Question 1

8051 Architecture — I/O Port Functions Which 8051 I/O port does not have any dual-purpose or alternate function pins and serves only as general-purpose digital I/O?

Accepted Answer

Introduction / Context: The classic 8051 has four 8-bit ports named P0, P1, P2, and P3. Several pins have alternate roles such as address/data bus, control strobes, or serial I/O. Knowing which port lines are pure GPIO simplifies interface design and pin planning. Given Data / Assumptions: Standard 8051 pinout is assumed. P0 and P2 can support external memory interfacing. P3 carries multiple special functions per pin. Concept / Approach: Identify ports with multiplexed functions: P0 is AD0–AD7 when external memory is used; P2 is A8–A15; P3 houses control and serial functions (INT, T, RXD, TXD, WR, RD). The only port that remains pure GPIO without built-in alternates is P1. Step-by-Step Solution: 1) List each port and its known alternates.2) Exclude ports with address/data or control roles.3) The remaining port without alternates is P1, therefore it is the correct answer. Verification / Alternative check: Inspect any standard 8051 block diagram and pin table; only P1 lacks an alternate function column. Why Other Options Are Wrong: port 0: Multiplexed address/data bus AD0–AD7. port 2: Higher address bus A8–A15 in external memory mode. port 3: Special functions such as RXD, TXD, INT0, INT1, T0, T1, WR, RD. Common Pitfalls: Confusing the ability to use a port as GPIO with the presence of built-in alternates; although P0 and P2 can be GPIO, they have additional roles that complicate external hardware. Final Answer: port 1

Question 2

8051 Pin Functions — P3.1 Alternate Role For the 8051 microcontroller, what is the alternate function of port pin P3.1?

Accepted Answer

Introduction / Context: Port 3 pins on the 8051 double as special function signals. Correctly recognizing these saves hardware and enables on-chip peripherals. P3.0 and P3.1 are particularly important for the built-in UART. Given Data / Assumptions: P3.0 is RXD (receive data) and P3.1 is TXD (transmit data). Standard UART operation is assumed. No external memory control signals are mapped to P3.1. Concept / Approach: The 8051 serial port uses two pins: RXD for input and TXD for output. TXD is mapped to P3.1, which means P3.1 drives the serial transmit data stream when the UART is enabled. Step-by-Step Solution: 1) Recall Port 3 alternates: P3.0=RXD, P3.1=TXD, P3.2=INT0, P3.3=INT1, P3.4=T0, P3.5=T1, P3.6=WR, P3.7=RD.2) Match P3.1 with TXD.3) Therefore its function is serial port output. Verification / Alternative check: Any 8051 datasheet pin table confirms P3.1=TXD. Why Other Options Are Wrong: serial port input: That is P3.0 (RXD), not P3.1. memory write strobe: WR is P3.6. memory read strobe: RD is P3.7. Common Pitfalls: Swapping RXD and TXD pins or confusing serial pins with external memory control signals. Final Answer: serial port output

Question 3

8051 Internal Data Memory Capacity What is the size of the internal RAM (lower internal data memory) available in a standard 8051 microcontroller?

Accepted Answer

Introduction / Context: Memory organization is fundamental to programming the 8051. The classic 8051 family divides internal memory into lower internal RAM and a separate SFR space, with some derivatives adding upper RAM. Correctly identifying capacities avoids addressing errors and stack overflow problems. Given Data / Assumptions: Reference to the original 8051 architecture. Lower internal RAM includes register banks, bit-addressable area, and general RAM. Upper internal RAM (if present) is device dependent and not universal. Concept / Approach: The original 8051 implements 128 bytes of lower internal RAM from addresses 00H to 7FH. This space comprises four register banks (00H–1FH), a bit-addressable region (20H–2FH), and general purpose RAM (30H–7FH). SFRs occupy 80H–FFH and are not counted as RAM bytes. Step-by-Step Solution: 1) Identify address map: 00H–7FH is lower RAM.2) Count size: 7FH–00H + 1 = 128 bytes.3) Conclude that classic 8051 internal RAM is 128 bytes. Verification / Alternative check: Many modern variants add extra internal RAM up to 256 bytes or more, but the canonical answer for the base 8051 remains 128 bytes. Why Other Options Are Wrong: 32 bytes, 64 bytes: Too small, do not match the documented map. 256 bytes: Applies to some derivatives with upper RAM, not to the baseline device. Common Pitfalls: Counting SFR space as RAM or assuming all 8051 derivatives share the same RAM size. Final Answer: 128 bytes

