EE1A2 Microprocessors Revision

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EE1A2 MicroprocessorsRevision

• Dr Sandra I. Woolley

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Contents

• Revision notes
• Revision and exam suggestions
• A note about writing code in the exam
• Examples

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EE1A2 Revision Notes

• What should you be expected to do in a typical
  exam question ?
• Understanding of basic principles.
• Ability to perform simple circuit analysis and/or
  design. Write or interpret samples of code.
• Apply skills to less familiar problems.

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EE1A2 Part A

• Number Systems
• Decimal, Binary, Hex.
• Binary Arithmetic
• Arithmetic Logic Units
• Addition circuits
• Subtraction circuits
• Flags
• ALUs
• Registers
• Using a Working Register with an ALU
• Tri-state ports Busses
• Microcontrollers
• Basic building blocks
• Special purpose registers and programs

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EE1A Part B

• An understanding of the basic principles of
  microprocessors and appreciation of the
  difference between microprocessors,
  microcontrollers and DSPs (digital signal
  processors).
• A working knowledge of the PIC16F84
  microcontroller.
• Experience of assembler programming and a basic
  appreciation of embedded systems.

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Revision Suggestions

• Read/work through course materials and examples.
• Look at past exam questions so that you are
  familiar with the style of questions.
• See on-line revision materials and the University
  exam paper database http//www.exampapers.bham.ac.
  uk.
• For this subject a keyword search may not work
  because of varied abbreviations for the long
  title. When using this database always be very
  careful that you are reading the right
  examination paper.
• Note in previous years a different number of
  questions were required.
• Time your attempts at questions.
• Work out your exam technique in advance. e.g.,
  allocate a set amount of time to read through all
  questions carefully.

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Exam Suggestions

• Take a calculator.
• Read questions carefully.
• Keep a close eye on the time.
• Keep descriptive answers short and to the point.
  
• Take note of how many marks are assigned to each
  question and dont spend long on questions with
  few marks.
• Use diagrams, tables or bullet-point lists if
  useful.

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Writing Code in the Exam

• We have referenced different code structures
  during the course. These include code segments,
  templates, subroutines and programs.
• Segments are lines of code.
• Templates are code frameworks (define the
  structure of components but dont contain any
  actual instructions)
• Subroutines are the component program tasks
  completed by a return instruction.
• Programs are full program listings including
  header with device and function fields.
• You will not be asked to write a full program
  listing in the exam.
•  

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Writing Code in the Exam

• Use the instruction set and/or register file map
  and information provided.
• READ THE QUESTION VERY CAREFULLY
• (dont write components that are NOT required)
• You could use a simple flow diagram to get you
  started.
• Comment your code (keep comments clear and
  concise)
• Do you need supporting equates or org statements?
  If the question asks for them they will be worth
  marks.
•  

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Common Mistakes

• that lose marks
• Misreading the question.
• Not using the register file list or instruction
  list.
• Fundamental errors (e.g., errors of the type
  advantage of TMR0 delay over software delay is
  that it is faster)
• Most common major loss of marks occurs when no
  attempt at all is made of a component.
• that dont lose marks
• Slightly different time delay times (our
  approximation is only an approximation - slightly
  different counter values are fine)
• Misspelling generally loses no marks unless, for
  example, a valid instruction is misspelt as a
  different instruction
• E.g., no lost marks for misspelling interrupt.

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Revision Examples (A)
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Revision Examples

• Convert the decimal numbers 53 and 21 into binary
  (base-2) form using eight digits.
• Calculate the twos complement representation of
  the decimal number 21.

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Revision Examples

• Using the circuit shown below, what is the
  relationship between the three bit output number,
  Q0-2, and the pair of inputs A0-2 and B0-2 when
• The control input is low.
• The control input is high.

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Revision Examples

• The figure below shows a positive edge triggered
  D-type flip-flop acting as a simple one bit
  information store. Copy the D and CLK signals
  and draw the missing Q signal.

Answer At first rising edge of CLK we know that
Q must be low (because D is low here) at 2nd
rising edge D is high so Q must now go high. At
last rising CLK edge D is still high so Q must
stay high I(and remain high while D goes low
because there has not been a new rising edge of
CLK.
Q
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Revision Examples

• In the context of a microcontroller, briefly
  describe the functions of
• i. The Status Register.
• ii. The Program Counter.
• iii. The Working Register.

• Answer
• The status register is used to store the states
  of several flags (e.g. carry flag, zero flag)
  generated as a result of an arithmetic or logical
  operation.
• ii. The program counter holds the address, in
  program memory, of the next instruction to be
  executed.
• iii. The working register is used to store one of
  the operands to be inputted into the ALU and also
  stores the result of an arithmetic or logical
  operation.

