8051 Microcontroller MCQs with Answers
Basic Questions
1. The 8051 microcontroller was originally developed by Intel under which family?
A) ARM
B) AVR
C) MCS-51
D) PIC
Answer: C) MCS-51
2. How many pins are available in the standard 8051 microcontroller package?
A) 28
B) 40
C) 20
D) 64
Answer: B) 40
3. How many I/O ports are present in 8051 and what is their size?
A) 2 ports of 8-bit
B) 3 ports of 8-bit
C) 4 ports of 8-bit
D) 4 ports of 4-bit
Answer: C) 4 ports of 8-bit
4. The internal RAM size of 8051 microcontroller is:
A) 64 bytes
B) 256 bytes
C) 128 bytes
D) 512 bytes
Answer: C) 128 bytes
5. Which register is mainly used for arithmetic and logical operations?
A) PSW
B) Accumulator (A)
C) DPTR
D) SP
Answer: B) Accumulator (A)
Registers & Addressing Modes
6. Which register contains flags like carry, parity, and overflow?
A) IE
B) TMOD
C) SCON
D) PSW
Answer: D) PSW
7. The Data Pointer (DPTR) is used for which purpose in 8051?
A) Interrupt handling
B) Stack operation
C) External memory addressing
D) Timer control
Answer: C) External memory addressing
8. The instruction “MOV A, #20H” uses which addressing mode?
A) Immediate addressing
B) Direct addressing
C) Indirect addressing
D) Register addressing
Answer: A) Immediate addressing
9. Which instruction performs subtraction along with borrow?
A) SUB
B) DEC
C) ADD
D) SUBB
Answer: D) SUBB
10. Which flag is set when an arithmetic overflow occurs?
A) Carry flag
B) Overflow flag
C) Parity flag
D) Auxiliary carry
Answer: B) Overflow flag
Timers & Control
11. Which register is used to configure timer modes in 8051?
A) TCON
B) TMOD
C) IE
D) PCON
Answer: B) TMOD
12. How many timers are available in standard 8051?
A) One
B) Two
C) Three
D) Four
Answer: B) Two
13. The TCON register is used for:
A) Arithmetic operations
B) Serial communication
C) Timer control and interrupt flags
D) Memory management
Answer: C) Timer control and interrupt flags
Serial Communication
14. Which register is used for serial communication control?
A) SCON
B) TMOD
C) PSW
D) IE
Answer: A) SCON
15. Baud rate generation in 8051 is commonly done using:
A) RAM
B) ALU
C) Timer
D) Port
Answer: C) Timer
16. Which pin is used for receiving serial data?
A) TXD
B) RXD
C) ALE
D) EA
Answer: B) RXD
Interrupts
17. The IE register is used to:
A) Store data
B) Enable/disable interrupts
C) Control timers
D) Manage memory
Answer: B) Enable/disable interrupts
18. Total number of interrupts in 8051 is:
A) 4
B) 5
C) 6
D) 3
Answer: B) 5
19. External interrupts are provided on which pins?
A) P1.0, P1.1
B) INT0, INT1
C) TXD, RXD
D) P2.0, P2.1
Answer: B) INT0, INT1
Instruction Set
20. Which instruction increments a register by one?
A) DEC
B) ADD
C) INC
D) MOV
Answer: C) INC
21. Which instruction decrements a register value?
A) INC
B) DEC
C) SUB
D) ADD
Answer: B) DEC
22. SJMP instruction is used for:
A) Data transfer
B) Arithmetic
C) Short jump
D) Interrupt
Answer: C) Short jump
Memory & Pins
23. Which memory loses data when power is switched OFF?
A) ROM
B) Flash
C) RAM
D) EPROM
Answer: C) RAM
24. ALE pin is used for:
A) Reset
B) Address latch enable
C) Interrupt
D) Timer control
Answer: B) Address latch enable
25. PSEN pin is used for:
A) Data write
B) Program read from external memory
C) Reset
D) Interrupt
Answer: B) Program read from external memory
26. EA pin is used to select:
A) RAM and ROM
B) Internal and external program memory
C) Timer and counter
D) Input/output ports
Answer: B) Internal and external program memory
27. Reset pin becomes active when signal is:
A) LOW
B) HIGH
C) Pulse
D) Analog
Answer: B) HIGH
Advanced Concepts
28. Why is 11.0592 MHz crystal commonly used?
A) Faster processing
B) Easy baud rate calculation
C) More memory
D) Less heat
Answer: B) Easy baud rate calculation
29. One machine cycle consists of:
A) 6 clock cycles
B) 24 clock cycles
C) 12 clock cycles
D) 48 clock cycles
Answer: C) 12 clock cycles
30. Which instruction rotates accumulator left without carry?
A) RLC
B) RL
C) RR
D) RRC
Answer: B) RL
31. Which instruction rotates accumulator right without carry?
A) RR
B) RL
C) RLC
D) ADD
Answer: A) RR
Stack & Registers
32. Stack in 8051 is stored in:
A) ROM
B) Flash
C) RAM
D) Register
Answer: C) RAM
33. Which register holds next instruction address?
A) SP
B) DPTR
C) PC
D) PSW
Answer: C) PC
34. Stack Pointer (SP) points to:
A) Current instruction
B) Next free RAM location
C) Timer value
D) Port data
Answer: B) Next free RAM location
35. Which port has no internal pull-up resistors?
A) Port 1
B) Port 2
C) Port 3
D) Port 0
Answer: D) Port 0
Instructions & Logic
36. MOVX instruction is used for:
A) Internal data transfer
B) Arithmetic operation
C) External memory access
D) Bit manipulation
Answer: C) External memory access
37. DJNZ instruction means:
A) Jump if zero
B) Decrement and jump if not zero
C) Add and jump
D) Compare and jump
Answer: B) Decrement and jump if not zero
38. Default register bank after reset is:
A) Bank 1
B) Bank 2
C) Bank 3
D) Bank 0
Answer: D) Bank 0
39. Which instruction clears accumulator?
A) CLR A
B) MOV A,#00H
C) SUB A,A
D) All of the above
Answer: D) All of the above
40. Which instruction complements accumulator?
A) ANL A
B) ORL A
C) CPL A
D) CLR A
Answer: C) CPL A
Conclusion
These 8051 microcontroller MCQs with answers are highly useful for:
ECE / EEE students
Embedded systems interviews
Competitive exams & viva
