Verilog Interview Questions

How to model Transport and Inertial Delays in Verilog?

Author : Rajesh Bawankule
Following simple example can illustrate the concept.

module delay(in,transport,inertial);
input in;
output transport;
output inertial;

reg transport;
wire inertial;

// behaviour of delays
always @(in)
begin
transport <= #10 in;
end

assign #10 inertial = in;

endmodule // delay

The timing Diagram for input and outputs

_______ __
in _____| |_____||_______

_______ __
transport _________| |_____||_____

_______
inertial _________| |____________

Non blocking assignment gives you transport delay. Whenever input changes, output is
immediately evaluated and kept in a event queue and assigned to output after specified
"transport" delay.

In Continuous assign statement the latest event overrides the earlier event in the queue.

I am attaching rudimentary testbench and its output. Hope this helps.

module test;
reg in;
wire transport, inertial;

// instantiate delay module

delay my_delay(in,transport,inertial);

// apply inputs
initial
begin
in = 0;
 #20 in = 1;
#20 in = 0;
#30 in = 1;
#5 in = 0;
#30 in = 1;
#30 $finish;
end
// monitor signals
initial
begin
$monitor($time," in = %b transport = %b inertial = %b",
in,transport, inertial);
end

endmodule // test

log file
Compiling source file "delay.v"
Highest level modules:
test

0 in = 0 transport = x inertial = x
10 in = 0 transport = 0 inertial = 0
20 in = 1 transport = 0 inertial = 0
30 in = 1 transport = 1 inertial = 1
40 in = 0 transport = 1 inertial = 1
50 in = 0 transport = 0 inertial = 0
70 in = 1 transport = 0 inertial = 0
75 in = 0 transport = 0 inertial = 0
80 in = 0 transport = 1 inertial = 0
85 in = 0 transport = 0 inertial = 0
105 in = 1 transport = 0 inertial = 0
115 in = 1 transport = 1 inertial = 1
L35 "delay.v": $finish at simulation time 135
81 simulation events

How to display the system date in $display or $write?

(Answers contributed by Swapnajit Mittra and Noman Hassan)
Support of $system() task in Verilog-XL, NC-Verilog and VCS not only allows you to display the
system date but also gives you the ability to call any command that you would normally type on
the UNIX prompt (C executable, script, standard UNIX command etc.), and would make sense in
executing from within Verilog source code.

$system is not an IEEE Standard(1364-1995), but is supported by both XL and VCS.

You could read back in the output of $system, by writing it to another file and reading it back in
using $readmemh() as illustrated in following example.

module top;

reg [23:0] today [0:1];

initial
begin
$system("date +%m%d%y > date_file");
// output is 073199 for july 31st 1999
$readmemh("date_file", today);
$display("Today is: %x", today[0]);
end
endmodule

How to display bold characters?

Using following program bold characters can be displayed. Note that this program takes help of
UNIX facilities. This may not work on PC based simulators.

module bold;

initial begin

$display ("Normal Text");

$display ("\033[1mBold Text");
$display ("\033[mSwitch back to Normal Text.....");
$display ("\033[7mInverse Text.");
$display ("\033[mSwitch back to Normal Text.....");

$display ("\033[1mBold Text \033[mfollowed by \033[7mInverse text

\033[m");
end

endmodule

Sample Verilog Questions asked in Interviews. Please contribute with your questions. If
you are looking for answers please refer to website Site FAQ

Differentiate between Inter assignment Delay and Inertial Delay.

What are the different State machine Styles ? Which is better ? Explain
disadvantages and advantages.
What is the difference between the following lines of code ?

• reg1<= #10 reg2 ;

• reg3 = # 10 reg4 ;

What is the value of Var1 after the following assignment ?

reg Var1;

initial begin

Var1<= "-"
end

In the below code, Assume that this statement models a flop with async reset. In
this, how does the synthesis tool, figure out which is clock and which is reset. Is the
statements within the always block is necessary to find out this or not ?
1 module which_clock (x,y,q,d);
2 input x,y,d;
3 output q;
4 reg q;
5
6 always @ (posedge x or posedge y)
7 if (x)
8 q <= 1'b0;
9 else
10 q <= d;
11
12 endmodule

What is the output of the two codes below ?

1 module quest_for_out();
2
3 integer i;
4 reg clk;
5
6 initial begin
7 clk = 0;
8 #4 $finish;
9 end
10
11 always #1 clk = ! clk;
12
13 always @ (posedge clk)
14 begin : FOR_OUT
15 for (i=0; i < 8; i = i + 1) begin
16 if (i == 5) begin
17 disable FOR_OUT;
18 end
19 $display ("Current i : ‰g",i);
20 end
21 end
22 endmodule

1 module quest_for_in();
2
3 integer i;
4 reg clk;
5
6 initial begin
7 clk = 0;
8 #4 $finish;
9 end
10
11 always #1 clk = ! clk;
 12
13 always @ (posedge clk)
14 begin
15 for (i=0; i < 8; i = i + 1) begin : FOR_IN
16 if (i == 5) begin
17 disable FOR_IN;
18 end
19 $display ("Current i : ‰g",i);
20 end
21 end
22 endmodule

Why cannot initial statement be synthesizeable ?