Question 4

8051 Instruction Set — Indirect Addressing to Port Which instruction copies the byte from internal RAM at the address contained in R0 to Port 1 (P1)?

Accepted Answer

Introduction / Context: Indirect addressing with @R0 or @R1 allows the 8051 to access a RAM location whose address is held in a register. Understanding legal source and destination forms is key to moving data between RAM and special function registers like P1. Given Data / Assumptions: R0 holds a valid internal RAM address. P1 is the Port 1 special function register at a direct address. Standard 8051 MOV instruction forms apply. Concept / Approach: Legal forms include MOV direct, @Ri and MOV direct, A and MOV direct, @Ri (source indirect to direct). To copy RAM[@R0] to P1, the destination must be direct (P1) and the source must be indirect (@R0). Thus MOV P1, @R0 is the correct encoding. Step-by-Step Solution: 1) Determine direction: RAM[@R0] → P1.2) Select instruction form: MOV direct, @Ri.3) Substitute operands: direct=P1, @Ri=@R0.4) The legal statement is MOV P1, @R0. Verification / Alternative check: Assembler manuals list MOV direct, @Ri as a valid form. Simulators or assemblers will accept MOV P1, @R0 and produce the corresponding opcode. Why Other Options Are Wrong: MOV @ P1, R0: Invalid syntax; destination cannot be @P1. MOV @ R0, P1: Moves in the opposite direction P1 → RAM[@R0]. MOV P1, R0: Copies the contents of register R0, not RAM at address in R0. Common Pitfalls: Confusing the contents of R0 with the byte pointed to by @R0; forgetting that port SFRs are direct addressed destinations. Final Answer: MOV P1, @ R0

Question 5

8051 Timers — Loading TH0 Which instruction correctly loads the immediate value 35H into the high byte of Timer 0 (TH0)?

Accepted Answer

Introduction / Context: Timer registers in the 8051 are split into THx (high) and TLx (low) bytes. Correct immediate loading syntax ensures predictable starting values for counting or timing operations. Given Data / Assumptions: Target register is TH0 (timer 0 high byte). Value to load is 35H. Assembler syntax uses # for immediate data. Concept / Approach: Immediate moves use the form MOV destination, #immediate. TH0 is a valid SFR symbol. Therefore the correct instruction is MOV TH0, #35H. Step-by-Step Solution: 1) Select the register: TH0.2) Use immediate notation: prefix with #.3) Write the instruction: MOV TH0, #35H. Verification / Alternative check: Assembler reference confirms that omitting # interprets the operand as a direct address, not a literal, which is not intended here. Why Other Options Are Wrong: MOV TH0, 35H: Treats 35H as a direct address, not a literal value. MOV T0, #35H and MOV T0, 35H: T0 is not a valid 8051 SFR name for the high byte; the correct symbol is TH0. Common Pitfalls: Forgetting the # for immediate data and mixing up TH0 with TL0. Final Answer: MOV TH0, #35H

Question 6

ADC Basics — ADC0804 Resolution The ADC0804 analog to digital converter provides what resolution in bits per conversion?

