VLSI Design Viva Voice Short Questions and Answers
VLSI Design Viva Voice Short Questions and Answers
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| VLSI Design Viva Voice Short Questions and Answers |
1.What is demarcation line?
The Demarcation line is an imaginary line used in stick diagram, to separate p-MOS and n-MOS transistors. All p-MOS transistors are placed above demarcation line and n-MOS below the demarcation line
2. What are the two types of Layout design rules?
Lambda design rules and micron rules are the major types of layout design rules.
3. What is Lay-out design rule?
The rules followed to prepare the photo mask are known as Layout design rules.
4. What are LVS and DRL tools?
LVS means Layout Versus Schematic. It checks the layout against schematic diagram. It is very important to verify layout.
DRC means Design Rule Checker. This tool checks every occurrence of the design rule list on layout. Width, spacing of every metal line in the layout is checked with this tool.
5. What is instance? What is instancing?
To construct big, complex circuit, the basic cells (small cells) can be copied. This process is known as Instancing. The cell which is copied is known as Instance.
6. What is flat cell?
The cell which is independent and not related to other objects is known as flat cell.
7. Give the advantages of CMOS IC?
• Size is less
• High Speed
• Less Power Dissipation
8. What are four generations of Integrated Circuits?
• SSI (Small Scale Integration)
• MSI (Medium Scale Integration)
• LSI (Large Scale Integration)
• VLSI (Very Large Scale Integration)
9. Give the variety of Integrated Circuits?
• More Specialised Circuits
• Application Specific Integrated Circuits (ASICs)
• Systems-On-Chips
10. Why NMOS technology is preferred more than PMOS technology?
N-channel transistors have greater switching speed when compared to PMOS transistors. Hence, NMOS is preferred than PMOS.
11. what is DIGITAL GATE?
Digital gates are basically electronic components which are used for switching and manipulating binary data
12 what do you mean by a universal gate?
The universal gates are those gate from which we can make any gate by using them. The universal gates are NAND & NOR
13 what is a truth table?
Truth table is a table from which we can get o/p of different gates
14.Make truth table of NAND gate?
A
B
O/P
0
0
1
0
1
1
1
0
1
1
1
0
15. What is different between Ex-or & Ex-nor gate?
The basic difference between this two gate is that Ex-or gate gives o/p when both the i/p is different & Ex-nor gate give o/p when both i/p same.
16. What is the Universal gates?
The universal gates are the gate by the help of which we can make other all gates.
17. What is/are universal gate(s)?
There are two universal gates:-
1. NAND
2. NOR
18. How would you make a NOT gate from a NOR gate?
19. How would you make an OR gate from a NOR gate?
20. Draw the NAND gate by using the NOR gate?
21. what is the use of half adder?
It is used for adding 2 bit data
22. in Half adder how many inputs are used?
Two
23. In o/p of Half adder what we get?
SUM, CARRY
24. in Half adder SUM=?
SUM= A (+) B.
25. In half adder Carry=?
-AB
26. how many AND gate(s) required to make a Half adder?
1 one
27. In Half adder how many types of gates are required?
two types NAND & Ex-or
28. What is the difference between the half and full adder?
In half adder only 2 bits can be used, but in full adder we can use 3 bit data.
29. What is the difference between half adder and half subs tractor?
The only difference is in carry & borrows expression.
30. what is use of Full adder?
Full adder is used for adding three bit data.
31.In full adder how many inputs are used?
Three
32. In o/p of full adder what we get?
SUM & Carry
33. In full adder SUM=?
SUM= A (+) B (+) C.
34. What is the use of Full Sub tractor?
Full sub tractor is used for differentiating three bit data.
35 . In Full Subs tractor how many inputs are used?
three
36. In output of Full Subs tractor what we gate?
Difference & borrow
37 .In Full Subs tractor Difference=?
Difference=A (+) B (+) C. Borrow=A’B+A’C+BC?
38 . how many NAND gates required to make a Full Subs tractor?
Nine Q. In Full Subs tractor how many Half Subtractor are required? A:- two
39. What is the difference between full & half substractor?
In half Subtractor we can subtract only 2 bit data, but in full Subtractor we can subtract 3 bit data.