Consider a 2:1 mux; what will the output F be if the Select (sel) is "X" ?

What is the difference between blocking and nonblocking assignments ?

What is the difference between wire and reg data type ?
Write code for async reset D-Flip-Flop.
Write code for 2:1 MUX using different coding methods.
Write code for a parallel encoder and a priority encoder.
What is the difference between === and == ?
What is defparam used for ?
What is the difference between unary and logical operators ?
What is the difference between tasks and functions ?
What is the difference between transport and inertial delays ?
What is the difference between casex and case statements ?
What is the difference between $monitor and $display ?
What is the difference between compiled, interpreted, event based and cycle based
simulators ?
What is code coverage and what are the different types of code coverage that one
does ?

How do I generate clock in Verilog ?

There are many ways to generate clock in Verilog; you could use one of the following
methods:

Method #1

1 initial begin
2 clk = 0;
3 end
4
5 always begin
6 #5 clk = ~clk;
7
8 end

Method #2

1 initial begin
2 clk = 0;
3 forever begin
4 #5 clk = ~clk;
5 end
6 end

Method #3

1 initial begin
2 clk = 0;
3 end
4
5 always begin
6 #5 clk = 0;
7 #5 clk = 1;
8 end

There are many ways to generate clocks: you may introduce jitter, change duty cycle.

How do I test my design xyz ?

To test or verify or validate any design, you need to have a test bench; writing test
benches is as difficult as designing itself. Please refer to the Verilog tutorial section in
"Art of Writing Test Bench" for more details.

What is the difference between wire and reg ?

Please refer to tidbits section for the difference between wire and reg.

What is the difference between blocking and nonblocking assignment ?

Please refer to tidbits section for difference between blocking and nonblocking statement.

How do I write a state machine in Verilog ?

Please refer to tidbits section for "writing FSM in Verilog".

How do I avoid Latch in Verilog ?

Latches are always bad (I don't like that statement); latches are caused when all the
possible cases of assignment to variable are not covered. Well this rule applies to
combinational blocks (blocks with edge sensitive lists are sequential blocks); let's look at
the following example.

Bad Code

1 always @ (b or c)
2 begin
3 if (b) begin
4 a = c;
5 end
6 end

In the code above, value of a is retained, and it gets changed only when b is set to '1'. This
results in a latch. (Need to phrase it right)

Good Code #1

1 always @ (b or c)
2 begin
3 a = 0;
4 if (b) begin
5 a = c;
6 end
7 end

In the code above, no matter what the value of b is, a gets value of '0' first and if b is set
to '1' and c is set to '1', only then a gets '1'. This is the best way to avoid latches.

Good Code #2

1 always @ (b or c)
2 begin
3 if (b) begin
4 a = c;
5 end else begin
6 a = 0;
7 end
8 end
In the above code, all the possible cases are covered (i.e. b = 1 and b = 0 case).

How does this xyz code get synthesized ?

Well it is a long story; let me cover that in the synthesis part of Verilog tutorial. You can
refer to Actel HDL coding Style. One simple logic is: any code inside always blocks with
edge sensitive sensitivity list, results in flip-flops and assign; inside level sensitive always
blocks results in combo logic.

How do I implement Memories in Verilog ?

You can implement them by declaring 2-dimension arrays. More details can be found in
the Verilog tutorial section "Modeling memories and FSM".

How do I read and write from a file ?

To Read from a file we use $readmemh, where h stands for hex decimal. For writing we
use $writememh, $fdisplay, $fmonitor. You could refer to the Verilog tutorial section for
more details.

What is this `timescale compiler directive ?

`timescale is used for specifying the reference time unit for the simulator. Syntax of the
`timescale is as below:

`timescale <reference_time_unit>/<time_precision>

example : `timescale 10ns/1ns

Timescale directive tends to make more sense at gatelevel simulation than at RTL
simulation.

Can we mix blocking and nonblocking in one always block ?

Yes, we can have both blocking and nonblocking code in same always block. Some
things that one should know to use this are:

• Blocking assignments are treated as combinational logic.

• One should not assign a variable in the same always block with both blocking and
nonblocking assignments.
• Not all synthesis tools support this. (Design compiler supports this).
 What is the output of AND gate in the circuit below, when A and B are as in
waveform? Tp is the gate delay of respective gate.

Identify the circuit below, and its limitation.

What is the current through the resistor R1 (Ic) ?

 Referring to the diagram below, briefly explain what will happen if the propagation
delay of the clock signal in path B is much too high compared to path A. How do we
solve this problem if the propagation delay in path B can not be reduced ?

