March 2014

6

2 Embedded Systems Lab

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:

ADC

3 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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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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• 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 2-

bits 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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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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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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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.

10 Embedded Systems Lab

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.

Lab Work 1

11 Embedded Systems Lab

Proteus

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.

- Variable resistor:

POT-LIN (or POT-HG)

- As its value changes,

voltage changes

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You can use voltmeter device to check your work:

Proteus

1. Repeat the previous part but with 2-analog inputs.

2. Simulate the circuit using Proteus ISIS program.

Lab Work 2

13 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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Proteus

In order to change from voltage to temperature:

Voltage degree

10m 1

X ? => degree =x/10m = x*100

1. Interface LM35 Temperature Sensor with PIC

Microcontroller. (LM35 at proteus)

2. Simulate the circuit using Proteus ISIS program.

Lab Work 3

15 Embedded Systems Lab

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 extra-

searching for

them by yourself

16 Embedded Systems Lab

… Good Luck …

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).

Homework