PYQ Analysis — Module 6: Applications

Topic Distribution Analysis

Module 6 carries 8–12 marks in the exam. Focus on:

High-Weightage Questions (Expected Tomorrow)

Q1: Explain stepper motor interfacing with 8051. Write a program to rotate a stepper motor 90 degrees clockwise. (5 marks)

Stepper Motor Basics:

• Step angle: 1.8 degrees per step → 200 steps/revolution
• 4 stator coils require driver circuit (ULN2003 Darlington array)
• Full-step sequence: 4 states per complete electrical cycle

Full-Step Sequence (Clockwise):

Program for 90 degrees (50 steps):

1MOV R0, #50        ; 50 full steps = 90 degrees
2MOV DPTR, #TABLE
3LOOP: MOV R1, #04  ; 4 states per step
4CYCLE: MOV A, #04H
5SUBB A, R1
6MOVC A, @A+DPTR
7MOV P1, A
8CALL DELAY
9DJNZ R1, CYCLE
10DJNZ R0, LOOP
11RET
12TABLE: DB 09H, 0CH, 06H, 03H

Q2: Calculate resistor value for an LED (2V forward drop, 10 mA current). (3 marks)

R = (Vcc - Vf) / I = (5 - 2) / 0.01 = 300 ohms (use 330 ohms standard)

Q3: Write a program to generate PWM for DC motor speed control. (4 marks)

1; 50% duty cycle PWM on P1.0
2MOV TMOD, #01H    ; Timer 0, Mode 1
3LOOP:
4SETB P1.0        ; Motor ON
5MOV TH0, #0FCH   ; Load for 1 ms ON time
6MOV TL0, #18H
7SETB TR0
8WAIT1: JNB TF0, WAIT1
9CLR TR0
10CLR TF0
11CLR P1.0         ; Motor OFF
12MOV TH0, #0FCH   ; Load for 1 ms OFF time
13MOV TL0, #18H
14SETB TR0
15WAIT2: JNB TF0, WAIT2
16CLR TR0
17CLR TF0
18SJMP LOOP

Q4: Explain matrix keyboard interfacing with 8051. (4 marks)

4x4 Matrix = 16 keys using only 8 pins (4 rows + 4 columns):

1. Rows connected to output port with pull-up resistors
2. Columns connected to input port
3. Scan algorithm: Activate one row LOW at a time, read columns
4. Key press detected when column reads LOW (row-column shorted)
5. Debounce: Wait 20 ms after first detection, verify again

Scanning logic:

1; Scan all 4 rows
2MOV P1, #0FH       ; Rows as output, Columns as input
3MOV R0, #04        ; 4 rows to scan
4MOV R1, #00        ; Row counter
5SCAN:
6MOV A, #0FH
7CPL A
8RL A              ; Rotate to next row
9MOV P1, A         ; Activate row
10CALL DELAY_20MS
11MOV A, P1         ; Read columns
12ANL A, #0FH       ; Mask rows
13CJNE A, #0FH, KEY_DETECTED
14INC R1
15DJNZ R0, SCAN

Q5: Interface an LCD with 8051 in 8-bit mode. (4 marks)

LCD Connections (16x2 alphanumeric LCD):

Initialization sequence:

1MOV A, #38H     ; 8-bit, 2 lines, 5x7 font
2CALL CMD_WR
3MOV A, #0EH     ; Display ON, cursor ON
4CALL CMD_WR
5MOV A, #06H     ; Auto-increment cursor
6CALL CMD_WR
7MOV A, #01H     ; Clear display
8CALL CMD_WR

Command write subroutine:

1CMD_WR:
2MOV P2, A       ; Send command to data lines
3CLR P1.0        ; RS = 0 (command mode)
4CLR P1.1        ; RW = 0 (write mode)
5SETB P1.2       ; E = 1
6CALL DELAY
7CLR P1.2        ; E = 0 (falling edge latches data)
8RET

Q6: Interface a temperature sensor (LM35) with 8051. (3 marks)

LM35: Output = 10 mV per degree Celsius. At 25 degrees C, output = 250 mV.

Interfacing:

1. LM35 output → ADC0804 analog input (Vin+)
2. ADC0804 digital output → 8051 Port 0
3. ADC reference: Vref/2 = 2.56V (for 0-5V range)
4. Temperature = (Digital_Value x 5) / 255 x 100 = (Digital_Value x 500) / 255

Quick Revision Notes

• Stepper motor: 4-state sequence (09H, 0CH, 06H, 03H) for clockwise
• Reverse sequence (03H, 06H, 0CH, 09H) for counter-clockwise
• ULN2003 Darlington driver needed for motor coils (high current)
• LED resistor: Always calculate using Ohm's Law
• LCD commands: 38H (init), 0EH (display on), 01H (clear), 80H+ (set cursor position)
• PWM duty cycle: ON_time / (ON_time + OFF_time) x 100%
• H-bridge (L298N): Controls DC motor direction and speed
• Debounce: 20 ms delay after first key detection