What is the function of a D flip-flop, whose inverted output is connected to its

input ?

Design a circuit to divide input frequency by 2.

Design a divide-by-3 sequential circuit with 50&#37; duty cycle.

 Design a divide-by-5 sequential circuit with 50&#37; duty cycle.

What are the different types of adder implementations ?

Draw a Transmission Gate-based D-Latch.

Give the truth table for a Half Adder. Give a gate level implementation of it.

What is the purpose of the buffer in the circuit below, is it necessary/redundant to

have a buffer ?

What is the output of the circuit below, assuming that value of 'X' is not known ?
 Consider a circular disk as shown in the figure below with two sensors mounted X,
Y and a blue shade painted on the disk for an angle of 45 degree. Design a circuit with
minimum number of gates to detect the direction of rotation.

Design an OR gate from 2:1 MUX.

Design an XOR gate from 2:1 MUX and a NOT gate

 What is the difference between a LATCH and a FLIP-FLOP ?

• Latch is a level sensitive device while flip-flop is an edge sensitive device.

• Latch is sensitive to glitches on enable pin, whereas flip-flop is immune to
glitches.
• Latches take less gates (also less power) to implement than flip-flops.
• Latches are faster than flip-flops.

Design a D Flip-Flop from two latches.

Design a 2 bit counter using D Flip-Flop.

What are the two types of delays in any digital system ?

Design a Transparent Latch using a 2:1 Mux.

Design a 4:1 Mux using 2:1 Muxes and some combo logic.
 What is metastable state ? How does it occur ?

What is metastability ?

Design a 3:8 decoder

Design a FSM to detect sequence "101" in input sequence.

Convert NAND gate into Inverter, in two different ways.

Design a D and T flip flop using 2:1 mux; use of other components not allowed,
just the mux.

Design a divide by two counter using D-Latch.

 Design D Latch from SR flip-flop.

Define Clock Skew , Negative Clock Skew, Positive Clock Skew.

What is Race Condition ?

Design a 4 bit Gray Counter.

Design 4-bit Synchronous counter, Asynchronous counter.

Design a 16 byte Asynchronous FIFO.

What is the difference between an EEPROM and a FLASH ?

What is the difference between a NAND-based Flash and a NOR-based Flash ?

You are given a 100 MHz clock. Design a 33.3 MHz clock with and without
50&#37; duty cycle.

Design a Read on Reset System ?

Which one is superior: Asynchronous Reset or Synchronous Reset ? Explain.

 Design a State machine for Traffic Control at a Four point Junction.

What are FIFO's? Can you draw the block diagram of FIFO? Could you modify it
to make it asynchronous FIFO ?
How can you generate random sequences in digital circuits?

Q: What is the difference between a Verilog task and a Verilog function?

A:The following rules distinguish tasks from functions:

A function shall execute in one simulation time unit;

a task can contain time-controlling statements.

A function cannot enable a task;

a task can enable other tasks or functions.

A function shall have at least one input type argument

and shall not have an output or inout type argument;
a task can have zero or more arguments of any type.

A function shall return a single value;

a task shall not return a value.

Verilog Answer 2

Q: Given the following Verilog code, what value of "a" is displayed?

always @(clk) begin

a = 0;
a <= 1;
$display(a);
end

A: This is a tricky one! Verilog scheduling semantics basically imply a

four-level deep queue for the current simulation time:
 1: Active Events (blocking statements)
2: Inactive Events (#0 delays, etc)
3: Non-Blocking Assign Updates (non-blocking statements)
4: Monitor Events ($display, $monitor, etc).

Since the "a = 0" is an active event, it is scheduled into the 1st
"queue".
The "a <= 1" is a non-blocking event, so it's placed into the 3rd queue.
Finally, the display statement is placed into the 4th queue.

Only events in the active queue are completed this sim cycle, so the "a
= 0"
happens, and then the display shows a = 0. If we were to look at the
value of
a in the next sim cycle, it would show 1.

Verilog Answer 3

Q: Given the following snipet of Verilog code,

draw out the waveforms for clk and a

always @(clk) begin

a = 0;
#5 a = 1;
end

A:

10 30 50 70 90 110 130
___ ___ ___ ___ ___ ___ ___
clk ___| |___| |___| |___| |___| |___| |___| |___

a ___________________________________________________________

This obviously is not what we wanted, so to get closer, you could use
"always @ (posedge clk)" instead, and you'd get

10 30 50 70 90 110 130
___ ___ ___ ___ ___ ___ ___
clk ___| |___| |___| |___| |___| |___| |___| |___

___ ___
a _______________________| |___________________| |_______

Verilog Answer 4

Q: What is the difference between the following two lines of Verilog

code?

#5 a = b;
a = #5 b;

A:
#5 a = b; Wait five time units before doing the action for "a = b;".
The value assigned to a will be the value of b 5 time units
hence.

a = #5 b; The value of b is calculated and stored in an internal temp

register.
After five time units, assign this stored value to a.

