Here, we are to see VLSI Interview Questions and answers.
Introduction
Kudos! You’ve received the most awaited call of your life to be the Lynn Conway of the VLSI world. There is just one step more between you and the seat you have been aiming for – “Interview”.
Interviews can be taxing. It’s your knowledge that’s been put to scrutiny by the interviewers, and this can be your only chance to prove to them your impression. (I don’t believe first impressions last, but interviews do need your first impression to be best).
My hack for the interview: Brush your basics along with your hair, get that lint of your advanced concepts like your clothes, and wear your knowledge as a smile. Trust me you are going to nail it.
To get you a little closer to your dream job, here are some questions that you might be asked on the day of the interview.
- Can you point out how Verilog differs from a normal coding language?
Verilog is an HDL that is a hardware descriptive language and deals with the hardware aspect of the processor. On the contrary, programming languages like C, C++, and python work on the software aspects.
The programs in Verilog are synthesized directly into hardware and often are loaded simultaneously. While in software programming languages the code is loaded one by one.
- MOSFETs operate in different regions, can you name them?
MOSFETs operate mainly in three regions, namely the Cut off region, saturation region, and Linear region.
Cut-off region: The region is void of any flow of current in the device. The device in this region can act as a basic switch and is often used for the same operation wherever necessary.
Saturation region: The saturation region is attained when the voltage in the source terminal exceeds the pinch-off voltage. This eliminates the region of contact between the source and acts as a closed switch.
The drain to source current is constant and any further enhancement in the voltage is not possible. The device is implemented where there is a need for switching performance.
Linear region: The region is responsible for the device acting as an amplifier. The current in this region increased linearly with the increase in the drain terminal.
- What are the procedural blocks in Verilog?
Verilog has two procedural blocks, initial and always.
Initial: The initial block is executed only once at zero time. It does not require triggering and often works with a single time domain.
Always: The always block incorporates a loop that executes every time a trigger occurs for that particular loop. It often is triggered with the rising edge of the clock.
- VHDL and Verilog, what should be used for better results?
VHDL is quite a verbose language compared to VLSI and has more lines of coding that resemble the C language. On the other hand, Verilog is compact and has a better grip over the hardware aspects.
For the question of what is better, both languages resemble each other and personal preference is a crucial deciding spot for an individual to decide whether they prefer VHDL or Verilog.
- Name the different skews in VLSI?
Skew is the difference in the arrival time of two clocks in a sequential circuit. Skews often differ in two categories: Local skew and Global skew.
Local skew: This is the difference in the clock signal arrival at two clock pins that are related to the same flip flop.
Global skews: Global skews deal with the difference in the clock signal arrival at two non-related flip flops.
Useful skew: The useful skew is given to calculate the delay in the original capturing paths. These could be further implemented to control the timing of the system efficiently.
They can also be further bifurcated on positive and negative skews inside the local and global skews.
- What is the ideal number of transistors in Static RAM?
SRAM often uses 6 transistors for their operations, some inventions deal with 8 as well as 4 transistors as well, but 6 is still considered to be the sweet spot for accurate working of the device.
- Could you tell us about the Antenna Effect in VLSI technology?
The antenna Effect in electronic devices deals with the excess collection of charge at the Gate terminal of the device. The wire which is diffused into the device acts as an antenna. This could occur during plasma etching and other processes in the fabrication of the device.
This could adversely affect the current-voltage characteristic of the device.
- What are FSM and name some types of FSM?
FSM or Finite State Machines are implemented in VLSI to decide the logic and to control the device. The state-space usually includes the present and the past state of the device which leads to decision-making in the future aspects of the hardware.
Talking about the different types, FSMs are classified into two categories.
Mealy state machine: The output in this state depends on present input and state. It deals with combinational logic and a memory block that combines the results to give the output at the end.
Moore state machine: The output in this state depends entirely on the present state of the machine. The device includes a combinational device and a memory device that determines the final output of the machine.
- Could you tell us the main difference between FPGA and ASIC and when you prefer which technology?
ASIC stands for Application-Specific Integrated Circuit. They are used for a single application and do not allow the user to reprogram the device.
An FPGA is a Field Programmable Field Array. They deal with general use and are implemented for a purpose of general industrial application. These can be reprogrammed in multiple ways and can be used many times as needed.
The question of when to use which technology depends upon the application one is applying the technology for. ASICs can be used for application-specific operations where there are restraints on time and power conservation. FPGAs can be used for applications where there is a constant need to reprogram the device.
- Why do you think BJTs are given an upper hand over CMOS in VLSI?
Even though CMOS has a lot more advantages over BJTs in terms of size, BJTs are preferred in some applications due to the high-speed switching of the device.
The response of BJT is quite superior to any MOS device which marks its presence in the still competitive environment.
- Name some logic gates that are primary in VLSI?
Even though VLSI has advanced to a lot of direct implementation gates, an entire device for the required operations can be derived from three basic gates: AND gate, OR gate, and NOT gate.
AND gate: The AND gate gives the logic a high output when both the logic is given at the input of the gate.
OR gate: The OR gate gives the logic 1 if any one of the input signals is 1, else 0.
NOT gate: The NOT gate gives the complement of the given input. It changes the logic 0 to 1 and 1 to 0.
- What do you understand about sequential circuits?
A sequential circuit is implemented by logical gates and takes into consideration the past inputs as well as outputs to decide the output at the present state.
- Could you elaborate on the timescale 1ns/ 1 ps in Verilog?
The timescale depicts the ratio of accuracy to the precision of the device. 1ns here is the accuracy while 1ps is the precision.
- What is the maximum number of gates that could be used in CMOS and why is it limited to that number?
The maximum number of gates in CMOS is limited to 4. The number is limited to 4 because as we keep on increasing the number of the gates the device will be slower. So the ideal number is taken to be four to conserve power as well as timing constraints of the device.
- What do you mean by SCR?
SCR, also known as Silicon Controlled Rectifier, is a rectifier that is derived from logical gates. It consists of four stacked layers that control the current flowing in the device. The device has 3 terminals for input and output.
- What are TTL chips?
TTL, Transistor transistor logic are bi-polar junction chips that are incorporated in logic designs using logic gates.
They are often kept in the control of the current passing through them with the help of various resistors inside the device.
The power consumption of TTL is very huge and thus it cannot be used in an environment where power conservation is required.
TTL has a major presence in computers.
- What are CMOS chips?
CMOS, Complementary Metal Oxide Semiconductor, is a chip designed using various FETs.
They consume extremely low power and are often implemented in devices where power needs to be conserved.
The major application includes mobile phones.
The density to accommodate the logic gates is also great in the given device.
- Suppose you come across setup and hold issues in VLSI how would you solve the violation?
To solve setup and hold violations there are a set of predefined steps that one needs to follow.
- Start with optimizing and reconstructing the logic for the flip flops used in the construction of the setup.
- Use a launch-flop to have a better grip on the clock as this will help in solving the setup violation. This allows the device to operate at faster speeds and provide lesser delays.
- Adjust the clock in such a manner that it captures the action of the flip flop with minimum delay. This can be achieved by slowing down the speed of the clock.
- Add buffers to avoid delay.
Refer to the questions above for your upcoming interview and get a step closer to your dream job.