verilog interview questions answers






What is difference betweenandin verilog?
There are two types of shift operators, the logical shift operators, and , and the arithmetic shift operators, and . The left shift operators, and , shall shift their left operand to the left by the number by the number of bit positions given by the right operand. In both cases, the vacated bit positions shall be ?lled with zeroes. The right shift operators, and , shall shift their left operand to the right by the number of bit positions given by the right operand. The logical right shift shall ?ll the vacated bit positions with zeroes. The arithmetic right shift shall ?ll the vacated bit positions with zeroes if the result type is unsigned. It shall ?ll the vacated bit positions with the value of the most-signi?cant (i.e., sign) bit of the left operand if the result type is signed. If the right operand has an unknown or high impedance value, then the result shall be unknown. The right operand is always treated as an unsigned number and has no effect on the signedness of the result. In this example, the reg result is assigned the binary value 0100, which is 0001 shifted to the left two positions and zero-?lled.
module sh; reg [3:0] st, result; initial begin st = 1; result = (st2); end endmodule
In this example, the reg result is assigned the binary value 1110, which is 1000 shifted to the right two positions and sign-?lled.
module shift; reg signed [3:0] st, result; initial begin st = 4’b1000; result = (st2); end endmodule
What are different types of looping statements in verilog?
There are four types of looping statements. These statements provide a means of controlling the execution of a statement zero, one, or more times. forever- Continuous execution a statement. repeat- Executes a statement a ?xed number of times. If the expression evaluates to unknown or high impedance, it shall be treated as zero, and no statement shall be executed. while- Executes a statement until an expression becomes false. If the expression starts out false, the statement shall not be executed . for- Controls execution of its associated statement(s) by a three-step process, as follows: a) Executes an assignment normally used to initialize a variable that controls the number of loops executed. b) Evaluates an expression-if the result is zero, the for-loop shall exit, and if it is not zero, the for-loop shall execute its associated statement(s) and then perform step c. If the expression evaluates to an unknown or high-impedance value, it shall be treated as zero. c) Executes an assignment normally used to modify the value of the loop-control variable, then repeats step b. what are different types of Procedural timing controls in verilog? different types of Procedural timing controls in verilog are - A delay control, which is introduced by the symbol # - Anevent control, which is introduced by the symbol- The wait statement, which operates like a combination of the event control and the while loop what is actually meant by a high impedance state as in digital circuits?
In a Digital circuit, A Tri State logic gate would have 3 output states, high, low and high Impedance State. In a high impedance state the output acts like a resistor with infinite resistance, which means that the output is disconnected from the rest of the circuit. An example of a situation where these gates are used is, when you try to tie 2 outputs together, at any time only one output should be connected. At such a time the other output can go into high impedance.
Why sub threshold current is there in CMOS?
1) A MOS biased in weak inversion is said to be operating in sub-threshold region; 2) The ideal current-voltage relationship are not valid from 0VgsVt, the so-called sub-threshold region, normally there can be no current in the MOS region for VgsVt which is not correct. MOS experiences weak inversion when the surface potential is greater than the Fermi potential and in this condition, there exists a channel beneath the oxide interface, a tiny channel that can conduct current although VgsVt. 3) In the sub threshold region, the conduction mechanism is based on diffusion rather than drift.
What is PRBS generator?
Pseudo Random binary sequences are useful in many areas of digital design and science. Among these are encryption, CDMA code generation, and verification and maintenance of digital devices. A compact way to generate a PRBS is to use a shift register with linear feedback (Linear feed back shift registers) you can refer to: http://vnsnes.freeshell.org/lfsr/LFSR.html
List some of Tool used to do PNR?
P&R tools are many. EDA vendors like Synopsys, Cadence, Magma and so on… are the leading players in the field. Astro/Apollo/PhysicalCompiler : Synopsys Silicon Ensemble : Cadence Blast Fusion : Magma Silicon Ensemble : Cadence is pretty outdated tool in the Silicon Industry Cadence has Released Nanoroute /SoC Encounter as their Next Generation Physical Design Platform tools. Please visit cadence.com to know more about the tool.
What are the 5 factors taken into consideration while choosing an fpga device?
The factors are: 1. The voltage (2.5v, 3.3v) 2. The density (no of slices) 3. The no of I/O pins. 4. The package type. 5. The internal memory. I would worry about these conditions 1. OTP (one time programmable) or reprogrammable 2. Number of gates and I/O s. 3. Package type, size etc. 4. Radiation hardened or not (because most of the FPGA applications are for space). 5. Cost
What is significance of latches and flip-flops?
They are the heart of any design Latches: Level sensitive. Utilizes only 60% of a flop area. Less delay. Time borrowing support (borrowing time from the previous stage) Almost asynchronous design. Flops: Edge sensitive. Complete synchronous design. No time borrowing. Sensitive to glitches
Design a circuit that can divide clock rate half but "without using flip-flops". In other words it must be design by using combinational logic.?
The method involves use of a 2:1mux and an inverter. The select signal of the 2:1 mux is the CLK. One input to the mux (when select is 0) is the output of the mux. And the other input to the mux (when the select is 1) is the inverted signal of the output of the mux. There might be many other ways of doing it..Please let me know.
Can this function glitch: A.B + A . C + B.C, If so, can it be reduced?
This expression (A.B + A . C + B.C) will not produce any glitch. If u solve this exp. using K map, u will find that the term "B.C" is redundant here. This term (B.C) is added to the expression to suppress the glitch.
What is a State Machine?
A finite state machine (FSM) is a mathematical model of a system that attempts to reduce the model complexity by making simplifying assumptions. Specifically, it assumes · The system being modeled can assume only a finite number of conditions, called states. · The system behavior within a given state is essentially identical. · The system resides in states for significant periods of time. · The system may change these conditions only in a finite number of well-defined ways, called transitions. · Transitions are the response of the system to events. · Transitions take (approximately) zero time. A Mealy Finite State Machine (FSM) associates actions with the transitions between the states. A Mealy FSM can use different transitions to the same state and execute different actions. A Moore FSM associates actions with the states themselves rather than the transitions. In general, Moore FSMs require more states to model the same system. A Moore FSM must use different states to represent conditions in which different actions are performed. State Machines are circuits that depends on the previous input states and the current input to the states.
What are combinational and Sequential Circuits?What is a flip-flop?
A combinational system is one that has its output depending on the inputs. A sequential system is one that has its output depending on the inputs and the current state. Combinational Circuit: The outputs of a combinational system are functions of only the present circuit inputs. Sequential Circuit: The outputs are functions of the present circuit inputs and also the present state or the past sequence of inputs. Employs Feedback in the circuit. Flip-flop: Binary storage device used to store a bit of information and it is a sequential device that samples its inputs and produces its outputs on a clock transition (either posedge or negedge). Built from latches
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