Verilog Answer 6

Q: What is the difference between:

c = foo ? a : b;

and

if (foo) c = a;
else c = b;

A:

The ? merges answers if the condition is "x", so for instance if foo =

1'bx, a = 'b10, and b = 'b11,
you'd get c = 'b1x.

On the other hand, if treats Xs or Zs as FALSE, so you'd always get c =

b.

(Back)

Verilog Answer 7

Q: Using the given, draw the waveforms for the following

versions of a (each version is separate, i.e. not in the same run):

reg clk;
reg a;

always #10 clk = ~clk;

(1) always @(clk) a = #5 clk;

(2) always @(clk) a = #10 clk;
(3) always @(clk) a = #15 clk;

Now, change a to wire, and draw for:

(4) assign #5 a = clk;

(5) assign #10 a = clk;
(6) assign #15 a = clk;
A:

10 30 50 70 90 110 130
___ ___ ___ ___ ___ ___ ___
clk ___| |___| |___| |___| |___| |___| |___| |___

___ ___ ___ ___ ___ ___ ___

(1)a ____| |___| |___| |___| |___| |___| |___| |_

___ ___ ___ ___ ___ ___ ___

(2)a ______| |___| |___| |___| |___| |___| |___|

(3)a __________________________________________________________

Since the #delay cancels future events when it activates, any delay
over the actual 1/2 period time of the clk flatlines...

With changing a to a wire and using assign, we

just accomplish the same thing...

10 30 50 70 90 110 130
___ ___ ___ ___ ___ ___ ___
clk ___| |___| |___| |___| |___| |___| |___| |___

___ ___ ___ ___ ___ ___ ___

(4)a ____| |___| |___| |___| |___| |___| |___| |_

___ ___ ___ ___ ___ ___ ___

(5)a ______| |___| |___| |___| |___| |___| |___|

(6)a __________________________________________________________

Vera Answer 1

Q: What is the difference between a Vera task and a Verilog task?

A:

Vera Answer 2

Q: What is the difference between running the following snipet of code

on Verilog vs Vera?

fork {
task_one();
#10;
task_one();
}

task task_one() {
 cnt = 0;
for (i = 0; i < 50; i++) {
cnt++;
}
}

A:

(Back)

Programming Answer 1

Q: Given $a = "5,-3,7,0,-5,12";

Write a program to find the lowest number in the string.

A:

// BEGIN PERL SNIPET

$a = "5,-5,-1,0,12,-3";
(@temp) = split (/,/, $a);
$lowest = $temp[0];

for ($i=0; $i<6; $i++) {

if ($temp[$i] < $lowest) { $lowest = $temp[$i]; }
}

print "Lowest value found was: $lowest\n";

// END PERL SNIPET

NOTE: You could also replace the for loop with this:

foreach $value (@temp) {

if ($value < $lowest) { $lowest = $value; }
}

Programming Answer 2

Q: Write the code to sort an array of integers.

A:

/* BEGIN C SNIPET */

void bubblesort (int x[], int lim) {

int i, j, temp;

for (i = 0; i < lim; i++) {

for (j = 0; j < lim-1-i; j++) {
 if (x[j] > x[j+1]) {
temp = x[j];
x[j] = x[j+1];
x[j+1] = temp;

} /* end if */
} /* end for j */
} /* end for i */
} /* end bubblesort */

/* END C SNIPET */

Some optimizations that can be made are that a single-element array does
not need to be sorted; therefore, the "for i" loop only needs to go from
0 to lim-1. Next, if at some point during the iterations, we go through
the entire array WITHOUT performing a swap, the complete array has been
sorted, and we do not need to continue. We can watch for this by adding
a variable to keep track of whether we have performed a swap on this
iteration.

Programming Answer 3

Q: Write the code for finding the factorial of a passed integer.

Use a recursive subroutine.

A:

// BEGIN PERL SNIPET

sub factorial {
my $y = shift;
if ( $y > 1 ) {
return $y * &factorial( $y - 1 );
} else {
return 1;
}
}

// END PERL SNIPET

Programming Answer 4

Q: In C, explain the difference between the & operator and

the * operator.

A:

& is the address operator, and it creates pointer values.

* is the indirection operator, and it dereferences pointers
to access the object pointed to.

Example:
 In the following example, the pointer ip is assigned the
address of variable i (&i). After that assignment,
the expression *ip refers to the same object denoted by i:

int i, j, *ip;
ip = &i;
i = 22;
j = *ip; /* j now has the value 22 */
*ip = 17; /* i now has the value 17 */

Programming Answer 5

Q: Write a function to determine whether a string is a palindrome (same

forward as reverse, such as "radar" or "mom").