40. what is the use of half Subtractor?
It is used for subtract 2 bit data.
41. in Half Subs tractor how many inputs are used?
Two
42 In o/p of Half Subs tractor what we gate?
Difference & Borrow
43. in Half Subs tractor Difference=?
Difference=A (+) B. Borrow=A’B?
44. how many AND gate required to make a Half Subs tractor?
one
45. in Half Subs tractor how many types of gates are required?
Two ANAD & Ex-or
46. What is latch?
The latch is asynchronous, and the outputs can change as soon as the inputs do (or at least after a small propagation delay)
47 What is flip-flop?
Flip-flop is a 1 bit storing element.
48 How many types of flip-flop are used?
4 types of flip –flop, S-R, JK,D, T
49. What is disadvantage of SR flip-flop?
When both the input is one then it gives invalid output.
50. What is disadvantage of JK flip-flop?
Race around condition.
51 To remove race around condition what we use?
Master slave
52 What is race around condition?
when pulse width is more then signal width then for signal change of pulse width many no of times signal changes its state that is called race around condition.
53.what are the characteristic equation for T flip-flop?
Q = TQ’+ QT’
54.which Gates are used in SR flip flops to a JK?
Nand Gates
55.D flip-flop is usedfor?
Providing delay.
56.what is full form of flipflop?
Toggle flip-flop
57 what is flip-flop?
Flip-flop is a 1 bit storing element.
58 how many types of flip-flop are used?
4 types of flip –flop, S-R, J-K, D, T
59. what is the disadvantage of SR flip-flop?
When both the input is one, then it gives invalid output.
60. what is the disadvantage of JK flip-flop?
race around condition
61. to remove race around condition what we use?
master slave Flip-flop.
62. what is a race around condition?
when pulse width is more than signal width then for signal change of pulse width many no of times signal changes its state that is called race around condition.
63. What is the characteristic equation for T flip-flop?
Q = TQ’+ QT’
64. Which Gates are used in SR flip flops to a JK flip-flop?
NAND Gates
65. D flip-flop is used for?
providing delay.
66. What is the full form of T flip-flop?
toggle flip-flop
67. What is the operation of D flip-flop?
In D flip-flop during the occurrence of clock pulse if D=1, the output Q is set and if D=0,the output is reset.
68. Explain the flip-flop excitation tables for D flip-flop
In D flip-flop the next state is always equal to the D input and it is independent of the present state.
Therefore D must be 0 if Qn+1 has to 0, and if Qn+1 has to be 1 regardless the value of Qn.
69. What do you mean by present state?
The information stored in the memory elements at any given time define.s the present state of the sequential circuit.
70. What do you mean by next state?
The present state and the external inputs determine the outputs and the next state of the sequential circuit.
71. State the types of sequential circuits?
1. Synchronous sequential circuits 2. Asynchronous sequential circuits
72. What is a counter?
a counter is a device which stores (and sometimes displays) the number of times a particular event or process has occurred, often in relationship to a clock signal.
72 what is the types of counter?
In practice, there are two types of counters:
1. Up counters, which increase (increment) in value
2. Down counters, which decrease (decrement) in value
73. what is the basic type of counter made by flip-flop or resistor?
Asynchronous (ripple) counter – changing state bits are used as clocks to
subsequent state flip-flops
Synchronous counter – all state bits change under control of a single clock
Decade counter – counts through ten states per stage
Up–down counter – counts both up and down, under command of a control input
Ring counter – formed by a shift register with feedback connection in a ring
Johnson counter – a twisted ring counter
Cascaded counter
74. What is asynchronous counter?
An asynchronous (ripple) counter is a single K-type flip-flop, with its J (data) input fed from its own
inverted output. This circuit can store one bit, and hence can count from zero to one before it overflows (starts over from 0). This counter will increment once for every clock cycle and takes two clock cycles to overflow, so every cycle it will alternate between a transition from 0 to 1 and a transition from 1 to 0.
Notice that this creates a new clock with a 50% duty cycle at exactly half the frequency of the input clock. If this output is then used as the clock signal for a similarly arranged D flip-flop (remembering to invert the output to the input), you will get another 1 bit counter that counts half as fast.
75. make diagram of Asynchronous counter?
