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Top 20 VLSI Interview Questions and Answers for 2022
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VLSI Design Interview Questions
1. What Are Four Generations Of Integration Circuits?
2. Give The Advantages Of Ic?
3. Give The Variety Of Integrated Circuits?
4. Give The Basic Process For Ic Fabrication?
5. What Are The Various Silicon Wafer Preparation?
6. Different Types Of Oxidation?
7. What Is The Transistors Cmos Technology Provides?
N-type transistors & p-type transistors.
8. What Are The Different Layers In Mos Transistors?
9. What Is Enhancement Mode Transistor?
The device that is normally cut-off with zero gate bias.
10. What Is Depletion Mode Device?
The Device that conduct with zero gate bias.
11. When The Channel Is Said To Be Pinched – Off?.
The Inversion layer will be drained by this voltage if a high Vds is applied. This Voltage effectively pinches off the channel near the drain.
12. Give The Different Types Of Cmos Process?
13. What Are The Steps Involved In Twin-tub Process?
14. What Are The Advantages Of Silicon-on-insulator Process?
15. What Is Bicmos Technology?
It is the combination of bipolar technology & CMOS technology.
16. How Does The Bicmos Process Work? Additional masks defining the P base region
17. What Are The Advantages Of Cmos Process?
18. What Is The Fundamental Goal In Device Modeling?
to determine the functional relationship between the model-relevant device’s terminal electrical variables
19. Define Short Channel Devices?
Short channel devices are transistors with channel lengths of less than three to five microns. The ratio of the lateral and vertical dimensions is lowered with short channel devices.
20. What Is Pulling Down Device?
A pull-down device is one that is connected to lower supply voltage, typically 0V, in order to pull the output voltage to that level.
21. What Is Pulling Up Device?
A pull up device is one that is connected to raise the supply voltage, typically VDD, from the output voltage.
22. Why Nmos Technology Is Preferred More Than Pmos Technology?
When compared to PMOS transistors, the switching speed of N-Channel transistors is faster.
23. What Are The Different Operating Regions Foes An Mos Transistor?.
24. What Are The Different Mos Layers?
25. What Is Stick Diagram?
It makes use of color coding to communicate information. Also it is the cartoon of a chip layout.
26. What Are The Uses Of Stick Diagram?
27. Give The Various Color Coding Used In Stick Diagram?
28. Compare Between Cmos And Bipolar Technologies?
CMOS Technology:Low static power dissipation. High input impedance (low drive current). Scalable threshold voltage. High noise margin. High packing density. High delay sensitivity to load (fanout limitations). Low output drive current. Low gm (gm a VIN). Bidirectional capability. A near ideal switching device.
Bipolar technology:High power dissipation. Low input impedance (high drive current). Low voltage swing logic. Low packing density. Low delay sensitivity to load. High output drive current. High gm (gm an eVin). High ft at low current. Essentially unidirectional.
29. Define Threshold Voltage In Cmos?
The voltage applied between a MOS transistor’s gate and source that causes the drain to source current, IDS, to effectively drop to zero is known as the threshold voltage, or VT.
30. What Is Body Effect?
The threshold voltage VT is not a constant w. r. the voltage difference between the MOS transistor’s source and substrate This effect is called substrate-bias effect or body effect.
31. What Is Channel-length Modulation?
No matter what voltage is applied to the terminals, the current flowing between the drain and source terminals remains constant. This is not entirely correct. The applied VDS actually affects the conductive channel’s effective length; as VDS rises, the depletion region at the drain junction expands and the effective channel length decreases.
32. What Is Latch – Up?
When parasitic elements cause the formation of low resistance conducting paths between VDD and VSS, a condition known as latch up occurs, which has disastrous effects. Careful control during fabrication is necessary to avoid this problem.
33. Define Rise Time?
The time it takes for a waveform to increase from 10% to 90% of its steady-state value is known as the rise time (tr).
34. Define Fall Time?
A waveform’s fall time, tf, is the amount of time it takes for it to go from 90% to 10% of its steady-state value.
35. Define Delay Time?
The time interval between the 50% input transition and the 50% output level is known as the delay time (td). The amount of time needed for a logic transition to go from input to output is shown here.
36. What Are Two Components Of Power Dissipation?
The amount of power dissipated in a CMOS circuit is determined by two factors. These are: .
- Static dissipation brought on by leakage current or another continuous current drawn from the power source
- Dynamic dissipation due to. Switching transient current. Charging and discharging of load capacitances.
37. Name A Few Important CAD Tools? A few essential CAD tools are:
-
- Layout editors
- Design Rule checkers (DRC)
- Circuit extraction
38. What Is Verilog?
Verilog is a general purpose hardware descriptor language. It is similar in syntax to the C programming language. At many different levels of abstraction, from the algorithmic level to the switch level, it can be used to model a digital system.
