Bachelor of Engineering in Computer and Electronic Engineering Computer Engineering 1 Year 2 Semester 3 Autumn 08 Niall O Keeffe Instructions to Candidates: - 2 hours duration Answer 4 out of 6 questions.
Question 1. a) List the 4 elements required to make up any computer system. b) Explain the difference between a microcontroller and a microprocessor. c) Explain the difference between code and data memory in a computer system. d) Explain the function of interrupts in a computer system. (6 marks) e) Figure Q1 overleaf shows a minimum 8051 circuit schematic. (i) What is the function of the RC circuit connected to the RST pin? (3 marks) (ii) What is the function of the EA pin? (2 marks) (iii) Explain the significance of the 12MHz crystal. (2 marks) Question 2. a) List 2 advantages and 2 disadvantages of programming the 8051 in C rather than assembly language. (6 marks) b) How much data memory will be allocated for the following variable declarations? int array1[4]; char a,b,c,d; unsigned char array2[] = 1,2,3,4,5,6; bit x,y,z; c) The C code below may not always produce the correct result. Explain the problem with the code and provide a solution. void main() char num1, num2, result; num1 = P1; num2 = P2; result = num1 + num2; d) Change the code of part (c) to add a bit variable called overflow. The overflow variable should be set to 1 if the addition result exceeds and 0 otherwise. e) Write C code to set the upper 2 bits of Port 3 without affecting the lower 6 bits of the port.
Question 3. An 8051 application board has 2 LEDs that are used to provide status information. The status of the board is dependent on 2 input switches SW1 and SW2. The switch and LED information for each state is shown in the table below. State Switch Status LED Status SW2 SW1 LED2 LED1 0 0 0 Off Off 1 0 1 Off On 2 1 0 On Off 3 1 1 Flash 1Hz Flash 1Hz a) Draw a circuit diagram of the application using a 74LS244 buffer to drive the LEDs. The pin-out for the 74LS244 buffer is shown below. (10 marks) b) Write a C program to implement the application. (15 marks) To implement the time delays your program can call a function delay(x); The parameter x will yield a delay of x * 50msec. e.g. delay(2) will generate a 0.1 second delay
Question 4. Figure Q4 overleaf shows an 8051 microcontroller interface to 2 7-segment displays. Software multiplexing is used to control the displays. a) Explain the functionality provided by the 74LS47 I.C. b) A single 74LS47 I.C. is used to drive 2 7-segment displays. Explain why the same number will not be displayed on both displays. c) List 2 advantages of using a 74LS47 instead of a 74LS244 buffer to interface to the 7- segment displays. d) What are the advantages of software multiplexing? e) The C program below may be used by the 8051 microcontroller in figure Q4. (i) What does the code do? (ii) Why is the function of the 10msec delay? (3 marks) sbit displaya = P3^0; sbit displayb = P3^1; void main() unsigned char data units, tens; for (tens = 0; tens < 10; tens++) for (units = 0; units < 10; units++) for (z = 0; z < 50; z++) P2 = tens; displaya = 0; displayb = 1; delay_10msec(); displaya = 1; displayb = 0; P2 = units; delay_10msec();
Question 5. An excerpt from the 74C922 keypad encoder datasheet is attached to the rear of the exam paper. a) Explain the functionality provided by the 74C922 keypad encoder chip (6 marks) b) What is the function of the capacitors connected to the Oscillator and Key Bounce Mask pins of the 74C922. c) The 74C922 keypad encoder is connected to the 4x4 keypad shown below. What is the 4-bit binary code output by the 74C922 if the keys 2 and C are pressed. d) Draw a schematic diagram to connect the 4x4 keypad to an 8051 microcontroller using the 74C922 decoder chip. (7 marks) e) Explain how the keypad may be interfaced to an 8051 microcontroller without the use of the 74C922 encoder chip. Y1 Y2 Y3 Y4 1 2 3 A 4 5 6 B 7 8 9 C * 0 # D X1 X2 X3 X4
Question 6. Figure Q6 overleaf shows an 8051 connected to an ADC0804 8-bit analog to digital converter. a) Complete the following table for an analog to digital converter (ADC). (6 marks) Analog Input Voltage Full Scale Voltage No. of Output Bits 1V 0 to +5V 8 3V -5V to +5V 8-1V -5V to +5V 8 Digital Output (Hex) b) What functionality do the ADC0804 WR and INTR pins provide? c) The code below interfaces an 8051 microcontroller to an ADC0804 ADC. Draw a timing diagram showing the interface signals between the 8051 and ADC. (6 marks) sbit WR = P3^0; sbit RD = P3^1; sbit INTR = P3^2; void main() unsigned char data adc; WR = 1; RD = 1; while(1) WR = 0; //start conversion by pulsing WR pin low WR = 1; while(intr); //wait for end of conversion RD = 0; adc = P1; RD = 1; d) Change the program in part C so that the maximum and minimum voltage values read by the ADC are stored in 2 variables called max and min. (8 marks)
74C922 Keypad Encoder Datasheet Extract