THE UNIVERSITY OF THE WEST INDIES

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1 THE UNIVERSITY OF THE WEST INDIES EXAMINATIONS OF MOCK 2004 Code and Name of Course: EE25M Introduction to Microprocessors Paper: Date and Time: Duration: Three Hours INSTRUCTIONS TO CANDIDATES: This paper has 12 pages and 35 questions Answer ALL questions in Sections A and B. Marks for each question are given in the margin. Approved calculators may be used. This exam does not contribute towards your nal course-mark. Section A (33 marks) contains 33 multiple-choice questions. Questions in Section A should be answered on this exam script. Choose the most appropriate answer for each question, and circle your chosen answer. You should spend about 45 minutes on Section A. Section B (100 marks) contains 2 questions. Section B should be answered on your own paper. Any code written for Section B should be appropriately commented for clarity. You should spend about 2 hours on Section B. At the end of the exam, please staple your paper inside the back cover of your exam script. You should make use of your 'green book' and the datasheets for the 80C51 microcontroller with this mock exam paper. c The University of the West Indies

2 page 2. Q1. Indicate which one of the following statements is correct: (a) Registers are a form of memory which exist inside the CPU. (b) The CPU co-ordinates all operations within the CU. (c) The Program Counter (PC) contains the address of the instruction being executed. (d) The ALU is responsible for decoding instructions. (e) All of the above statements are false. Q2. Choose the best answer. The memory map size aects the nature of the microprocessor-based system bus, because: (a) the number of data lines must match the memory bit-width. (b) the number of address lines must allow the entire address range to be addressed. (c) if the number of data and address lines are the same, the lines can be multiplexed. (d) Q2.a and Q2.b are true but Q2.c is false. (e) Q2.a, Q2.b and Q2.c are all true. Q3. A device has support for addressing 8 locations (i.e. it has 3 address lines). It is connected to an 8 bit bus. The upper 3 bus address lines go through a decoder which asserts the device select if the lines read 101 2, the next 2 bus address lines are unconnected, and the lower 3 bus address lines are tied to the device address lines. Which one of the following addresses (if any) will access the device? (a) (b) (c) (d) Q4. Indicate which one of the following statements is correct (a) Harvard architecture CPU's must have a stack. (b) Choosing between RISC and CISC is an organisational decision (c) Accumulator-based architectures do not work without a stack. (d) The reset vector is not always located at address 0x0000. (e) Von Neumann architectures have separate data and program memories

3 page 3. Q5. The memory space of a 4 bit, accumulator-based microprocessor contains the following values: Address 0x8 0x9 0xA 0xB 0xC 0xD Value 0x3 0x8 0xD 0x2 0xD 0xA What value will be in the working register, at the end of the following sequence of instructions LOAD DIRECT 0x09 ADD IMMEDIATE 0x0D ADD INDIRECT 0x0A (a) 0 (b) F (c) D (d) C (e) 7 Q6. Within the development tool-chain, the simulator is best described as: (a) software which runs on the host processor, and converts text to machine language. (b) software which runs on the host processor, and interprets target machine instructions (c) software which runs on the target processor, and communicates information about the state of the target to the host. (d) software which runs on the target processor, and provides a common platform for higher-level applications to operate on. Q7. Which one of the following (if any) is NOT a valid purpose for a host computer in a microprocessor based development system (a) Compilation of software for target. (b) Display of information reported by the debug kernel. (c) Download of the software to the target (programming). (d) Simulation/Emulation of target operation. (e) None of the above Q8. Manufacturer X supplies an IDE which is appropriate for all the microprocessors that he supplies. We can presume that: (a) all microprocessors supplied by Manufacturer X have the same instruction format. (b) all microprocessors supplied by Manufacturer X are supported by the same assembler/compiler. (c) microprocessors supplied by Manufacturer X operate at the same clock speed. (d) microprocessors supplied by Manufacturer X all support on-line debugging.