A:

/* BEGIN C SNIPET */

#include <string.h>

void is_palindrome ( char *in_str ) {

char *tmp_str;
int i, length;

length = strlen ( *in_str );

for ( i = 0; i < length; i++ ) {
*tmp_str[length-i-1] = *in_str[i];
}
if ( 0 == strcmp ( *tmp_str, *in_str ) ) printf ("String is a
palindrome");
else printf ("String is not a palindrome");
}

/* END C SNIPET */

Programming Answer 6

Q: Write a function to output a diamond shape according to the given

(odd) input.

Examples: Input is 5 Input is 7

* *

*** ***

***** *****

*** *******

* *****
 ***

A:

### BEGIN PERL SNIPET ###

for ($i = 1; $i <= (($input * 2) - 1); $i += 2) {

if ($i <= $input) {
$stars = $i;
$spaces = ($input - $stars) / 2;
while ($spaces--) { print " "; }
while ($stars--) { print "*"; }
} else {
$spaces = ($i - $input) / 2;
$stars = $input - ($spaces * 2);
while ($spaces--) { print " "; }
while ($stars--) { print "*"; }
}
print "\n";
}
### END PERL SNIPET ###

General Answer 1

Q: Given the following FIFO and rules, how deep does the FIFO need to
be to

prevent underflowing or overflowing?

RULES:

1) frequency(clk_A) = frequency(clk_B) / 4

2) period(en_B) = period(clk_A) * 100

3) duty_cycle(en_B) = 25%

A:

Assume clk_B = 100MHz (10ns)

From (1), clk_A = 25MHz (40ns)

From (2), period(en_B) = 40ns * 100 = 4000ns, but we only output for
1000ns,
due to (3), so 3000ns of the enable we are doing no output work.

Therefore, FIFO size = 3000ns/40ns = 75 entries.

General Answer 2

Q: Draw the state diagram to output a "1" for one cycle

if the sequence "0110" shows up (the leading 0s cannot be

used in more than one sequence).

A:
General Answer 3

Q: Explain the differences between "Direct Mapped", "Fully Associative",

and "Set Associative" caches.

A:
If each block has only one place it can appear in the cache, the cache
is said to be direct mapped. The mapping is usually (block-frame
address)
modulo (number of blocks in cache).

If a block can be placed anywhere in the cache, the cache is said to be

fully associative.

If a block can be placed in a restricted set of places in the cache,

the cache
is said to be set associative. A set is a group of two or more
blocks in the cache. A block is first mapped onto a set, and then the
block
can be placed anywhere within the set. The set is usually chosen by bit
selection; that is, (block-frame address) modulo (number of sets in
cache).
If there are n blocks in a set, the cache placement is
called n-way set associative.
General Answer 4

Q: Design a four-input NAND gate using only two-input NAND gates.

A:

Basically, you can tie the inputs of a NAND gate together to get an
inverter, so...

(Back)

General Answer 5

Q: Draw the state diagram for a circuit that outputs a "1" if the
aggregate serial

binary input is divisible by 5. For instance, if the input stream

is 1, 0, 1, we

output a "1" (since 101 is 5). If we then get a "0", the aggregate
total is 10, so

we output another "1" (and so on).

A:

We don't need to keep track of the entire string of numbers - if

something
is divisible by 5, it doesn't matter if it's 250 or 0, so we can just
reset to 0.
So we really only need to keep track of "0" through "4".

From ASIC.co.in
1) Write a verilog code to swap contents of two registers with and without a
temporary register?

With temp reg ;

always @ (posedge clock)

begin
temp=b;
b=a;
a=temp;
end

Without temp reg;

always @ (posedge clock)

begin
a <= b;
b <= a;
end

2) Difference between blocking and non-blocking?(Verilog interview questions that is

most commonly asked)

The Verilog language has two forms of the procedural assignment statement: blocking
and non-blocking. The two are distinguished by the = and <= assignment operators. The
blocking assignment statement (= operator) acts much like in traditional programming
languages. The whole statement is done before control passes on to the next statement.
The non-blocking (<= operator) evaluates all the right-hand sides for the current time unit
and assigns the left-hand sides at the end of the time unit. For example, the following
Verilog program

// testing blocking and non-blocking assignment

module blocking;
reg [0:7] A, B;
initial begin: init1
A = 3;
#1 A = A + 1; // blocking procedural assignment
B = A + 1;

$display("Blocking: A= %b B= %b", A, B ); A = 3;
#1 A <= A + 1; // non-blocking procedural assignment
B <= A + 1;
#1 $display("Non-blocking: A= %b B= %b", A, B );
end
endmodule

produces the following output:

Blocking: A= 00000100 B= 00000101
Non-blocking: A= 00000100 B= 00000100

The effect is for all the non-blocking assignments to use the old values of the variables at
the beginning of the current time unit and to assign the registers new values at the end of
the current time unit. This reflects how register transfers occur in some hardware systems.

blocking procedural assignment is used for combinational logic and non-blocking

procedural assignment for sequential

Click to view more

3) Difference between task and function?