Accepted Answer

Introduction / Context: Resolution defines how many discrete digital levels an ADC provides. The ADC0804 is a widely used microcontroller compatible ADC, often paired with 8-bit systems for simple voltage measurements. Given Data / Assumptions: Device family: ADC080x. Single ended input with typical reference and successive approximation architecture. Parallel 8-bit data output bus. Concept / Approach: An N-bit converter resolves 2^N quantization levels. The ADC0804 outputs 8 data lines, hence N = 8 and the number of codes is 2^8 = 256. Resolution step size equals Vref / 256 when full scale equals Vref. Step-by-Step Solution: 1) Identify number of output bits: 8.2) Compute levels: 2^8 = 256 codes.3) Conclude resolution: 8-bit. Verification / Alternative check: Datasheets show D7..D0 outputs and 8-bit timing diagrams consistent with an 8-bit SAR core. Why Other Options Are Wrong: 4-bit: Too coarse; would only have 16 levels. 16-bit: Far beyond the ADC0804 capability. 32-bit: Not applicable to this device class. Common Pitfalls: Confusing resolution with accuracy or linearity; believing that oversampling changes the native resolution without digital filtering and scaling. Final Answer: 8-bit

Question 7

8051 External Memory Interface — Address/Data Mapping Which ports of the 8051 are used as the multiplexed address/data bus for external memory interfacing?

Accepted Answer

Introduction / Context: When the 8051 accesses external memory, it exposes address and data signals on its I/O pins. Understanding which ports carry which signals is essential for latching, decoding, and bus transceiving. Given Data / Assumptions: P0 is multiplexed AD0–AD7 (low address and data). P2 provides A8–A15 (high address). Control strobes ALE, PSEN, RD, and WR handle timing. Concept / Approach: The 8051 uses a multiplexed bus to reduce pin count. The low address byte appears on P0 and is latched with ALE; during later phases the same P0 pins carry data. P2 continuously drives the high address lines during the cycle. Step-by-Step Solution: 1) Associate P0 with AD0–AD7.2) Associate P2 with A8–A15.3) Conclude that ports 0 and 2 together implement the external address/data interface. Verification / Alternative check: Standard timing diagrams show ALE latching P0 into an external latch such as 74HC373 or 74HC573 to separate address from data. Why Other Options Are Wrong: ports 1 and 2 / ports 1 and 3: P1 and P3 are not assigned to the external address/data bus. ports 0 and 3: P3 provides control signals, not the address/data bus itself. Common Pitfalls: Forgetting to demultiplex P0 with a latch; assuming P3 carries address bits. Final Answer: ports 0 and 2

Question 8

8051 External EPROM Interfacing — Demultiplexing Requirement When interfacing an external EPROM to an 8051, it is necessary to demultiplex the address/data lines (AD0–AD7) from Port 0. State whether this requirement is true or false.

Accepted Answer

Introduction / Context: The 8051 saves pins by multiplexing the low address byte and data on Port 0. External memories like EPROM require a stable address while data is read, which demands demultiplexing with a latch controlled by ALE. Given Data / Assumptions: P0 = AD0–AD7 is time multiplexed. ALE provides an address latch enable pulse each external cycle. Typical latch parts are 74HC373 or 74HC573. Concept / Approach: During the first phase of the cycle, P0 carries A0–A7 and ALE is high. A transparent latch captures the address. When ALE goes low, P0 switches to data D0–D7 while the latched address remains at the memory pins. Without demultiplexing, the EPROM would see address corruption when the bus turns to data. Step-by-Step Solution: 1) Use ALE to clock the low address into the latch.2) Hold the latched address on the memory A0–A7 pins.3) Allow P0 to drive data lines without disturbing the address. Verification / Alternative check: Refer to standard 8051 memory interface diagrams showing P0 through a latch to memory address lines and P2 directly to high address lines. Why Other Options Are Wrong: False: Ignores the multiplexed nature of P0. Only necessary above 8 KB: Size does not change the bus requirement. Not required if Port 2 is unused: Port 2 handles high address bits; it does not eliminate the need to separate A0–A7 from D0–D7. Common Pitfalls: Connecting P0 directly to memory address and data without a latch; forgetting to tie ALE to the latch enable. Final Answer: True

Question 9

8051 Port Count — Parallel I/O Availability How many parallel 8-bit I/O ports are available on a standard 8051 microcontroller?

