Question 1

Q.1. List of benefits of subroutine.

Accepted Answer

Subroutines offer benefits of code reusability, reduced program size and memory usage, easier debugging/maintenance, and modular program structuring.

Question 2

Q.2. Explain A/D converter interfaced with 8051 microcontroller.

Accepted Answer

An A/D converter (e.g., ADC0808/0809) is interfaced with the 8051 via its data bus (typically Port 0 or Port 1 for the 8-bit digital output) and control lines (START, ALE, EOC, OE) connected to 8051 I/O pins, which the program toggles to initiate conversion and read the digital result.

Question 3

Q.3. List the 8051 interrupts with its priority.

Accepted Answer

The 8051 has 5 interrupt sources with default priority order (highest to lowest): External Interrupt 0 (INT0), Timer 0 overflow (TF0), External Interrupt 1 (INT1), Timer 1 overflow (TF1), and Serial Port interrupt (RI/TI) — all can be reprioritized into two levels using the IP register.

Question 4

Q.4. What is the drawback of memory mapped I/O.

Accepted Answer

The main drawback of memory-mapped I/O is that it consumes part of the available memory address space for I/O devices, reducing the memory available for actual program and data storage, and typically requires more complex/longer address-decoding instructions since only memory-reference instructions (not dedicated, shorter I/O instructions) can access the devices.

Question 5

Q.5. Explain the instruction LHLD of 8085 microprocessor.

Accepted Answer

LHLD (Load H and L registers Direct) is an 8085 instruction that loads the L register with the contents of a specified 16-bit memory address and the H register with the contents of the next memory address, effectively loading a 16-bit value from memory into the HL register pair.

Question 6

Q.6. Draw the timing diagram of STA.

Accepted Answer

The STA (Store Accumulator Direct) timing diagram shows an opcode fetch machine cycle followed by two memory-read cycles (to fetch the 16-bit address) and one memory-write cycle (to store the accumulator content at that address), spanning a total of 4 machine cycles / 13 T-states.

Question 7

Q.7. Draw PIN diagram of 8051.

Accepted Answer

The 8051 is a 40-pin IC with four bidirectional 8-bit I/O ports (P0-P3), pins for power (Vcc, Vss), crystal oscillator (XTAL1, XTAL2), reset (RST), and control signals (ALE, PSEN, EA).

Question 8

Q.8. Draw internal diagram of CPU.

Accepted Answer

The internal CPU architecture comprises the ALU (arithmetic/logic operations), control unit (instruction decoding and sequencing), register array (general-purpose and special-purpose registers), and internal buses connecting these blocks to memory and I/O.

Question 9

Q.9. Give the difference between ADC and DAC.

Accepted Answer

An ADC (Analog-to-Digital Converter) converts a continuous analog voltage into a discrete digital code, while a DAC (Digital-to-Analog Converter) performs the reverse conversion, converting a digital code into a corresponding continuous analog voltage.

Question 10

Q.10. Briefly explain working of LCD and LED.

Accepted Answer

An LCD (Liquid Crystal Display) displays alphanumeric characters by selectively polarizing liquid crystal segments/dot-matrix cells to block or pass light, driven by an onboard controller (e.g., HD44780) accepting command/data bytes from the microcontroller, while an LED (Light Emitting Diode) emits light directly when forward-biased current flows through its semiconductor junction.

Question 11

Q.1. Explain various timers and interrupts and their functions of 8051.

Accepted Answer

The 8051 has two 16-bit timers (Timer 0 and Timer 1) configurable in four operating modes for timing/counting/baud-rate generation, and five interrupt sources (two external, two timer overflow, one serial port) each individually enabled and prioritized via the IE and IP special function registers.

Question 12

Q.2. Draw T-states & Machine cycles of STAX & CMP.

Accepted Answer

STAX (an 8085 register-indirect memory-write instruction) takes 2 machine cycles (Opcode Fetch + Memory Write, 7 T-states total), while CMP (a register-to-accumulator comparison instruction) takes just 1 machine cycle (Opcode Fetch only, 4 T-states), since CMP operates entirely on internal registers with no external memory access beyond the opcode fetch itself.

Question 13

Q.3. Compare 8-bit, 16-bit & 32-bit microcontroller.

