About this previous year paper
Subject Code: BEC502
Maximum Marks: 70
Time: 3 Hours
Paper ID: 310481
Question Paper Overview
SECTION A (2 × 7 = 14 Marks)
(Short conceptual questions – definitions and fundamentals)
a. Enumerate the term Memory.
b. Define Memory-Mapped I/O.
c. Enumerate the term Flow Chart.
d. Determine the use of Interrupts.
e. Discuss the term Interfacing.
f. Clarify the term Microcontrollers.
g. Outline the functions of the Serial Port in 8051.
SECTION B (Attempt any three × 7 = 21 Marks)
a. Demonstrate the Address–Data Bus Demultiplexing process in 8085, with a neat diagram.
b. Explain the types of interrupts in 8085.
c. Interpret the functions associated with the Bus Interface Unit (BIU) in 8086.
d. Demonstrate the Special Function Registers (SFRs) in 8051.
e. Explain the functions and programming associated with the Interrupt Controller block in 8051.
SECTION C (Attempt one part from each question × 7 = 35 Marks)
Q3
(a) Explain the circuit diagram to connect one RAM of 32KB and one ROM of 32KB to 8085.
OR
(b) Discuss the functions associated with 8085 pins:
Vpp, CLK(OUT), RESET IN’, X1–X2, INTA’, RD’, and IO/M’.
Q4
(a) Demonstrate the output value, Register A content, and Flag Register content after executing this program on 8085:
MVI A, 02H MVI C, A4H CMP C ADD C HLT
OR
(b) Discuss the functions of the 8085 instructions:
STAX, PUSH, SBI, CMA, RAR, RSTn, and JNC.
Q5
(a) Explain different Addressing Modes associated with 8086.
OR
(b) Interpret the functions of bits in the Control Word of 8253/54 Timer Chip.
Q6
(a) Discuss the functions associated with all four I/O Port pins in 8051.
OR
(b) Explain the functions of the following 8051 pins:
PSEN’, T1, Vcc, RST, AD5, TxD, and WR’.
Q7
(a) Demonstrate interfacing of a 4×4 (16-key) keyboard with 8051, with a neat circuit diagram.
OR
(b) Discuss the functions of 8051 instructions:
XCH, DIV, XRL, SETB, JZ, MOV, and SWAP.
Key Topics for Revision
1. Memory and Memory-Mapped I/O
Memory: Stores instructions and data for execution.
Memory-Mapped I/O: Assigns I/O device addresses within memory address space — same instructions used for both memory and I/O operations.
2. Flow Chart
Graphical representation of algorithmic steps using standard symbols (rectangles for processes, diamonds for decisions, arrows for flow).
3. Interrupts (8085)
| Type | Description | Example |
|---|---|---|
| Hardware | External pins trigger interrupt | INTR, RST7.5, RST6.5, RST5.5 |
| Software | Generated by instructions | RST 0–7 |
Used for real-time response in embedded systems.
4. Interfacing
Process of connecting microprocessor with memory and I/O devices.
Types: Memory Interfacing (RAM/ROM), I/O Interfacing (ADC, DAC, sensors).
Involves address decoding, control signals, and timing coordination.
5. Microcontrollers (8051)
Single-chip CPU + RAM + ROM + I/O ports + Timers + Serial Port.
Common features: 4KB ROM, 128B RAM, 2 Timers, 32 I/O pins, 1 serial port, 64K external memory support.
6. Serial Port Functions (8051)
Supports full-duplex serial communication.
Uses TxD (P3.1) and RxD (P3.0) pins.
Configurable in 4 modes (asynchronous and synchronous).
7. Bus Demultiplexing (8085)
AD0–AD7 are multiplexed for both address and data.
Latch (74LS373) separates address from data using ALE (Address Latch Enable) signal.
8. 8086 Addressing Modes
| Mode | Description |
|---|---|
| Immediate | Operand is part of instruction |
| Register | Operand in register |
| Direct | Address directly specified |
| Indirect | Memory address in register pair |
| Indexed / Based Indexed | Offset added to base/index register |
| Relative | Address relative to instruction pointer |
9. 8253/54 Control Word Bits
Controls Timer Mode, BCD/Binary counting, and Read/Write operations.
| Bit | Function |
|------|-----------|
| D7–D6 | Select Counter (0–2) |
| D5–D4 | Read/Write Control |
| D3–D1 | Mode Select (0–5) |
| D0 | BCD/Binary Selection |
10. 8051 I/O Port Functions
| Port | Alternate Functions |
|---|---|
| P0 | AD0–AD7 (Address/Data) |
| P1 | General I/O |
| P2 | A8–A15 (Address High) |
| P3 | TxD, RxD, INT0, INT1, T0, T1, WR’, RD’ |
11. Key 8051 Pins
PSEN’ (Program Store Enable): Reads external program memory.
T1: Timer 1 input. RST: Resets microcontroller.
WR’/RD’: Write/Read control signals. AD5: Multiplexed address/data line.
TxD: Serial data transmission.
12. 8051 Instructions
| Instruction | Function |
|---|---|
| XCH | Exchange accumulator and register/memory |
| DIV AB | Divide A by B |
| XRL | Exclusive OR logical operation |
| SETB | Set bit to 1 |
| JZ | Jump if A = 0 |
| MOV | Move data |
| SWAP | Swap nibbles of A |
13. Keyboard Interfacing (4×4 Matrix)
16 keys arranged in 4 rows × 4 columns.
Rows scanned sequentially; columns read for pressed key.
Connected to Port 1 or Port 2 of 8051.
Each key corresponds to unique row–column combination.
14. Special Function Registers (SFRs)
Dedicated control registers in 8051.
| Register | Purpose |
|-----------|----------|
| ACC (A) | Arithmetic/Logic operations |
| B | Multiplication/Division |
| PSW | Flags and status |
| SP | Stack pointer |
| DPTR | 16-bit data pointer |
| TMOD/TCON | Timer control |
| SCON/SBUF | Serial communication |
15. 8085 Pin Functions
| Pin | Function |
|---|---|
| Vpp | For programming EPROM |
| CLK(OUT) | System clock output |
| RESET IN’ | Initializes microprocessor |
| X1–X2 | Crystal oscillator inputs |
| INTA’ | Acknowledge interrupt |
| RD’ / IO/M’ | Control signals for read and I/O selection |
Exam Tips
Write pin diagrams neatly and label signals clearly.
For programs, include register contents and flag status after each instruction.
Practice bus timing diagrams and address decoding for interfacing questions.
Revise SFR list, 8051 instruction set, and 8085 interrupt priorities.