Accepted Answer

Introduction / Context: Knowing the number of available GPIO ports is essential for system pin budgeting. The baseline 8051 exposes multiple 8-bit ports that can be read and written in software for digital interfacing. Given Data / Assumptions: Standard 8051 package and feature set. Ports are named P0, P1, P2, P3. External memory mode may repurpose some pins, but the count as ports remains unchanged. Concept / Approach: The 8051 provides four parallel ports, each 8 bits wide, totaling 32 general purpose lines. Although some pins have alternate roles, software can still configure and read or write port latches as appropriate to the application and hardware wiring. Step-by-Step Solution: 1) List ports: P0, P1, P2, P3.2) Count them: 4 ports.3) Conclude that there are four parallel ports. Verification / Alternative check: Pinout diagrams show 32 I/O lines divided into four groups, even though P0 and P2 may serve the external bus in some designs. Why Other Options Are Wrong: 2 or 3: Underestimate the available ports. 5: Exceeds the number present on the baseline 8051. Common Pitfalls: Confusing the presence of alternate bus functions with the absence of general purpose I/O capability; overlooking the need for external pull-ups on P0 when used purely as GPIO. Final Answer: 4

Question 10

8051 assembly immediate vs direct addressing for moving an 8-bit constant into A Which instruction correctly loads the hexadecimal constant 27H into the accumulator A?

Accepted Answer

Introduction / Context: In 8051 assembly programming, understanding addressing modes is fundamental. The accumulator A is the primary register for arithmetic and logic operations. Loading a constant value into A requires the correct use of the immediate addressing mode, which is denoted by the number sign. This question checks the ability to distinguish immediate, direct, and indirect addressing syntaxes for the MOV instruction. Given Data / Assumptions: Target value is the 8-bit hexadecimal constant 27H. Destination register is the accumulator A. Instruction family is the 8051 MOV with various addressing modes. Concept / Approach: Immediate addressing loads a literal constant into a register using the format MOV dest, #immediate. Direct addressing without the number sign reads from an internal RAM or SFR address. Indirect addressing uses a register pointer such as @R0 or @R1, not a literal number. Step-by-Step Solution: Identify the needed mode: loading a constant requires immediate addressing.Correct syntax: MOV A, #27H places the literal 27H into A.Reject direct addressing: MOV A, 27H fetches the byte stored at RAM address 27H, not the constant 27H.Reject invalid forms: MOV A, P27 is not a valid operand. MOV A, @27 is invalid because @ must reference R0 or R1. Verification / Alternative check: Assemble and simulate: MOV A, #27H results in A = 27H immediately after execution. Using MOV A, 27H would depend on memory contents at address 27H, which may differ from 27H. Why Other Options Are Wrong: MOV A, P27: operand is not a valid 8051 symbol here; incorrect syntax. MOV A, 27H: direct addressing reads from data memory address 27H, not a literal constant. MOV A, @27: @ requires R0 or R1 as a pointer, not a literal numeric address. Common Pitfalls: Forgetting the number sign in immediate mode, misreading direct address values as constants, and using incorrect indirect syntax are frequent mistakes. Final Answer: MOV A, #27H

Question 11

8051 SFR naming vs addressing The special function registers of the 8051 can be referred to by their hexadecimal addresses or by their symbolic register names. Choose the correct statement.

Accepted Answer

Introduction / Context: Special Function Registers, or SFRs, implement core control and status features of the 8051 microcontroller, such as port latches, timer registers, serial control, and the accumulator. Toolchains generally allow programmers to access these registers either by their absolute addresses or by standardized symbolic names. This question validates understanding of how SFRs are referenced in assembly and C toolchains for 8051. Given Data / Assumptions: Device family is classic 8051 and compatible derivatives. Assembler supports standard SFR symbols like ACC, B, PSW, P0, P1, TMOD, TH0, etc. All SFRs reside in the upper data space 80H to FFH and are byte addressable. Concept / Approach: In assembly, programmers may write either a name (for example, MOV A, P1) or an explicit address (for example, MOV A, 90H) to refer to the same SFR. In C, header files typically map names to addresses via definitions so code can remain readable and portable. Step-by-Step Solution: Recognize that SFRs have fixed addresses in the range 80H–FFH.Assemblers map symbolic names to these addresses, enabling two equivalent reference forms.Therefore the statement that both addresses and names are usable is correct. Verification / Alternative check: Consult any standard 8051 assembler manual or device header: it shows SFR address tables paired with names. Assembling small test code with both forms yields identical opcodes. Why Other Options Are Wrong: False: denies a widely supported feature. Only by hexadecimal addresses: reduces readability and contradicts standard practice. Only by register names: ignores that low level code can still refer to raw addresses. Common Pitfalls: Confusing bit-addressable SFR bits with byte-wide SFR addresses, and forgetting that not all SFRs are bit-addressable. Final Answer: True

