Railway Foundation

Electronic, Electrical and Processor Engineering

2

Microprocessor Systems

• Four main components– Microprocessor– Memory– Inputs– Outputs

• Memory – ROM types – program and fixed data– RAM (Read & Write) – Data variables

3

Microprocessor

• Circuit is driven by a clock signal

• The microprocessor has internal registers.

• The action performed is determined by a set of binary instructions stored in ROM

• A reset starts the microprocessor at a predetermined point in the program (usually location 0)

4

Inputs & Outputs

• Normally Digital I/O ( two levels ‘0’ & ‘1’)• Normally parallel i.e grouped – 8bit PortsWhat about analogue signals?

– Analogue to Digital Converter (ADC)– Digital to Analogue Converter (DAC)

• Other devices include hardware timers and counters

• Digital data can also be in a serial format (e.g. RS232, RS 485 are serial standards)

5

Microcontrollers

• Integration of all required components onto one chip.

• Many manufacturers – Microchip, Freescale, Intel, Infineon, Philips, ARM etc. producing different microprocessors

• Many microcontrollers with same microprocessor but differ in other components.

• Used in embedded products.

Examples

6

Rail - Points Heating Control Systems

7

Programming• Can be done at different levels

– Object (machine) binary code– Assembly language– High level language ( e.g. ‘C’ language)– Graphical (e.g. LabVIEW)

• Internal architecture • Memory Map• Programmers Model – different for

programming at different levels

8

EXAMPLES

Machine code –

0110111100001000 means move the value from W reg. to file register 00001000 i.e 8

Assembly –

Count EQU 8

MOVWF Count

A program called an assembler converts it to the binary object code.

An example machine instruction

9

00050 ;Constants 00051 LED equ 3 ;LED bit 3 on PORTB

00053 ;Reset vector 00054 ; This code will start executing when a reset occurs. 00055 000000 00056 ORG 0x0000 00057

00058 ;Start of main program000000 00059 Start:000000 9693 00060 bcf TRISB,LED ;Set PortB bit 3 as an o/p000002 9681 00061 bcf PORTB,LED ;set LED off000004 00062 Loop: ;while(1)000004 8681 00063 bsf PORTB,LED ; turn led on 000006 9681 00064 bcf PORTB,LED ; turn led off000008 D7FD 00065 bra Loop ;endwhile

CommentsBegin with ;Assembly

instructions

Program Line Numbers

Object code in HEX format

ROM location

Labels

An Assembler program

10

C Programming

• Portable • High level – Abstract• Standard constructs

– Variables – various data types– Selection ( if statements)– Loops (while, for, do)– Standard operations (+-*/)– Logical and bit-wise operations (AND OR

XOR etc.)

11

Simple ‘C’ outlineSelection - two typesif (comparison is true ){

Do this once;}

if (comparison is true){

do this;}else {

do that;}

Loops

while (comparison is true )

{

KEEP Doing this;

}

Comparisons:-

== is equal to

!= is not equal to

> is greater than

< is less than

>= is greater or equal to

<= is less than or equal to

Defining variables

unsigned char i; //8 bit value

int x; // 16 bit signed

unsigned int y; // 16 bit value

Misc.

// starts a comment

i++; // increment by one

i--; // decrement by one

&& // logical AND

|| // logical OR

Graphical Programming• LabVIEW is a graphical programming language that

uses icons instead of lines of text to create applications.• In contrast to text-based programming languages,

where instructions determine program execution, LabVIEW uses dataflow programming, where data determine execution.

12

13

Practical approach

• Treat as a programmable digital device• Choose device based on number and types

of input and outputs• Write program:-

– Define inputs and outputs– Read input data, process data and generate

outputs

• Requires knowledge of a programming language and microcontroller specific features.

14

Analogue to Digital Converter

• n bits – determines the resolution

• Reference voltage sets the input range

• often have an analogue multiplexer to allow several input channels to use a single ADC

ADC

n bits

Analogue input voltage

Reference voltages (one is usually analogue ground)

15

Example 8 bit ADC

• Reference voltage of 0v and +5v• input voltage range = 5v - 0v = 5V• Number of digital values (steps) = 2n=28=256• Note! values range from 0 to 255• Resolution = Reference voltage range = 5

Number of digital values 256

=0.01953125 = 19.53125mV• This is the smallest change in voltage that can be

detected

160

255

4.98046875 Volts

Remember max digital value = 255.

Max convertible input = 255 × resolution in volts = 255 × 0.01953125

= 4.980468755

Input voltage

17

Digital Conversion

0

1

2

3

Digital value

Analogue input voltage

0.01953125

0.0390625

0.05859375