76. what is Synchronous counter?
A simple way of implementing the logic for each bit of an ascending counter (which is what is depicted in the image to the right) is for each bit to toggle when all of the less significant bits are at a logic high state. For example, bit 1 toggles when bit 0 is logic high; bit 2 toggles when both bit 1 and bit 0 are logic high; bit 3 toggles when bit 2, bit 1 and bit 0 are all high; and so on.
Synchronous counters can also be implemented with hardware finite state machines, which are more complex but allow for smoother, more stable transitions.
77. what is the ring counter?
A ring counter is a shift register (a cascade connection of flip-flops) with the output of the last one connected to the input of the first, that is, in a ring. Typically a pattern consisting of a single 1 bit is circulated, so the state repeats every N clock cycles if N flip-flops are used. It can be used as a cycle counter of N states.
78. what is the Johnson counter?
A Johnson counter (or switch tail ring counter, twisted-ring counter, walking-ring counter, or Moebius counter) is a modified ring counter, where the output from the last stage is inverted and fed back as input to the first stage. A pattern of bits equal in length to twice the length of the shift register thus circulates indefinitely.
These counters find specialist applications, including those similar to the decade counter, digital to analog conversion, etc. it can be established by D flip flop and JK flip flop.
79. what is the decade counter?
A decade counter is one that counts in decimal digits, rather than binary. A decade counter may have each digit binary encoded (that is, it may count in binary-coded decimal, as the 7490 integrated circuit did) or other binary encoding (such as the bi-unary encoding of the 7490 integrated circuit).
Alternatively, it may have a "fully decoded" or one-hot output code in which each output goes high in turn; the 4017 is such a circuit. The latter type of circuit finds applications in multiplexers and demultiplexers, or wherever a scanning type of behaviour is useful. Similar counters with different numbers of outputs are also common. The decade counter is also known as a mod-counter.
80. what do you mean by up down counter?
A counter that can change state in either direction, under the control of an up–down selector input, is known as an up–down counter. When the selector is in the up state, the counter increments its value; when the selector is in the down state, the counter decrements the count Machine cycle- the time taken by data/ opcode / operand from memory/ peripheral devices to acknowledge the external hardware. It takes 1to 6 T-state.
81. Write short note on Frequency Counter?
A frequency counter is a digital instrument that can be used to measure the frequency of any periodic waveform.
82. What is the difference between primary & secondary storage device?
A. In primary storage device the storage capacity is limited. It has a volatile memory. In secondary storage device the storage capacity is larger. It is a nonvolatile memory. Primary devices are: RAM / ROM. Secondary devices are: Floppy disc / Hard disk.
83. Difference between static and dynamic RAM?
Static RAM: No refreshing, 6 to 8 MOS transistors are required to form one memory cell, Information stored as voltage level in a flip flop.
Dynamic RAM: Refreshed periodically, 3 to 4 transistors are required to form one memory cell; Information is stored as a charge in the gate to substrate capacitance.
84. What is cache memory?
Cache memory is a small high-speed memory. It is used for temporary storage of data & information between the main memory and the CPU (central processing unit). The cache memory is only in RAM.
85. What is called “Scratch pad of computer”?
Cache Memory is scratch pad of computer.
86. Differentiate between RAM and ROM?
RAM: Read / Write memory, High Speed, Volatile Memory. ROM: Read only memory, Low Speed, Non-Volatile Memory.
86. What is stack?
Stack is a portion of RAM used for saving the content of Program Counter and general purpose registers
87. Can ROM be used as stack?
ROM cannot be used as stack because it is not possible to write to ROM.
88. What is NV-RAM?
Nonvolatile Read Write Memory, also called Flash memory. It is also known as shadow RAM.
89. What is cache memory?
Cache memory is a small high-speed memory. It is used for temporary storage of data & information between the main memory and the CPU (central processing unit). The cache memory is RAM.
90. Define address of a memory.
The location of a unit of data in a memory is called address.
91. What is Read and Write operation?
The Write operation stores data into a specified address into the memory and the Read operation takes data out of a specified address in the memory.
92. Why RAMs are called as Volatile?
RAMs are called as Volatile memories because RAMs lose stored data when the power is turned OFF.