Question 12

8051 internal RAM bit-addressable area Identify the byte address range that is bit-addressable in the internal data memory.

Accepted Answer

Introduction / Context: The 8051 architecture supports direct bit manipulation for selected areas of memory. Bit-addressability enables instructions such as SETB, CLR, JB, and JNB to operate on single bits efficiently. Correctly identifying the bit-addressable byte range is essential for mapping application flags and control bits into the low internal RAM area. Given Data / Assumptions: The question pertains to the internal data RAM of a standard 8051. Byte addresses are in hexadecimal. Bit-addressable bytes correspond to specific bit addresses from 00H to 7FH. Concept / Approach: In a classic 8051, the 16 bytes from 20H to 2FH form a 128-bit region that is bit-addressable. Each byte contains 8 individually addressable bits, making 16 * 8 = 128 bit locations. Bit addresses 00H to 7FH map directly onto this region. Step-by-Step Solution: Recall mapping: bit address 00H corresponds to byte 20H bit 0; bit address 07H is byte 20H bit 7.Continuing upward, bit address 7FH maps to byte 2FH bit 7.Therefore, the bit-addressable byte range is 20H–2FH inclusive. Verification / Alternative check: Device memory maps and assembler include files confirm that only 20H–2FH in low RAM and selected SFR bits are bit-addressable. Attempts to set a bit at bytes outside this range with direct bit addresses will assemble to SFR bit operations, not low RAM bits. Why Other Options Are Wrong: 10H–1FH and 30H–3FH and 40H–4FH are normal byte-addressable areas in low RAM and are not low RAM bit-addressable blocks. Common Pitfalls: Confusing bit-addressable low RAM with bit-addressable SFR bits, and assuming the entire low RAM supports single-bit operations. Final Answer: 20H through 2FH

Question 13

Feasibility with an 8051 microcontroller A centigrade (Celsius) thermometer and a PWM motor speed-control circuit can both be implemented using the 8051 platform. Choose the correct statement.

Accepted Answer

Introduction / Context: The 8051 microcontroller is a general purpose 8-bit platform widely used in embedded control. By interfacing sensors and using timers, it can implement a variety of applications. This question tests conceptual understanding of system level feasibility rather than exact circuit values, focusing on a Celsius thermometer and PWM speed control. Given Data / Assumptions: 8051 core with I/O ports, timers, and interrupts. Temperature sensor provides an analog or digital signal that can be read through a suitable interface. PWM generation can be achieved through timers or software toggling. Concept / Approach: For temperature, a sensor such as LM35 or a digital sensor can be interfaced. If the sensor is analog, an external ADC is used because the classic 8051 lacks an internal ADC. For PWM, the 8051 timers can generate periodic toggles at desired duty cycles to control motor speed via a driver stage such as a MOSFET and an appropriate power supply. Step-by-Step Solution: Use a sensor interface: either direct digital sensor read or external ADC channel for analog sensors.Convert the measured value to degrees Celsius using the sensor transfer function and calibration offsets.Configure a timer to produce a periodic interrupt, update an output pin with the desired duty cycle to create PWM.Use a driver circuit and flyback protection to control a DC motor safely. Verification / Alternative check: Simulations or bench tests confirm that temperature readings match known references and that PWM varies motor speed smoothly. Numerous reference designs demonstrate both tasks on 8051 derivatives. Why Other Options Are Wrong: False: both applications are achievable with proper interfacing. Only PWM or only temperature: each ignores the flexible I/O and timer capabilities of 8051 plus simple external components. Common Pitfalls: Neglecting the need for level shifting, ADC resolution, sensor calibration, and proper motor driver design. Final Answer: True

Question 14

8051 clock input pins Pins XTAL1 and XTAL2 of the 8051 are used to connect an external crystal or oscillator source. Choose the correct statement.