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Revision Examples

• Briefly describe the three possible states of a
  tri-state port and their use in connecting
  devices to a common bi-directional bus.

Answer Low impedance, high (equivalent to a
ve d.c. voltage source.) Low impedance, low
(equivalent to a short circuit to ground.) High
impedance (equivalent to a large resistor
connected to ground.) In the two low impedance
states, the port can write to a common bus as
long as all other ports sharing the bus are in
the high impedance state. In the high impedance
state, a port can read from the bus or simply
ignore all bus activity.
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Revision Examples

• Outline the differences and the relationships
  between assembly language and machine code.

Answer A program written in assembly language is
a list of human readable code. A machine code
program, on the other hand, is a list of numbers
understood by the control unit of a
microcontroller. All assembly language
instructions map onto a unique machine code
equivalent. To convert from assembly language to
machine code, an assembler is used.
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Revision Examples

• The figure shows an ALU, working register and two
  general-purpose registers, R0 and R1, all
  connected to a common system bus.
• List the data transfers required to add together
  the contents of R0 to itself and store the answer
  in register R1.
• Answer
• R0 outputs to the bus, W inputs and stores.
• R0 outputs to the bus (again), the ALU adds
  (storing the result in W)
• W outputs to the bus, R1 inputs and stores.

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Revision Examples

• List the data transfers required to compare for
  equality the contents of registers R0 and R1, the
  result appearing in the zero flag.
• Answer
• R0 outputs to the bus, W inputs and stores.
• R1 outputs to the bus, the ALU subtracts (storing
  the difference in W)
• The zero flag will now be high only if R0 R1

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Revision Examples (B)
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Revision Examples

• Briefly describe the PIC16F84? (A descriptive
  summary of the most important points)
• Write a simple template for a PIC assembler
  program that shows the reader the order of the
  basic contents of PIC assembler file. (For the
  program header comments, list only the important
  fields. ) (Think of the content of a typical
  program what does it contain and in what
  order?)
• Write a simple program segment (with supporting
  equates and comments) to configure PORTA pins as
  inputs and PORTB pins as outputs.
• What is the purpose of a data table how is it
  implemented in PIC assembler and why is it done
  this way?
• What are the contents of the INTCON register and
  what are their function? (exact bit names and
  locations not required).
• Briefly explain the advantages of TMRO over a
  software delay.
• Most PIC16F84 instructions take 1 clock cycle to
  execute. Identify all the instructions which
  take always and which sometimes take longer.
  Explain why they take longer.
• Write a 1/20 s second nested loop software time
  delay.

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Notes on Selected Questions

• TMR0 is the hardware timer. The software time
  delay ties up the processor. Using TMR0 means
  the processor is free to do other things. (It
  isnt faster! )
• TMR0 overflow is one of the 4 interrupts on our
  processor.
• The instructions all take one clock cycle except
  instructions which involve a program branch.
  Identifying these is often an exam question.
• BTFSS and BTFSC and DECFSZ and INCFSZ all take
  one or two clock cycles.
• BTFSS and BTFSC are useful bit test
  instructions. Test is a bit is set or clear and
  skip (the next line in the program) if the
  condition is satisfied.

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The Basic Nested-Loop Time Delay

• This is the 0.2s nested loop time delay from your
  lecture notes.
• Several exam questions in the past have required
  a time delay that is a simple fraction of 0.2s
  (1/5 s), for example, 0.1s or 1/20 s. These can
  be achieved by simply halving or quartering
  MCOUNT.
• For 0.2s delay MCOUNT255 (0xFF)
• For 0.1s delay MCOUNT128 (0x80)
• For 0.05s delay, MCOUNT64 (0x40)
• etc.

DELAY MOVLW 0XFF MOVWF MCOUNT GET_N MOVLW 0
XFF MOVWF NCOUNT DEC_N DECFSZ NCOUNT,F GOTO
DEC_N DECFSZ MCOUNT,F GOTO GET_N RETURN
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Time Delay with Flowchart

• DELAY MOVLW 0XFF
• MOVWF MCOUNT
• GET_N MOVLW 0XFF
• MOVWF NCOUNT
• DEC_N DECFSZ NCOUNT,F
• GOTO DEC_N
• DECFSZ MCOUNT,F
• GOTO GET_N
• RETURN

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Summary

• Revision notes
• Revision and exam suggestions
• A note about writing code in the exam
• Examples

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Thank You and Good Luck