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Railway Foundation

Railway Foundation. Electronic, Electrical and Processor Engineering . Microprocessor Systems. Four main components Microprocessor Memory Inputs Outputs Memory ROM types – program and fixed data RAM (Read & Write) – Data variables. Microprocessor. Circuit is driven by a clock signal

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Railway Foundation

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  1. 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 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)

  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.

  6. Programming • Can be done different levels • Object (machine) binary code • Assembly language • High level language ( e.g. ‘C’ language) • Internal architecture • Memory Map • Programmers Model – different for programming at different levels

  7. An example machine instruction 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.

  8. An Assembler program Program Line Numbers 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 program 000000 00059 Start: 000000 9693 00060 bcf TRISB,LED ;Set PortB bit 3 as an o/p 000002 9681 00061 bcf PORTB,LED ;set LED off 000004 00062 Loop: ;while(1) 000004 8681 00063 bsf PORTB,LED ; turn led on 000006 9681 00064 bcf PORTB,LED ; turn led off 000008 D7FD 00065 bra Loop ;endwhile ROM location Comments Begin with ; Assembly instructions Object code in HEX format Labels

  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 Loops while (comparison is true ) { KEEP Doing this; }Example: while ( y < 10) Selection - two types if (comparison is true ){ Do this once;} if (comparison is true ){ Do this once;}else{ do this once;} Example: if (x == 9) 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 valueint x; // 16 bit signedunsigned int y; // 16 bit value Misc. // starts a commenti++; // increment by onei--; // decrement by one&& // logical AND|| // logical OR Assignment(=) and operators(+-*/) x = 9;i = 7;y = x/4 + (5*i); //integer division!

  11. 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 ‘C’ language and knowledge of microcontroller specific features.

  12. Microchip PIC 18F452 & C18 ‘C’ Compiler • Access to Special Function Registers by #include <p18f452.h> • Defines names for all component parts. • Example : PORTB an 8 bit digital I/O Port • TRISB - Port B direction register (0 = output, 1 = input) • PORTB - all 8 bits • PORTBbits.RBx - individual bits where x is bit number (numbering starts at 0)

  13. What does this do? //PortA is 7 bits TRISA = 15; TRISB = 0; PORTBbits.RB0 = 0; while(1) { while( PORTAbits.RA4 == 0) { } while( PORTAbits.RA4 == 1) { } PORTBbits.RB0 = 1; while( PORTAbits.RA4 == 0) { } while( PORTAbits.RA4 == 1) { } PORTBbits.RB0 = 0; }

  14. n bits Analogue voltage ADC Reference voltages (one is usually analogue ground) Analogue to Digital Converter(ADC) • n bits – determines resolution • Reference voltage sets the input range • Often have an analogue multiplexer to allow several input channels to a single ADC

  15. Example: 8 bit ADC • Reference voltages 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 / 256 = 0.01953125 volts = 19.53125mV Number of digital values

  16. Remember max. digital value = 255. Max. convertible input = 255 x resolution in volts= 255 x 0.019531250.01953125 = 4.98046875 V Digital value 255 Input voltage 0 4.98046875Volts 0

  17. Digital Conversion Digital value 3 2 0.05859375 1 0.0390625 Analogue Input voltage 0 0.01953125

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