Accepted Answer

Introduction / Context: The system clock determines instruction timing for 8051 microcontrollers. Classic 8051 devices provide two pins, XTAL1 and XTAL2, to interface with a crystal resonator or an external clock source. Properly understanding these pins is essential for board level hardware design and reliable timing. Given Data / Assumptions: Standard 8051 pinout with XTAL1 and XTAL2 dedicated to the oscillator network. External components may include a crystal and load capacitors or a clock module. Reset and GPIO functions are on other pins, not on XTAL1 or XTAL2. Concept / Approach: XTAL1 is typically the input to the internal inverting amplifier, and XTAL2 is the output. A crystal connected across XTAL1 and XTAL2 with capacitors to ground can sustain oscillation. Alternatively, a single ended external clock can be injected on XTAL1 according to the device data sheet. Step-by-Step Solution: Connect the crystal across XTAL1 and XTAL2 with appropriate capacitors for load and stability.For an external clock module, drive XTAL1 while leaving XTAL2 open as recommended.Verify clock frequency meets device timing and baud rate requirements. Verification / Alternative check: Oscilloscope measurements at XTAL2 show oscillation when configured with a crystal. Serial communication accuracy also verifies proper clocking. Why Other Options Are Wrong: False: contradicts the standard 8051 design. XTAL1 only for GPIO: incorrect; these pins serve the oscillator function. XTAL2 as reset: reset is on a separate pin named RST. Common Pitfalls: Using incorrect load capacitors, long traces causing startup issues, and injecting clock levels that violate input requirements. Final Answer: True

Question 15

8051 bit set instruction and bit addresses Consider the 8051 statement: SETB 01H Does this instruction set the bit at bit address 01H to logic 1?

Accepted Answer

Introduction / Context: The 8051 includes instructions for single bit manipulation. SETB sets a specified bit to logic 1. Bits may refer to SFR bits or bits in the bit-addressable low RAM region. Understanding how numeric bit addresses map to byte addresses is key to using these instructions correctly. Given Data / Assumptions: Instruction is SETB 01H with a numeric bit address. Bit addresses 00H to 7FH correspond to low RAM bit space mapped onto bytes 20H to 2FH. No operand refers to R0 or R1, so this is not indirect addressing. Concept / Approach: SETB bit_address sets the specified bit to 1. The low RAM bit space uses a linear mapping: bit_address = (byte_address * 8) + bit_index. The lowest bit address 00H corresponds to byte 20H bit 0; therefore 01H corresponds to byte 20H bit 1. The instruction is valid and performs a set operation. Step-by-Step Solution: Recognize SETB with a direct bit address operand.Map 01H to the low RAM bit space: it is the second bit in byte 20H.Execute conceptually: the CPU sets that bit to logic 1.No SFR is involved unless an SFR bit address is specified. Verification / Alternative check: Assemble and single step in a simulator: memory byte 20H toggles bit 1 to 1; PSW flags are unaffected. Why Other Options Are Wrong: False: contradicts the defined behavior of SETB. It clears the bit: would be the effect of CLR, not SETB. Invalid address: 01H is valid within the 00H–7FH bit address range. Common Pitfalls: Confusing bit addresses with byte addresses and assuming all bytes are bit-addressable. Only the 20H–2FH byte range and specific SFRs support bit addressing. Final Answer: True

Question 16

8051 loop behavior with port polling Program fragment: STAT: MOV A, P0  MOV P2, A  JNB P2.3, STAT This code reads Port 0 to A, writes A to Port 2, and will stop looping when bit 3 of Port 2 becomes set. Choose the correct statement.

