Railway Foundation

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Railway Foundation

• Electronic, Electrical and Processor Engineering

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Microprocessor Systems

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

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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)

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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)

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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.

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Examples
Rail - Points Heating Control Systems
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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

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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.
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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
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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.)

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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 gt is greater than lt is less than gt is greater
or equal to lt 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
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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.

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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.

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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)
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Example 8 bit ADC

• Reference voltage of 0v and 5v
• input voltage range 5v - 0v 5V
• Number of digital values (steps) 2n28256
• 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

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Remember max digital value 255. Max convertible
input 255 resolution in volts 255
0.01953125 4.980468755
255
Input voltage
0
4.98046875 Volts
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Digital Conversion
Digital value
3
0.05859375
2
0.0390625
1
0.01953125
Analogue input voltage
0