4 page 4. Q9. Which of the following statements about the PIC16F877 is not true: (a) PIC16F877 machine cycle consists of 4 clock cycles. (b) Data memory is 8 bits wide with a 9 bit address. (c) The banking bits are located in the STATUS register. (d) The stack used to store the PC during a subroutine call is only 8 levels deep. (e) The PIC16F77 has a RISC-based Von Neumann architecture. Q10. Which ONE of the following statements about the MPASM assembler directives is correct: (a) the END directive appears at the end of each subroutine (b) the ORG directive indicates the data memory location that the variable is stored at. (c) the DT directive is used for lookup tables (d) the BANKSEL directive is used to set the tristate registers (e) the INCLUDE directive is used to indicate the options to be used by the assembler Q11. If the dataword represented a signed integer (two's complement representation scheme), the number would be: (a) (b) 1E 16 (c) +E2 16 (d) +1E 16 Q12. The working shows an attempt to perform addition on 8 bit signed numbers, using 4 bit unsigned addition Identify the aw (if any) in the method shown: (a) carry was not transferred from lower nibbles to upper nibbles (b) arithmetic error in lower nibble (c) arithmetic error in upper nibble (d) incorrect representation

5 page 5. Q13. In order to add two real numbers using oating point representation/arithmetic, we: (a) add the mantissas, and subtract the signicands. (b) equate the mantissas, and then add the signicands. (c) subtract the mantissas, and add the signicands. (d) equate the mantissas, and multiply the signicands. (e) equate the signicands, and add the mantissas. Q14. A string of characters needs to be stored in the PIC16F877. If the DE directive is used to store the string, to access the characters we: (a) use a lookup table, and pass the index of the character in W. (b) use the data EEPROM, and pass the index of the character in EEADR. (c) use the program EEPROM, and pass the index of the character in EEADR. (d) use indirect addressing, and place the index of the character in the FSR. (e) use bank switching, and use the index to decide which bank to switch to. Q15. my_sub movlw 0x66 addwf Reg,W btfsc STATUS,C retlw 1 btfsc STATUS,DC retlw 1 retlw 0 Choose the one incorrect statement. The above piece of code will: (a) run when the instruction call my_sub is executed. (b) return either 0 or 1 in the working register. (c) add 0x66 to the contents of the working register, and return 1 if there is a carry or digit carry. (d) take a maximum of 8 instruction cycles to execute. (e) take a minimum of 5 instruction cycles to execute. Q16. You are writing a program which loops continuously. Within the loop, you are required to alternately set and clear bit 4 in an 8-bit register. The shortest way to successfully do so at each loop iteration is to: (a) test the bit; if true bit-wise AND with EF 16 ; if false bit-wise OR with (b) test the bit; if true bit-wise OR with ; if false bit-wise AND with EF 16 (c) test the bit; if true bit-wise AND with ; if false bit-wise OR with (d) bitwise XOR with EF 16 (e) bitwise XOR with 08 16

6 page 6. Q17. You are writing a C program for the PIC16F877 which initialises locations in memory to the same default value. You are wondering which program would generate the most ecient assembly code when compiled. Your alternate choices are: [A:] [B:] [C:] for (i=0;i<5;i++) c[i]=-1; for (i=4;i!=0;i--) c[i]=0; c[0]=0;c[1]=0;c[2]=0;c[3]=0; Which one of the following statements does NOT represent a valid reason for choosing the respective C code: (a) Code [A:] should be chosen because we have an increment instruction, but no decrement instruction. (b) Code [C:] should be chosen; only a few locations need to be cleared, so it will take the least space when compiled. (c) Code [B:] or Code [C:] should be chosen; clearing a location is easily done with clrf. (d) Code [B:] should be chosen; testing for the ZERO ag is easily done with btfsc STATUS,Z. (e) Code [A:] or Code [B:] should be chosen; indirect addressing can be used in a loop. Q18. A normally open push button switch needs to be interfaced to the PIC16F877. Which one of the following methods (if any) is NOT a viable alternative: (a) tie the switch between PORTB pin and ground with internal pull-ups enabled; switch-o == pin-high (b) tie the switch between PORTB pin and ground with an external pull-up resistor; switch-o == pin-high (c) tie the switch between PORTB pin and V DD with a pull-down resistor to ground; switch-o == pin-low (d) tie the switch between PORTB pin and V DD with a pull-down resistor to ground; switch-o == pin-high Q19. Choose the word combination which best completes the following sentence(s): The interrupt Q19I is the address of the instruction executed when the interrupt occurs. Interrupts are typically used when the Q19II needs to communicate information to the Q19III. (a) Q19I: service routine; Q19II:CPU ; Q19III:peripheral (b) Q19I: vector; Q19II:CPU ; Q19III:peripheral (c) Q19I: service routine; Q19II:peripheral; Q19III:CPU (d) Q19I: vector; Q19II:peripheral; Q19III:CPU (e) Q19I: handler; Q19II:peripheral; Q19III:CPU

