5/1023 V
• The module has five registers:
-
- A/D Result High Register (ADRESH)
-
- A/D Result Low Register (ADRESL)
-
- A/D Control Register 0 (ADCON0)
-
- A/D Control Register 1 (ADCON1)
-
- A/D Control Register 2 (ADCON2)
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Embedded Systems Lab
The configuration of ADC module can be determined by configuring
ADCON0, ADCON1 and ADCON2 registers
ADCON0: (mainly Select A/D input channel)
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Embedded Systems Lab
ADCON1(Configure analog pins, voltage reference and digital I/O)
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A DCON2 (Select A/D conversion clock)
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Embedded Systems Lab
We will deal with adcon1
-
Bits 0-3: will be changed according to our needs of using analog inputs.
-
At the past, we have used to write adcon1=adcon1 or 0f => referring to the
table below => f (the value of 4 bits) means that all the inputs are digital
which was suitable with our work at that time.
Working with code
In order to deal with analog signals, we must read the signal and make its sampling
and quantization operations. ADC_READ function reads the channel signal and
returns its quantization level value.
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Embedded Systems Lab
Lab Work 1
1. Write a basic program for the simplified digital voltmeter
circuit for one analog input and show the results at the
LCD.
1. 2. Simulate the circuit using Proteus ISIS program.
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Embedded Systems Lab
Proteus
- Variable resistor:
POT-LIN (or POT-HG)
- As its value changes,
voltage changes
Mikrobasic
- Variable for quantization level as integer.
- Variable for voltage value after the equation.
- Variable to store values for view in LCD
For 1-analog input, from the table
Or adcon1 = 0x0E (00001110)
Read the signal at the first channel (AN0)
Function to change the type of float into
string to view it on LCD, take the float value
and the destination string value as
parameters.
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Embedded Systems Lab
 You can use voltmeter device to check your work:
Lab Work 2
1. Repeat the previous part but with 2-analog inputs.
2. Simulate the circuit using Proteus ISIS program.
Proteus
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Embedded Systems Lab
Mikrobasic
Interfacing LM35 Temperature Sensor with PIC Microcontroller
The are many cool sensors available now a days, ranging from IR distance sensor
modules, accelerometers, humidity sensors, temperature sensors and many
more(gas sensors, alcohol sensor, motion sensors, touch screens). Many of these
are analog in nature. That means they give a voltage output that varies directly
(and linearly) with the sensed quantity. For example in LM35 temperature sensor,
the output voltage is 10mV per degree centigrade. That means if output is 300mV
then the temperature is 30 degrees. At the following part we will learn how to
interface LM35 temperature sensor with PIC18F4550 microcontroller and display its
output on the LCD.
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Embedded Systems Lab
Lab Work 3
1. Interface
LM35
Temperature
Microcontroller. (LM35 at proteus)
Sensor
2. Simulate the circuit using Proteus ISIS program.
Proteus
In order to change from voltage to temperature:
Voltage
10m
X
14
degree
1
=> degree =x/10m = x*100
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Embedded Systems Lab
with
PIC
Mikrobasic
Light Dependent Resistor (LDR)
Used to measure Light Intensity, when the resistance lowers, the light intensity is
increased. LDR at proteus are:
-
LDR
-
Torch LDR
Do you’re extrasearching
for
them by yourself
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Embedded Systems Lab
Homework
1. Write a basic program for the simplified digital voltmeter
circuit for one 2- analog input (AN2, AN3) for any type of LDR
and variable resistor and show the results at the LCD.
2. Write a basic program that will turn on the led if the
temperature (analog input) is greater than 60 degrees.
Bonus: Implement a Light intensity measurement circuit using
LRD (output is not a voltage, but light measurement by Lux unit
of light).
… Good Luck …
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Embedded Systems Lab

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