Digital Circuits - Online Test
Q1. The clock frequency of an 8085 microprocessor is 5 MHz. If the time required to execute an instruction is 1.4 μs, then the number of T-states needed for executing the instruction is
Answer : Option C
Explaination / Solution:
Q2. 
In the latch circuit shown, the NAND gates have non-zero, but unequal propagation delays. The present input condition is: P = Q = '0'. If the input condition is changed simultaneously to P = Q = '1', the outputs X and Y are
Answer : Option B
Explaination / Solution:
Unequal propagation delay
Case I: Case II:
Gate 1⟶2ns Gate 1⟶1nsec
Gate 2⟶1ns Gate 2⟶2nsec
Either x = 1, y = 0 or x = 0, y =1
Q3. 
A finite state machine (FSM) is implemented using the D flip-flops A and B, and logic gates, as shown in the figure below. The four possible states of the FSM are QA QB = 00,01,10 and 11.
Assume that XIN is held at a constant logic level throughout the operation of the FSM. When the FSM is initialized to the state QA QB = 00 and clocked, after a few clock cycles, it starts cycling through
Answer : Option D
Explaination / Solution:
In given diagram
When Xin = 0 2 State
When Xin =1 3 State
Q4. The following FIVE instructions were executed on an 8085 microprocessor.
MVI A, 33H
MVI B, 78H
ADD B
CMA
ANI 32H
The Accumulator value immediately after the execution of the fifth instruction is
Answer : Option B
Explaination / Solution:
MVI A, 33H A⟵33H
MVI B, 78H B⟵78H
ADD B B⟵ABH
CMA A⟵54H
ANI 32H A⟵10H
A⟶0011 0011 A⟶1010 1011 0101 0100
B⟶0111 1000 B⟶0101 0100 0011 0010
1010 1011 0001 0000
Q5.
Which one of the following gives the simplified sum of products expression for the Boolean function F =
m0 + m2 + m3 + m5, where m0,
m2, m3
and m5 are minterms corresponding to the inputs A, B and C with A as the MSB and C as the LSB?
and m5 are minterms corresponding to the inputs A, B and C with A as the MSB and C as the LSB?
Answer : Option B
Explaination / Solution:
F =
m0 + m2 + m3 + m5, ⟶ minterm
Q6. In an 8085 microprocessor, the shift registers which store the result of an addition and the overflow bit are, respectively
Answer : Option B
Explaination / Solution:
The shift registers A and F store the result of an addition and the overflow bit.
Q7. 
In the circuit shown below, the op-amp is ideal, the transistor has VBE = 0.6 V and β= 150. Decide whether the feedback in the circuit is positive or negative and determine the voltage V at the output of the op-amp.
Answer : Option D
Explaination / Solution:
The circuit is shown in fig below
Q8. A 3-input majority gate is defined by the logic function M(a,b,c) = ab + bc + ca. Which one of the following gates is represented by the function 
Answer : Option B
Explaination / Solution:
3 input majority gate is given as
M(a,b,c) = ab + bc + ca
We have to obtain 
We obtain truth table for the function as
So, the function is odd number of 1’s detector. This function represent the 3-input XOR gate.
Q9. The Boolean expression 
converted into the canonical product of sum (POS) form is
converted into the canonical product of sum (POS) form is
Answer : Option A
Explaination / Solution:
We have the SOP Boolean form,
Hence, in POS form, we have
Q10. If X = 1 in logic equation 
Then
Then
Answer : Option D
Explaination / Solution:
Substituting X = 1 and
we get
1 + A = 1 and 0 + A = A
Substituting X = 1 and
we get 1 + A = 1 and 0 + A = A