7 page 7. Q20. Choose the word combination which best completes the following sentence(s): Q20I Modulation may be considered as a means of Q20II. The frequency/period of the signal is independent of the Q20I. The ratio of Q20I to period is referred to as the Q20III. (a) Q20I:pulse width; Q20II:digital-to-analog conversion; Q20III:duty cycle (b) Q20I:pulse width; Q20II:analog-to-digital conversion; Q20III:duty cycle (c) Q20I:pulse wave; Q20II:digital-to-analog conversion; Q20III:duty cycle (d) Q20I:pulse width; Q20II:digital-to-analog conversion; Q20III:cycle time (e) Q20I:pulse width; Q20II:analog-to-digital conversion; Q20III:cycle time Q21. You have a circuit in which a counter is connected to the output pin of the PIC16F877. The counter should count on each rising edge; however you notice that it counts twice on each rising edge. In order to discover the cause of the problem, the most appropriate action is to: (a) use a pull-up resistor on the output pin signal (b) connect an oscilloscope to the output pin and observe the waveform (c) replace the PIC16F877 (d) replace the counter (e) use a meter to check the supply voltage Q22. In order to use the Parallel Slave Port on the PIC16F877: (a) the PSPMODE bit in register TRISD must be cleared (b) the PSPMODE bit in register TRISE must be cleared (c) the PSPMODE bit in register TRISE must be set (d) the PSPMODE bit in register PORTE must be cleared (e) the PSPMODE bit in register TRISD must be set Q23. A student writes a PIC16F877 ISR in which he checks for the TMR0 interrupt, the external interrupt, and the Serial RX interrupt in that order. He observes that if the TMR0 interrupt and the Serial RX interrupt occur at the same time, sometimes the Serial RX interrupt gets processed rst. This is probably because: (a) Sometimes the interrupt signals occur when the ISR is already handling an external interrupt. (b) Sometimes the interrupt handlers, interrupt each other. (c) Sometimes TMR0 malfunctions and overows. (d) Sometimes the Serial peripheral receives alot of data. (e) Sometimes the PIC16F877 malfunctions and resets itself.

8 page 8. Q24. A PIC16F877 with a 4MHz oscillator is used with an external A/D converter which responds with the latest digital reading on PORTD<3:0> 1µs after the write line on PORTD<4> is raised. Assuming that PORTD has already been appropriately congured, which piece of code will successfully read the converter in the least instruction cycles? (a) bsf PORTD,4 movf PORTD,W andlw 0x0F movwf VAL_IN (b) bsf PORTD,4 nop movf PORTD,W andlw 0x0F movwf VAL_IN (c) bsf PORTD,4 movf PORTD,W andlw 0x0F movf VAL_IN (d) bsf PORTD,4 movlw 0x0F andwf PORTD,W movwf VAL_IN (e) bsf PORTD,4 movf nop PORTD,W andlw 0x0F movwf VAL_IN Q25. A PIC16F877 is to be interfaced to a device which has the following electrical characteristics: Max. current sunk/sourced by output (ignoring totals) 10µA / 5µA Max. input leakage current ±10µA Output Pin Load capacitance 25 pf Input Pin capacitance 10 pf Max. Output Low Voltage 1V Min. Output High Voltage 3.5 Volts Max. Input Low Voltage 2.5 Volts Min. Input High Voltage 4 Volts All ratings taken at 25. Which one of the following statements (if any) is NOT a correct analysis: (a) DC analysis PIC16F877 output to device input: OK ±10µA < ±20mA (b) DC analysis device output to PIC16F877 input: OK ±1µA < 10/ + 5µA (c) AC analysis PIC16F877 output to device input: OK 10pF < 50pF (d) AC analysis device output to PIC16F877 input: OK 5pF < 25pF