Accepted Answer

Introduction / Context: Polling a port bit with conditional jumps is a common technique on the 8051. Understanding the meaning of JNB, which stands for Jump if Not Bit, is essential for predicting control flow. This fragment mirrors a simple data transfer from Port 0 to Port 2 followed by a loop controlled by a single bit on Port 2. Given Data / Assumptions: MOV A, P0 copies the current Port 0 value into A. MOV P2, A outputs the accumulator value to Port 2 latch. JNB P2.3, STAT jumps back to STAT only if bit 3 of Port 2 is 0. Concept / Approach: JNB tests the specified bit, and if that bit equals 0, program control branches to the label. If the bit equals 1, the branch is not taken and execution falls through, thereby exiting the loop. The code therefore continues looping while P2.3 remains 0 and stops looping when P2.3 becomes 1. Step-by-Step Solution: Iteration start at STAT: read Port 0 into A.Write A to Port 2: P2 now reflects the input value or the last written latch value subject to hardware wiring.Evaluate JNB P2.3, STAT: if P2.3 equals 0, jump back to STAT; if P2.3 equals 1, do not jump and proceed.Therefore the loop terminates when P2.3 is set to 1. Verification / Alternative check: Simulate with different input patterns or manually set P2.3 through hardware. Observe that the loop persists while P2.3 is 0 and stops when P2.3 goes to 1. Why Other Options Are Wrong: False: contradicts JNB behavior. Loops until P2.3 becomes 0: it already loops when 0; termination occurs when it becomes 1. Never terminates: false if hardware or program logic can set P2.3 to 1. Common Pitfalls: Confusing JNB with JB, and assuming port pins always reflect latch writes even when configured as inputs with external pullups. Real hardware behavior depends on wiring and port mode. Final Answer: True

Question 17

8051 interrupt sources count How many distinct interrupt sources are supported in a standard classic 8051 implementation?

Accepted Answer

Introduction / Context: Interrupts allow asynchronous events to preempt normal execution and run dedicated service routines. Knowing how many interrupt sources are available informs peripheral planning and priority assignment in embedded systems using the 8051. Given Data / Assumptions: Classic 8051 without extended peripherals such as Timer 2. Default interrupt controller with two priority levels. Sources include external and internal events. Concept / Approach: The classic 8051 supports five interrupt sources: External 0 (INT0), Timer 0 overflow (TF0), External 1 (INT1), Timer 1 overflow (TF1), and the serial port (RI or TI). Some derivatives add a sixth source for Timer 2, but that is not present in the original core. Step-by-Step Solution: Enumerate sources: INT0, TF0, INT1, TF1, serial (RI/TI combined vector).Confirm no Timer 2 in the basic device; therefore total remains five.Map vectors to addresses to verify count if needed. Verification / Alternative check: Consult a standard 8051 datasheet or vector table; exactly five vectors exist for the listed sources. Why Other Options Are Wrong: 3 and 4: undercount by omitting legitimate sources. 6: applies to certain enhanced derivatives that include Timer 2, not the base 8051. Common Pitfalls: Assuming features of derivatives apply to all 8051 variants; conflating the serial port's receive and transmit flags with separate vectors when they share one vector. Final Answer: 5

Question 18

Address space implied by an 8-bit address bus With an 8-bit address bus, what is the accessible address range expressed in hexadecimal?