Function:
A function is unable to enable a task however functions can enable other functions.
A function will carry out its required duty in zero simulation time. ( The program time
will not be incremented during the function routine)
Within a function, no event, delay or timing control statements are permitted
In the invocation of a function their must be at least one argument to be passed.
Functions will only return a single value and can not use either output or inout
statements.

Tasks:
Tasks are capable of enabling a function as well as enabling other versions of a Task
Tasks also run with a zero simulation however they can if required be executed in a non
zero simulation time.
Tasks are allowed to contain any of these statements.
A task is allowed to use zero or more arguments which are of type output, input or inout.
A Task is unable to return a value but has the facility to pass multiple values via the
output and inout statements .

4) Difference between inter statement and intra statement delay?

//define register variables

reg a, b, c;

//intra assignment delays

initial
begin
a = 0; c = 0;
b = #5 a + c; //Take value of a and c at the time=0, evaluate
//a + c and then wait 5 time units to assign value
//to b.
end

//Equivalent method with temporary variables and regular delay control

initial
begin
a = 0; c = 0;
temp_ac = a + c;
#5 b = temp_ac; //Take value of a + c at the current time and
//store it in a temporary variable. Even though a and c
//might change between 0 and 5,
//the value assigned to b at time 5 is unaffected.
end

5) What is delta simulation time?

6) Difference between $monitor,$display & $strobe?

These commands have the same syntax, and display text on the screen during simulation.
They are much less convenient than waveform display tools like cwaves?. $display and
$strobe display once every time they are executed, whereas $monitor displays every time
one of its parameters changes.
The difference between $display and $strobe is that $strobe displays the parameters at the
very end of the current simulation time unit rather than exactly where it is executed. The
format string is like that in C/C++, and may contain format characters. Format characters
include %d (decimal), %h (hexadecimal), %b (binary), %c (character), %s (string) and
%t (time), %m (hierarchy level). %5d, %5b etc. would give exactly 5 spaces for the
number instead of the space needed. Append b, h, o to the task name to change default
format to binary, octal or hexadecimal.
Syntax:
$display (“format_string”, par_1, par_2, ... );
$strobe (“format_string”, par_1, par_2, ... );
$monitor (“format_string”, par_1, par_2, ... );

7) What is difference between Verilog full case and parallel case?

A "full" case statement is a case statement in which all possible case-expression binary
patterns can be matched to a case item or to a case default. If a case statement does not
include a case default and if it is possible to find a binary case expression that does not
match any of the defined case items, the case statement is not "full."
A "parallel" case statement is a case statement in which it is only possible to match a case
expression to one and only one case item. If it is possible to find a case expression that
would match more than one case item, the matching case items are called "overlapping"
case items and the case statement is not "parallel."

8) What is meant by inferring latches,how to avoid it?

Consider the following :

always @(s1 or s0 or i0 or i1 or i2 or i3)
case ({s1, s0})
2'd0 : out = i0;
2'd1 : out = i1;
2'd2 : out = i2;
endcase

in a case statement if all the possible combinations are not compared and default is also
not specified like in example above a latch will be inferred ,a latch is inferred because to
reproduce the previous value when unknown branch is specified.
For example in above case if {s1,s0}=3 , the previous stored value is reproduced for this
storing a latch is inferred.
The same may be observed in IF statement in case an ELSE IF is not specified.
To avoid inferring latches make sure that all the cases are mentioned if not default
condition is provided.

9) Tell me how blocking and non blocking statements get executed?

Execution of blocking assignments can be viewed as a one-step process:

1. Evaluate the RHS (right-hand side equation) and update the LHS (left-hand side
expression) of the blocking assignment without interruption from any other Verilog
statement. A blocking assignment "blocks" trailing assignments in the same always block
from occurring until after the current assignment has been completed

Execution of nonblocking assignments can be viewed as a two-step process:

1. Evaluate the RHS of nonblocking statements at the beginning of the time step. 2.
Update the LHS of nonblocking statements at the end of the time step.

10) Variable and signal which will be Updated first?

Signals

11) What is sensitivity list?

The sensitivity list indicates that when a change occurs to any one of elements in the list
change, begin…end statement inside that always block will get executed.

12) In a pure combinational circuit is it necessary to mention all the inputs in

sensitivity disk? if yes, why?

Yes in a pure combinational circuit is it necessary to mention all the inputs in sensitivity
disk other wise it will result in pre and post synthesis mismatch.

13) Tell me structure of Verilog code you follow?

A good template for your Verilog file is shown below.

// timescale directive tells the simulator the base units and precision of the simulation
`timescale 1 ns / 10 ps
module name (input and outputs);
// parameter declarations
parameter parameter_name = parameter value;
// Input output declarations
input in1;
input in2; // single bit inputs
output [msb:lsb] out; // a bus output
// internal signal register type declaration - register types (only assigned within always
statements). reg register variable 1;
reg [msb:lsb] register variable 2;
// internal signal. net type declaration - (only assigned outside always statements) wire net
variable 1;
// hierarchy - instantiating another module
reference name instance name (
.pin1 (net1),
.pin2 (net2),
.
.pinn (netn)
);
// synchronous procedures
always @ (posedge clock)
begin
.
end
// combinatinal procedures
always @ (signal1 or signal2 or signal3)
begin
.
end
assign net variable = combinational logic;
endmodule

14) Difference between Verilog and vhdl?