39. What Are The Various Modeling Used In Verilog?
- Gate-level modeling
- Data-flow modeling
- Switch-level modeling
- Behavioral modeling
40. What Is The Structural Gate-level Modeling?
Digital logic networks are described by structural modeling in terms of their constituent parts. The foundation of gate-level modeling is the use of simple logic gates and the specification of their wiring.
41. What Is Switch-level Modeling?
Switch-level modeling in Verilog is possible and is based on MOSFET behavior. Using MOSFET switches, digital circuits at the MOS-transistor level are described.
42. What Are Identifiers?
Module, variable, and other object names that can be used in the design are known as identifiers. Upper- and lower-case letters, the digits 0 through 9, the underscore character (_), and the dollar sign ($) make up identifiers. It must be a single group of characters. Examples: A014, a, b, in_o, s_out.
43. What Are The Value Sets In Verilog?
Verilog offers four levels of support for value sets, which are used to describe hardware. Value levels Condition in hardware circuits:
44. What Are The Types Of Gate Arrays In Asic?
- Channeled gate arrays
- Channel less gate arrays
- Structured gate arrays
45. Give The Classifications Of Timing Control?Methods of timing control:
-
- Delay-based timing control
- Event-based timing control
- Level-sensitive timing control
Types of delay-based timing control:
-
-
- Regular delay control
- Intra-assignment delay control
- Zero delay control
-
Types of event-based timing control:
-
-
-
- Regular event control
- Named event control
- Event OR control
- Level-sensitive timing control
-
-
46. What Are Gate Primitives?
Verilog supports basic logic gates as predefined primitives. The primitive logic function keyword offers the fundamentals for gate-level structural modeling. These primitives are instantiated similarly to modules but do not require a module definition because they are predefined in verilog. And, nand, or, xor, xnor, and buf (non-inverting drive buffer) are crucial operations.
47. Give The Two Blocks In Behavioral Modeling?
- Initial conditions and sequential data flow are set up by an initial block, which runs only once during the simulation.
- During the simulation, an always block runs in a loop and repeatedly.
48. What Kinds Of Conditional Statements Are There? If ([expression]) true – statement is the syntax;
One else statement:Syntax: if ([expression]) true – statement; else false-statement;
Nested if-else-if:
Syntax:
- If statement 1 is true, then statement 2 is true, then statement 3 is true, and statement 4 is the default statement;
The [expression] is evaluated. The true-statement is carried out if it is true (1 or a non-zero value). The false statement is executed if it is false (zero) or ambiguous (x).
49. Name The Types Of Ports In Verilog?
50. What Are The Types Of Procedural Assignments?
51. Give The Different Types Of Asic?1. Full custom ASICs 2. Semi-custom ASICs:
-
- Standard cell based ASICs.
- Gate-array based ASICs.
3. Programmable ASICs:
-
- Programmable Logic Device (PLD).
- Field Programmable Gate Array (FPGA).
52. What Is The Full Custom Asic Design?
An engineer creates all of the logic cells, circuits, or layout for a single ASIC when creating a fully customized ASIC. Only if there are no suitable existing cell libraries that can be used for the entire design does this course of action make sense.
53. What Is The Standard Cell-based Asic Design?
Standard cells are predesigned logic cells that are used in a cell-based ASIC (CBIC). In a CBIC, rows of standard cells are used to construct the standard cell areas, also known as flexible blocks. Only the location of the standard cells and the interconnect are specified by the ASIC designer in a CBIC. Every layer of a CBIC’s mask is unique to and customized for a specific customer.
54. Differentiate Between Channeled & Channel Less Gate Array?Channeled Gate Array: .
Only the interconnect is customized. The interconnect uses predefined spaces between rows of base cells. Routing is done using the spaces. Only the top few layers of the gate array’s mask are customized, resulting in a lower logic density. No predefined areas are set aside for routing between cells. Routing is done using the area of transistors unused. Logic density is higher.
55. What Does A 22V10 I/O Cell Consist Of? A 22V10 I/O cell consists of:
- A register
- An output 4:1 mux
- A tristate buffer
- A 2:1 input mux
It has the following characteristics:
- Product time 9 10 12 14 16 14 12 10 824 pins12 inputs10 I/Os
56. What Is A Fpga?
A programmable logic device called a field programmable gate array (FPGA) enables the implementation of relatively large logic circuits. A CPLD can implement circuits with up to about 20,000 equivalent gates, whereas FPGAs can implement logic circuits with more than 20,000 gates.
57. What Are The Different Techniques For Programming PALs? There are three main techniques for programming PALs:
58. What Is An Antifuse?
An antifuse is normally high resistance (>100MW). When the proper programming voltages are applied, the antifuse is permanently changed to a low-resistance structure (200–500W).
59. Which Levels Of Design Abstraction Are Used In Physical Design?
60. What Are Macros?
A gate-array library’s logic cells are frequently referred to as macros.
