This set of 8051 Micro-controller Multiple Choice Questions & Answers (MCQs) focuses on “Interrupt Programming”.
1. When any interrupt is enabled, then where does the pointer moves immediately after this interrupt has occurred?
a) to the next instruction which is to be executed
b) to the first instruction of ISR
c) to the first location of the memory called the interrupt vector table
d) to the end of the program
Explanation: When any interrupt is enabled, then it goes to the vector table where the address of the ISR is placed.
2. What are the contents of the IE register, when the interrupt of the memory location 0x00 is caused?
Explanation: When interrupt of 0x00 is caused (the reset interrupt) then all the other interrupts will be disabled or the contents of the IE register becomes null.
3. After RETI instruction is executed then the pointer will move to which location in the program?
a) next interrupt of the interrupt vector table
b) next instruction of the program after the IE instruction
c) next instruction after the RETI in the memory
d) none of the mentioned
Explanation: When the RETI instruction is executed, it will execute the instruction present at the top of the stack (which is the PC’s value i.e after the interrupt enable instruction).
4. Which pin of the external hardware is said to exhibit INT0 interrupt?
a) pin no 10
b) pin no 11
c) pin no 12
d) pin no 13
Explanation: INT0 interrupt is caused when pin no 12 in the hardware of the 8051 controller is enabled with a low levelled pulse.
5. Which bit of the IE register is used to enable TxD/RxD interrupt?
Explanation: IE.D4 is used to enable RS interrupt or the serial communication interrupt.
6. Which of the following combination is the best to enable the external hardware interrupt 0 of the IE register (assuming initially all bits of the IE register are zero)?
c) any of the mentioned
d) both of the mentioned
Explanation: For executing the EX0 interrupt, the EX0 and EA bits of the IE register should be set. EA is set to enable all the interrupts and EX0 is set to enable the external hardware interrupt 0 interrupt and mask the other enabled interrupts.
7. Why normally LJMP instructions are the topmost lines of the ISR?
a) so as to jump to some other location where there is a wider space of memory available to write the codes
b) so as to avoid overwriting of other interrupt instructions
c) both of the mentioned
d) none of the mentioned
Explanation: There is a small space of memory present in the vector table between two different interrupts so in order to avoid overwriting of other interrupts we normally jump to other locations where a wide range of space is available.
8. Which register is used to make the pulse a level or a edge triggered pulse?
Explanation: TCON register is used to make any pulse level or edge triggered one.
9. What is the disadvantage of a level triggered pulse?
a) a constant pulse is to be maintained for a greater span of time
b) difficult to analyse its effects
c) it is difficult to produce
d) another interrupt may be caused, if the signal is still low before the completion of the last instruction
Explanation: In a level triggered pulse, if the signal does not becomes high before the last instruction of the ISR, then the same interrupt will be caused again, so monitoring of pulse is required for a level triggered pulse.
10. What is the correct order of priority that is set after a controller gets reset?
a) TxD/RxD > T1 > T0 >EX1 > EX0
b) TxD/RxD < T1 < T0<EX1 < EX0
c) EX0 > T0 > EX1 >T1> TxD/RxD
d) EX0 < T0 < EX1 < T1 < TxD/RxD
Explanation: EX0 >T0 > EX1> T1>TxD/RxD. This is the correct order of priority that is set after a controller gets reset.
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