Compilation
VHDL. Multiple design-units (entity/architecture pairs), that reside in the same system
file, may be separately compiled if so desired. However, it is good design practice to keep
each design unit in it's own system file in which case separate compilation should not be
an issue.

Verilog. The Verilog language is still rooted in it's native interpretative mode.
Compilation is a means of speeding up simulation, but has not changed the original
nature of the language. As a result care must be taken with both the compilation order of
code written in a single file and the compilation order of multiple files. Simulation results
can change by simply changing the order of compilation.

Data types
VHDL. A multitude of language or user defined data types can be used. This may mean
dedicated conversion functions are needed to convert objects from one type to another.
The choice of which data types to use should be considered wisely, especially enumerated
(abstract) data types. This will make models easier to write, clearer to read and avoid
unnecessary conversion functions that can clutter the code. VHDL may be preferred
because it allows a multitude of language or user defined data types to be used.

Verilog. Compared to VHDL, Verilog data types a re very simple, easy to use and very
much geared towards modeling hardware structure as opposed to abstract hardware
modeling. Unlike VHDL, all data types used in a Verilog model are defined by the
Verilog language and not by the user. There are net data types, for example wire, and a
register data type called reg. A model with a signal whose type is one of the net data types
has a corresponding electrical wire in the implied modeled circuit. Objects, that is signals,
of type reg hold their value over simulation delta cycles and should not be confused with
the modeling of a hardware register. Verilog may be preferred because of it's simplicity.

Design reusability
VHDL. Procedures and functions may be placed in a package so that they are avail able
to any design-unit that wishes to use them.

Verilog. There is no concept of packages in Verilog. Functions and procedures used

within a model must be defined in the module. To make functions and procedures
generally accessible from different module statements the functions and procedures must
be placed in a separate system file and included using the `include compiler directive.

15) What are different styles of Verilog coding I mean gate-level,continuous level
and others explain in detail?

16) Can you tell me some of system tasks and their purpose?

$display, $displayb, $displayh, $displayo, $write, $writeb, $writeh, $writeo.

The most useful of these is $display.This can be used for displaying strings, expression or
values of variables.
Here are some examples of usage.
$display("Hello oni");
--- output: Hello oni
$display($time) // current simulation time.
--- output: 460
counter = 4'b10;
$display(" The count is %b", counter);
--- output: The count is 0010
$reset resets the simulation back to time 0; $stop halts the simulator and puts it in
interactive mode where the
user can enter commands; $finish exits the simulator back to the operating system

17) Can you list out some of enhancements in Verilog 2001?

In earlier version of Verilog ,we use 'or' to specify more than one element in sensitivity
list . In Verilog 2001, we can use comma as shown in the example below.
// Verilog 2k example for usage of comma
always @ (i1,i2,i3,i4)

Verilog 2001 allows us to use star in sensitive list instead of listing all the variables in
RHS of combo logics . This removes typo mistakes and thus avoids simulation and
synthesis mismatches,
Verilog 2001 allows port direction and data type in the port list of modules as shown in
the example below
module memory (
input r,
input wr,
input [7:0] data_in,
input [3:0] addr,
output [7:0] data_out
);

18)Write a Verilog code for synchronous and asynchronous reset?

Synchronous reset, synchronous means clock dependent so reset must not be present in
sensitivity disk eg:
always @ (posedge clk )

begin if (reset)
. . . end
Asynchronous means clock independent so reset must be present in sensitivity list.
Eg
Always @(posedge clock or posedge reset)
begin
if (reset)
. . . end

19) What is pli?why is it used?

Programming Language Interface (PLI) of Verilog HDL is a mechanism to interface

Verilog programs with programs written in C language. It also provides mechanism to
access internal databases of the simulator from the C program.
PLI is used for implementing system calls which would have been hard to do otherwise
(or impossible) using Verilog syntax. Or, in other words, you can take advantage of both
the paradigms - parallel and hardware related features of Verilog and sequential flow of C
- using PLI.

20) There is a triangle and on it there are 3 ants one on each corner and are free to
move along sides of triangle what is probability that they will collide?

Ants can move only along edges of triangle in either of direction, let’s say one is
represented by 1 and another by 0, since there are 3 sides eight combinations are possible,
when all ants are going in same direction they won’t collide that is 111 or 000 so
probability of collision is 2/8=1/4

21) Tell me about file I/O?

21)What is difference between freeze deposit and force?

