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Railway Foundation

Railway Foundation. Electronic, Electrical and Processor Engineering . Typical microprocessor system. Check if value exceeds limits Multiply by a scaling factor Check for a certain combination of inputs. Measure something or check an input . Display output switch something on/off.

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Railway Foundation

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  1. Railway Foundation Electronic, Electrical and Processor Engineering

  2. Typical microprocessor system • Check if value exceeds limits • Multiply by a scaling factor • Check for a certain combination of inputs Measure something or check an input Display output switch something on/off • Temperate sensors • Force sensors • Position switch • Control dial • Display on LCD • Switch on warning buzzer • Start/Stop a motor

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

  4. 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)

  5. 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)

  6. 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.

  7. Examples Rail - Points Heating Control Systems

  8. Microprocessor +DAQ • Data AcQuisition module (DAQ) • Signal • Terminal Block • Cable • DAQ Device • Computer 7

  9. DAQ Hardware – DAQ Device • Most DAQ devices have four standard elements: analog input, analog output, digital I/O, and counters • You can transfer the signal you measure with the DAQ device to the computer through a variety of different bus structures

  10. DAQ Hardware – Analog Input The process of measuring an analog signal and transferring the measurement to a computer for analysis, display, or storage • An analog signal is a signal that varies continuously • Analog input most commonly measures voltage or current

  11. DAQ Hardware – Analog Output The process of generating analog signals from your computer • Performing digital-to-analog (D/A) conversions generates analog output • The available analog output types are voltage and current • To perform a voltage or current output, a compatible device must be installed that can generate that type of signal

  12. DAQ Hardware – Digital I/O • Digital signals: • Electrical signals that transfer digital data (on/off, high/low, 1/0) using a wire • Used to control or measure digital or finite state devices, such as switches and LEDs • Used to transfer data • program devices • communicate between devices • Use digital signals as clocks or triggers to control or synchronize other measurements

  13. DAQ Hardware – Counters • A counter is a digital timing device typically used for event counting, frequency measurement, period measurement, position measurement, and pulse generation • A counter has a fixed number it can count to as determined by the resolution of the counter • For example, a 24-bit counter can count to: 2(Counter Resolution) – 1 = 224 – 1 = 16,777,215

  14. 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

  15. 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.

  16. 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

  17. 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.)

  18. Simple ‘C’ outline Loops while (comparison is true ) { KEEP Doing this; } Selection - two types if (comparison is true ) { Do this once; } if (comparison is true) { do this; } else { do that; } 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

  19. 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.

  20. 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.

  21. 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 n bits ADC Analogue input voltage Reference voltages (one is usually analogue ground)

  22. 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

  23. Remember max digital value = 255. Max convertible input = 255 × resolution in volts = 255 × 0.01953125 = 4.980468755 255 Input voltage 0 4.98046875 Volts

  24. Digital Conversion Digital value 3 0.05859375 2 0.0390625 1 0.01953125 Analogue input voltage 0

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