61. What Is Programmable Interconnects?
A PAL’s device is programmed by altering the switching element’s properties. An alternative would be to program the routing.
62. Give The Steps In Asic Design Flow?
63. List the gradations at which a chip can be tested.
64. What Are The Categories Of Testing?
65. Write Notes On Functionality Tests?
Functionality tests verify that the chip performs its intended function. These tests claim that the chip’s entire set of gates work together to perform the desired function. These tests are typically used early in the design cycle to confirm the circuit’s functionality.
66. Write Notes On Manufacturing Tests?
Manufacturing inspections confirm that each gate and register on the chip operates properly. These tests are used to ensure that the silicon is intact after the chip is manufactured.
67. Mention The Defects That Occur In A Chip?
68. Give Some Circuit Maladies To Overcome The Defects?
69. What Are The Tests For I/o Integrity?
70. What Is Meant By Fault Models?
A fault model describes how faults happen and what effect they have on circuits.
71. Give Some Examples Of Fault Models?
72. What Is Stuck – At Fault?
This model models a defective gate input as being “stuck at zero” or “stuck at one.” The most common causes of these faults are thin-oxide shorts or metal-to-metal shorts.
73. What Is Meant By Observability?
The degree to which a specific internal circuit node can be observed at an integrated circuit’s outputs is known as the node’s observability.
74. What Is Meant By Controllability?
An internal circuit node’s controllability in a chip is a gauge of how simple it is to change the node’s state from 0 to 1.
75. What Is Known As Percentage-fault Coverage?
The percentage-fault coverage is the total number of nodes that, when set to 1 or 0, do result in the detection of the fault, divided by the total number of nodes in the circuit.
76. What Is Fault Grading?
Fault grading consists of two steps. First, the node to be faulted is selected. Without adding any faults, a simulation is run, and the results are saved. The test vector set is applied after setting each node or line to be faulted to 0 and then 1. The simulation is terminated and the fault is said to have been discovered if and when there is a difference between the response of the defective circuit and the response of the intact circuit.
77. Mention The Ideas To Increase The Speed Of Fault Simulation?.
78. What Is Fault Sampling?
An approach to fault analysis is known as fault sampling. When it is impossible to fault every node in a circuit, this is used. Nodes are randomly selected and faulted. The number of faults found in the fault set and the size of the set can be used to statistically infer the resulting fault detection rate. The randomly selected faults are unbiased. It will decide if the fault coverage is greater than the desired level.
79. What Are The Approaches In Design For Test Ability?
80. Mention The Common Techniques Involved In Ad Hoc Testing?
81. What Are The Scan-based Test Techniques?
82. What Are The Two Tenets In Lssd?
The circuit is level-sensitive. Each register may be converted to a serial shift register.
83. What Are The Self-test Techniques?
84. What Is Known As Bilbo?
The scan technique can be combined with signature analysis to produce a structure known as BILBO, which stands for Built in Logic Block Observation.
85. What Is Known As Iddq Testing?
IDDQ, or current supply monitoring, is a well-liked technique for bridging fault testing. This is predicated on the fact that complementary CMOS logic gates do not draw any DC current when they are not switching. For some combinations of input conditions, a measurable DC IDD will flow when a bridging fault occurs.
86. What Are The Applications Of Chip Level Test Techniques?
87. What Is Boundary Scan?
System designers came to an agreement on a unified scan-based methodology for testing chips at the board due to the growing complexity of boards and the adoption of technologies like multichip modules and surface-mount technologies. This is called boundary scan.
88. What Is The Test Access Port (TAP)? The Test Access Port (TAP) is the interface that must be present in an integrated circuit (IC) in order for it to be used in boundary-scan architecture. There are four or five single bit connections on the port, as shown below:
It also has an optional signal:
89. What Does The Test Architecture Include? The test architecture includes:
90. What Is The Tap Controller?
The 16-state FSM used by the TAP controller switches between states based on the TCK and TMS signals. It provides signals for the instruction register and test data registers. These include serial-shift clocks and update clocks.
91. What Is Known As Test Data Register?
The inputs of the modules to be tested are set using the test-data registers, and the output of tests is collected using the test-data registers.
92. What Is Known As Boundary Scan Register?
A data register’s special case is the boundary scan register. It enables testing of the connections between circuit boards, testing of external components, and sampling of the state of chip digital I/Os.
Explain how binary number can give a signal or convert into a digital signal?Binary number consists of either 0 or 1, in simple words number 1 represents the ON state and number 0 represents OFF state. These binary numbers can combine billion of machines into one machines or circuit and operate those machines by performing arithmetic calculations and sorting operations.
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Explain how logical gates are controlled by Boolean logic?In Boolean algebra, the
These are the basic three types of gates where Boolean logic work, apart from these, other gates that are functional works with the combination of these three basic gates, they are XNOR gate, NAND gate, Nor gate and XOR gate.