9 page 9. Q26. You are told that the 80C51 supports register-register instructions, while the PIC16F877 only supports load/store to accumulator. Based on this information, which of the following statements is correct: (a) The 80C51 does not have an accumulator. (b) We can move information between registers using 1 instruction on the 80C51. (c) We can move information between registers using 1 instruction on the PIC16F877. (d) We can move information between registers in 1 machine cycle on the 80C51. (e) We can move information between registers in 1 machine cycle on the PIC16F877. Q27. You are told to choose a microcontroller based on it's ability to perform 16 bit multiplication. The PIC16F877 is under consideration. You could rule out the PIC16F877 because of which of the following (if any): (a) The PIC16F877 has 14 bit instructions (b) The PIC16F877 does not have a multiply instruction (c) The PIC16F877 instructions operate on 8 bit data (d) The PIC16F877 has Harvard architecture Q28. When writing routines for a microprocessor, you have been advised to always initialize your registers, even if the default value is the value you want. Which one of the following statements (if any) is NOT a valid reason for this guideline: (a) The microprocessor may malfunction and/or reset without powering up (b) The routine may be called after the value was changed subsequent to power-up (c) The microprocessor may not power-up correctly (d) The routine may attempt to access more memory than originally intended/allocated The following information is for Q A device uses a proprietary synchronous communication protocol that requires 3 lines, Signal, Ground, and an externally controlled clock. The maximum rate at which data can be transmitted is 2400 bits/s. A message transaction involves 8 data bits and 3 overhead bits (start, stop and parity). Q29. The eective bit rate at which data can be transferred using this protocol is: (a) 2400 bits/second (b) 2400 (8+3) 8 bits/second (c) (8+3) bits/second (d) (8+3) bits/second (e) 2400 (8+3) 3 bits/second

10 page 10. Q30. You are given the choice of using bit-banging or a dedicated peripheral to communicate with the device. Which of the following statements is NOT correct: (a) If we use a peripheral, then our program will need to provide the peripheral with commands about the required function. (b) If we use bit-banging, then our program will need to explicitly set and clear bits at the correct times. (c) Peripherals are always faster than bit-banging. (d) Monitoring communications for bit-banging may occupy most of the processor time. (e) We may use built-in timers and timer interrupts to perform bit-banging. Q31. If we need to communicate between a PC and this device, we could interface individual lines to the Parallel Port. To access the various lines of the Parallel Port, we use the inp and outp instructions. These instructions refer to AL (Register A). However, when moving information between register A and other registers we refer to AX. This is because of the following reasons EXCEPT: (a) AX is a 16 bit register, and AL is the lower byte. (b) The parallel port peripheral registers are 8 bits wide. (c) The outp instruction format only has room for 8 bits of data. (d) Of the bus lines used to communicate with the parallel port peripheral, only 8 are data lines. (e) The PC processor is backwards compatible with earlier members of the x86 family. Q32. We need to communicate between the PIC16F877 and this device. It has been suggested that we could use the UART to do so. Which of the following is a valid conguration value for the TXSTA register? (a) (b) (c) (d) (e) Q33. Four of these devices are to be interfaced to the PIC16F877. It is suggested that a parallel bus could be used to send/receive data from all devices simultaneously. Which of the following is NOT a viable alternative? (a) Clock lines: tied together; Ground lines: tied together; Data lines: tied together (b) Clock lines: individual; Ground lines: tied together; Data lines: tied together (c) Clock lines: tied together; Ground lines: tied together; Data lines: individual (d) Clock lines: tied together; Ground lines: individual; Data lines: individual (e) Clock lines: individual; Ground lines: individual; Data lines: tied together END OF SECTION A