Accepted Answer

Introduction / Context: An address bus of width N bits can uniquely select 2^N addresses. Embedded developers must translate bus widths into concrete ranges to reason about memory mapping and peripheral decoding. This question checks the ability to map an 8-bit bus to the correct hexadecimal range. Given Data / Assumptions: Address bus width is 8 bits. Addresses are zero based and inclusive at both ends when stated as ranges. Hexadecimal notation is used. Concept / Approach: Two raised to the power of eight equals 256. The lowest address is 00H and the highest is FFH, giving 256 total unique locations. Wider buses such as 16-bit would yield 0000H to FFFFH. Step-by-Step Solution: Compute capacity: 2^8 = 256 addresses.Translate to hex endpoints: minimum is 00H, maximum is FFH.State the inclusive range: 00H to FFH. Verification / Alternative check: Compare with a 16-bit case: 2^16 = 65536 addresses from 0000H to FFFFH, confirming the pattern. Why Other Options Are Wrong: 0000–FFFFH: corresponds to a 16-bit bus. 000–FFFH: corresponds to a 12-bit bus. 0–FH: corresponds to a 4-bit bus. Common Pitfalls: Confusing data bus width with address bus width, or assuming decimal endpoints rather than hexadecimal. Final Answer: 00 to FFH

Question 19

8051 direct addressing to read internal RAM Which instruction copies the byte stored at internal RAM address 04H into the accumulator A?

Accepted Answer

Introduction / Context: Direct addressing allows the 8051 to access internal RAM or SFR locations by their absolute addresses. Reading a byte from a low RAM address into the accumulator requires the correct operand order and addressing form. This question tests recognition of the direct address form for a memory to register transfer. Given Data / Assumptions: Internal RAM address of interest is 04H, within the lower 32 bytes used for register banks and general RAM. Destination register is A, the accumulator. MOV instruction supports register and memory operands. Concept / Approach: To read memory into A using direct addressing, use MOV A, direct_address. To write A back to a memory location, reverse the operands. Labels such as L4 represent assembler symbols, not numeric addresses, unless explicitly defined. Step-by-Step Solution: Select operation: we need a memory to register read.Use MOV A, 04H to fetch the byte at address 04H into A.Reject forms that imply label references or a write operation. Verification / Alternative check: In a simulator, preload 04H with a known value such as 5AH and execute MOV A, 04H. The accumulator updates to 5AH, confirming the correct semantics. Why Other Options Are Wrong: MOV A, L4: depends on a label and is not guaranteed to be 04H. MOV L4, A: writes from A to memory, opposite of what is required. MOV 04H, A: also writes A to address 04H rather than reading. Common Pitfalls: Reversing operand order, forgetting that direct addresses below 80H refer to internal RAM rather than SFRs, and confusing labels with numeric addresses. Final Answer: MOV A, 04H

Question 20

8051 instruction semantics — does 'RL A' rotate the eight bits of the Accumulator left by one position? State whether the mnemonic RL A performs a one-bit left rotate confined to A (bit7 wraps to bit0, carry flag unchanged).

Accepted Answer

Introduction / Context: The 8051 family provides two left-rotate instructions for the Accumulator: RL A and RLC A. Understanding the difference is essential for bit-level operations, look-up indexing, and simple arithmetic without disturbing the carry flag. Given Data / Assumptions: Target CPU: 8051/8052 architecture Mnemonic under discussion: RL A Accumulator A is 8 bits wide (A7…A0) Carry flag CY exists in PSW but may or may not be affected depending on the instruction Concept / Approach: RL A performs a pure 8-bit rotate left within A. Bit7 moves into bit0, and all other bits shift left by one. The carry flag is not used and not updated. In contrast, RLC A rotates through the carry: bit7 goes to CY and CY enters bit0. Step-by-Step Solution: Let A = b7 b6 b5 b4 b3 b2 b1 b0RL A result = b6 b5 b4 b3 b2 b1 b0 b7 (CY unchanged)RLC A result = b6 b5 b4 b3 b2 b1 b0 CY ; CY_new = b7 Verification / Alternative check: Examine the flags after RL A; CY remains whatever it was. After RLC A, CY reflects previous b7. This confirms that RL does not use the carry path. Why Other Options Are Wrong: False: contradicts the ISA definition. Only if CY = 1: CY is irrelevant for RL. Rotates through carry: that describes RLC, not RL. Shifts left filling 0: that would be a logical shift, not a rotate. Common Pitfalls: Confusing RL with RLC and assuming CY always participates. Expecting CY to capture bit7 on RL; it does not. Final Answer: True

The 8051 Microcontroller Questions