Railway Foundation Electronic, Electrical and Processor Engineering

1 Microprocessor Systems Four main components –Microprocessor –Memory –Inputs –Outputs Memory –ROM types – program and fixed data –RAM (Read & Write) – Data variables

2 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)

3 Inputs & Outputs Normally Digital I/O ( two levels ‘0’ & ‘1’) Normally parallel i.e grouped – 8bit Ports What 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)

4 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 5 Rail - Points Heating Control Systems

6 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

7 EXAMPLES Machine code – means move the value from W reg. to file register i.e 8 Assembly – Count EQU8 MOVWF Count A program called an assembler converts it to the binary object code. An example machine instruction

;Constants LED equ 3 ;LED bit 3 on PORTB ;Reset vector ; This code will start executing when a reset occurs ORG 0x ;Start of main program Start: bcf TRISB,LED ;Set PortB bit 3 as an o/p bcf PORTB,LED ;set LED off Loop: ;while(1) bsf PORTB,LED ; turn led on bcf PORTB,LED ; turn led off D7FD bra Loop ;endwhile Comments Begin with ; Assembly instructions Program Line Numbers Object code in HEX format ROM location Labels An Assembler program

9 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.)

10 Simple ‘C’ outline Selection - two types if (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. 11

12 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.

13 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)

14 Example 8 bit ADC Reference voltage of 0v and +5v input voltage range = 5v - 0v = 5V Number of digital values (steps) = 2 n =2 8 =256 Note! values range from 0 to 255 Resolution = Reference voltage range = 5 Number of digital values 256 = = mV This is the smallest change in voltage that can be detected

Volts Remember max digital value = 255. Max convertible input = 255 × resolution in volts = 255 × = Input voltage

16 Digital Conversion Digital value Analogue input voltage