11 page 11. Section B Q1. You are a member of a development team which is working on an automated parking system to help people locate their car in the supermarket parking lot. Persons entering the car park will be given two battery powered wireless parking "cards": one to be left in the car, and one which they carry while they shop. The pair of cards must be returned on exiting the car park. The cards each contain an array of DIP switches which are set to the same. The carry-card has a button, a circle of 8 LED's, a wireless transmitter, and an array of 4 directional wireless receivers. When the button is pressed, a query-message containing the is transmitted. The car-card has a single wireless transmitter/reciever. When the car-card receives the query message, it transmits a reply-message. The carry-card receives reply-message(s) with varying strength on each of the directional receivers; if a reply-message with the correct is received, the signal strength at the receiver is recorded. After waiting an appropriate time, the carry-card lights the LED closest to the receiver(s) which received the reply (i.e. in the direction of the car). (a) Draw circuit(s) to show: how a normally-closed push-button can be interfaced to the PIC16F877 pin PORTB<0> so that when the button is pressed, the PIC16F877 detects a high signal. how an LED can be interfaced to the PIC16F877 pin PORTD<0>, so that it lights when the PIC16F877 sends a low signal. Your answers should include details of how you would congure the PIC16F877 port pins, and choose appropriate component values. (b) The 8 LED's are connected to the pins of PORTD. In order to light a single LED, you need to write a byte with one bit set to PORTD. Your team is discussing whether to generate this byte from the bit # by using a lookup table, or by rotating the number 0x01 the appropriate number of bits. Write subroutines which will take the bit# in the working register, and return the appropriate byte in the working register, using each of these methods. Your code should return 0x00 if the number passed in the working register is not between 0x00 and 0x07. (c) You are using a module independent of the PIC16F877 to detect the RF for each receiver. Each of these modules has an output that will stay high for a time proportional to the signal strength received. The 4 module outputs are tied to PORTB<7:4>. i. Describe using a diagram how you will determine which LED needs to be lit based on the outputs of the 4 modules. ii. Implement this algorithm using polling on the PIC16F877. iii. Implement this algorithm using interrupts on the PIC16F877. You should make use of the same locations/sub-routines you wrote in the previous question. (d) Your initial prototype of the carry-card does nothing when the button is pressed. Outline how you would go about testing the system. Justify your answer. 5 marks 5 marks 10 marks 4 marks 10 marks 10 marks 6 marks

12 page 12. Q2. Your company is designing an independent device for monitoring RS232 trac. The device should passively monitor electrical signals on the TX/RX lines of an RS232 serial cable, and display current and cumulative estimates of communication trac. (a) You have been asked to choose between the PIC16F877 and the 80C51. Use your knowledge of the PIC16F877 characteristics and the data-sheets provided to identify at least 3 functional dierences which would be relevant for this application. Based on the functional dierences which you have previously identied, recommend one of the microcontrollers to your team. (b) After selecting the processor, the team discusses the support tools which will be needed for this project. Identify at least ve items which you believe are necessary in order to get this project working. Justify your answers. (c) The team has decided to use an LCD display module to show the trac estimates. Your job is to write the module which will send a single character to an LCD display which was initialised in 4-wire mode. Write appropriately commented code for the PIC16F marks 15 marks 15 marks END OF SECTION B Electrical Reference Notes for the PIC16F877 Output Pin Load resistance R L 464Ω Output Pin Load capacitance C L 50pF Input Pin capacitance C in 5 pf Max. External Clock rise/fall time T os 25ns Max. Port output rise/fall time T io 40ns Max. Delay port output T os H2 io 255ns Min. Input Hold time T os H2 io I 100 ns Max. current sunk/sourced by output (ignoring totals) 20mA Max. input leakage current I IL ±1µA Output Low Voltage V OL 0.6V Output High Voltage V OH V DD 0.7 Volts Max. Input Low Voltage V IL 0.15V DD Volts Min. Input High Voltage V IH 0.25V DD Volts Machine cycle (in 1 instruction cycle) Q1 Q2 Q3 Q4 Pipeline Fetch stage Increment PC Latch IR Pipeline Execute stage Decode Register Read Process Register Write Peripheral actions Output signal Input signal Set ags (interrupt/status) All voltages must lie between V dd and V ss. All ratings taken at 25.[PIC01] References [PIC01] PIC16F87X Data Sheet: 28/40-Pin 8-Bit CMOS FLASH Microcontrollers. Technical Reference Document 30292c, MicroChip Technology Inc., END OF QUESTION PAPER