Accepted Answer

8-bit microcontrollers (e.g., 8051) have simple architecture and low cost, suited to basic control tasks; 16-bit microcontrollers offer higher processing speed and larger address space for moderately complex applications; and 32-bit microcontrollers (e.g., ARM Cortex-M) provide high performance, large memory addressing, and advanced peripherals for complex, computation-intensive embedded applications.

Question 14

Q.4. Write short note on synchronous and Asynchronous communication.

Accepted Answer

Synchronous communication transmits data along with a shared clock signal for precise bit-timing synchronization between sender and receiver, suited to high-speed, continuous data transfer, while asynchronous communication transmits each data character independently framed with start/stop bits and no shared clock, suited to simpler, lower-cost, variable-timing serial links such as UART/RS-232.

Question 15

Q.5. Explain the protocols for Blue-Tooth and Zig-bee.

Accepted Answer

Bluetooth is a short-range (typically up to 10m), moderate-data-rate wireless protocol using frequency-hopping spread spectrum in the 2.4 GHz ISM band, suited to point-to-point/small-network device pairing, while Zigbee is a low-power, low-data-rate wireless protocol built on the IEEE 802.15.4 standard, designed for large mesh-networked sensor/control networks with long battery life.

Question 16

Q.6. What are SFRs, Explain it.

Accepted Answer

SFRs (Special Function Registers) are dedicated on-chip registers in the 8051's internal RAM address space (addresses 80H-FFH) that control and monitor the microcontroller's various on-chip peripheral functions — such as I/O ports, timers, serial port, and interrupts — rather than being available for general-purpose data storage.

Question 17

Q.7. How Microprocessor & Microcontroller are different from computer based c…

Accepted Answer

A microprocessor is a general-purpose CPU-on-a-chip requiring external memory and I/O support chips, a microcontroller integrates the CPU with on-chip memory, I/O ports, timers and peripherals for dedicated embedded control tasks, whereas a computer-based controller uses a full general-purpose computer system (with an operating system) for control, offering greater flexibility and processing power at much higher cost, size and power consumption than a microcontroller-based solution.

Question 18

Q.1. Show the interfacing circuit & functional pins of LCD.

Accepted Answer

A 16×2 character LCD module (HD44780-based) is interfaced with a microcontroller via 8 (or 4) data lines plus three control lines (RS, R/W, E), with the microcontroller writing command bytes (to configure/clear the display, position the cursor) and data bytes (ASCII character codes) following the module's specific timing/handshake sequence.

Question 19

Q.2. Give PSW of 8051 Microcontroller & describe each bit of PSW.

Accepted Answer

The Program Status Word (PSW) of the 8051 is an 8-bit special function register containing the Carry flag (CY), Auxiliary Carry (AC), F0 (user flag), register bank select bits (RS1, RS0), Overflow flag (OV), a reserved bit, and the Parity flag (P), used to record the outcome of arithmetic/logical operations and select the active register bank.

Question 20

Q.3. With the help of schematic diagram explain the interfacing of stepper mo…

Accepted Answer

A stepper motor is interfaced with the 8051 through a driver IC (such as ULN2003) that amplifies the microcontroller's low-current logic-level outputs (from a port, e.g., P1) to drive the motor's winding coils in the correct excitation sequence, with an assembly program cycling through the sequence table forward (for one rotation direction) or in reverse (for the opposite direction) and adjusting the delay between steps to control rotational speed.

Question 21

Q.4. Write a program to transfer one block of data into another block in same…

Accepted Answer

An 8085 assembly program to transfer a block of data from a source memory location to a destination in the same order uses register pairs HL and DE as source and destination pointers, register C (or B) as a byte counter, and a loop using LDAX/MOV, STAX/MOV and INX instructions to copy each byte sequentially, terminating with DCR/JNZ.

Question 22

Q.5. For an 8051 system of 11.0592MHz. Find the time delay for the following …

Accepted Answer

For an 8051 system running at 11.0592 MHz, the machine cycle time is 12/11.0592MHz ≈ 1.0851 μs; analyzing the given subroutine's instruction cycle counts (MOV=1, NOP=1 each, DJNZ=2) shows the loop executes 250 times at 4 cycles per iteration (1000 cycles total), plus 1 cycle for MOV and 2 for RET, giving 1003 total machine cycles and a total time delay of approximately 1.088 milliseconds.