93. Define ROM.
ROM is a type of memory in which data are stored permanently or semi permanently. Data can be read from a ROM, but there is no write operation.
94. Define RAM.
RAM is Random Access Memory. It is a random access read/write memory. The data can be read or written into from any selected address in any sequence.
95. Define Static RAM and dynamic RAM.
Static RAM use flip flops as storage elements and therefore store data indefinitely as long as dc power is applied. Dynamic RAMs use capacitors as storage elements and cannot retain data very long without capacitors being recharged by a process called refreshing.
96. List the two types of SRAM.
Asynchronous SRAMs and Synchronous Burst SRAMs
97. List the basic types of DRAMs.
Fast Page Mode DRAM, Extended Data Out DRAM (EDO DRAM), Burst EDO DRAM and Synchronous DRAM.
98. Define a bus.
A bus is a set of conductive paths that serve to interconnect two or more functional components of a system or several diverse systems.
99. Define Cache memory.
A. It is a relatively small, high-speed memory that can store the most recently used instructions or data from larger but slower main memory.
100. What is the technique adopted by DRAMs.
DRAMs use a technique called address multiplexing to reduce the number of address lines.
101. what do you mean by operational –amplifier?
an operational amplifier is a direct- coupled, high gain amplifier used for some mathematical-operation such as addition, subtraction, multiplication and integration.
102.List the ideal characteristics of an op-amp?
An ideal-opamp would have the following characteristics:-
1 Infinite voltage gain
2 Infinite input resistance
3 Zero o/p resistance
4 Zero o/p voltage when (i/p voltage is zero)
5 Infinite Band with
6 infinite common mode Rejection ratio
7 infinite slew rates.
103. A Differential Amplifier should have collector resistor’s value (RC1 & RC2) as
a) 5kΩ, 5kΩ
b) 5Ω, 10kΩ
c) 5Ω, 5kΩ
d) 5kΩ, 10kΩ
a The values of collector current will be equal in differential amplifier (RC1=RC2).
104. What is the purpose of the Differential Amplifier ?
The purpose of the differential amplifier is to amplify the difference between two signals.
105 If output is measured between two collectors of transistors, then the Differential amplifier with two input signal is said to be configured as Dual Input Balanced Output
When two input signals are applied to base of transistor, it is said to be Dual Input. When both collectors are at same DC potential with respect to ground, then it is said to be Balance Output.
106. A differential amplifier is capable of amplifying
AC & DC input signal Direct connection between stages removes the lower cut off frequency imposed by coupling capacitor; therefore it can amplify both AC and DC signal.
107. In ideal Differential Amplifier, if same signal is given to both inputs, then output will be
ZERO: In ideal amplifier, Output voltage ⇒ Vout = Vin1-Vin2.
108. When a differential amplifier is operated single-ended,
One input is grounded and signal is applied to the other
109. What is a ring Oscillator ?
A. Ring oscillator is a device composed of an odd number of NOT gates in a ring, whose output oscillates between two voltage levels, representing true and false. The NOT gates, or inverters, are attached in a chain and the output of the last inverter is fed back into the first.
109. Explain the commonly used technique to estimate the delay time of a CMOS inverter.
As the delay time of an inverter is very small, it cannot be measured accurately using conventional method with the help of an oscilloscope. Delay is usually measured by realizing a ring oscillator using a large number of inverters, say 101. Then the frequency of oscillation is measured using the expression where n is the number of inverters and td is the delay time.
110. What is Jitter?
Period of ring oscillator vibrates in a random manner T=T+T' where T' is a random value. In high-quality circuits, the range of T' is relatively small compared to T. This variation in oscillator period is called jitter.
111. What are the applications of Ring Oscillators?
- The voltage-controlled oscillator in most phase-locked loops is built from a ring oscillator.
- Jitter of ring oscillators is commonly used in hardware random number generators.
- A ring oscillator is often used to demonstrate a new hardware technology, analogous to the way a hello world program is often used to demonstrate a new software technology.
- Many wafers include a ring oscillator as part of the scribe line test structures. They are used during wafer testing to measure the effects of manufacturing process variations.
- Ring oscillators can also be used to measure the effects of voltage and temperature on a chip.