$deposit(variable, value);
This system task sets a Verilog register or net to the specified value. variable is the
register or net to be changed; value is the new value for the register or net. The value
remains until there is a subsequent driver transaction or another $deposit task for the
same register or net. This system task operates identically to the ModelSim
force -deposit command.

The force command has -freeze, -drive, and -deposit options. When none of these is
specified, then -freeze is assumed for unresolved signals and -drive is assumed for
resolved
signals. This is designed to provide compatibility with force files. But if you prefer
-freeze
as the default for both resolved and unresolved signals.

Verilog interview Questions

22)Will case infer priority register if yes how give an example?

yes case can infer priority register depending on coding style

reg r;
// Priority encoded mux,
always @ (a or b or c or select2)
begin
r = c;
case (select2)
2'b00: r = a;
2'b01: r = b;
endcase
end

Verilog interview Questions

23)Casex,z difference,which is preferable,why?

CASEZ :
Special version of the case statement which uses a Z logic value to represent don't-care
bits. CASEX :
Special version of the case statement which uses Z or X logic values to represent don't-
care bits.

CASEZ should be used for case statements with wildcard don’t cares, otherwise use of
CASE is required; CASEX should never be used.
This is because:
Don’t cares are not allowed in the "case" statement. Therefore casex or casez are
required. Casex will automatically match any x or z with anything in the case statement.
Casez will only match z’s -- x’s require an absolute match.

Verilog interview Questions

24)Given the following Verilog code, what value of "a" is displayed?
always @(clk) begin
a = 0;
a <= 1;
$display(a);
end

This is a tricky one! Verilog scheduling semantics basically imply a

four-level deep queue for the current simulation time:
1: Active Events (blocking statements)
2: Inactive Events (#0 delays, etc)
3: Non-Blocking Assign Updates (non-blocking statements)
4: Monitor Events ($display, $monitor, etc).
Since the "a = 0" is an active event, it is scheduled into the 1st "queue".
The "a <= 1" is a non-blocking event, so it's placed into the 3rd queue.
Finally, the display statement is placed into the 4th queue. Only events in the active
queue are completed this sim cycle, so the "a = 0" happens, and then the display shows a
= 0. If we were to look at the value of a in the next sim cycle, it would show 1.

25) What is the difference between the following two lines of Verilog code?
#5 a = b;
a = #5 b;

#5 a = b; Wait five time units before doing the action for "a = b;".
a = #5 b; The value of b is calculated and stored in an internal temp register,After five
time units, assign this stored value to a.

26)What is the difference between:

c = foo ? a : b;
and
if (foo) c = a;
else c = b;

The ? merges answers if the condition is "x", so for instance if foo = 1'bx, a = 'b10, and b
= 'b11, you'd get c = 'b1x. On the other hand, if treats Xs or Zs as FALSE, so you'd
always get c = b.

27)What are Intertial and Transport Delays ??

28)What does `timescale 1 ns/ 1 ps signify in a verilog code?

'timescale directive is a compiler directive.It is used to measure simulation time or delay

time. Usage : `timescale / reference_time_unit : Specifies the unit of measurement for
times and delays. time_precision: specifies the precision to which the delays are rounded
off.
29) What is the difference between === and == ?

output of "==" can be 1, 0 or X.

output of "===" can only be 0 or 1.
When you are comparing 2 nos using "==" and if one/both the numbers have one or more
bits as "x" then the output would be "X" . But if use "===" outpout would be 0 or 1.
e.g A = 3'b1x0
B = 3'b10x
A == B will give X as output.
A === B will give 0 as output.
"==" is used for comparison of only 1's and 0's .It can't compare Xs. If any bit of the
input is X output will be X
"===" is used for comparison of X also.

30)How to generate sine wav using verilog coding style?

A: The easiest and efficient way to generate sine wave is using CORDIC Algorithm.

31) What is the difference between wire and reg?

Net types: (wire,tri)Physical connection between structural elements. Value assigned by a

continuous assignment or a gate output. Register type: (reg, integer, time, real, real time)
represents abstract data storage element. Assigned values only within an always statement
or an initial statement. The main difference between wire and reg is wire cannot hold
(store) the value when there no connection between a and b like a->b, if there is no
connection in a and b, wire loose value. But reg can hold the value even if there in no
connection. Default values:wire is Z,reg is x.

32 )How do you implement the bi-directional ports in Verilog HDL?

module bidirec (oe, clk, inp, outp, bidir);

// Port Declaration
input oe;
input clk;
input [7:0] inp;
output [7:0] outp;
inout [7:0] bidir;
reg [7:0] a;
reg [7:0] b;
assign bidir = oe ? a : 8'bZ ;
assign outp = b;
// Always Construct
always @ (posedge clk)
begin
b <= bidir;
a <= inp;
end
endmodule

33)How to write FSM is verilog?

there r mainly 4 ways 2 write fsm code

1) using 1 process where all input decoder, present state, and output decoder r combine in
one process.
2) using 2 process where all comb ckt and sequential ckt separated in different process
3) using 2 process where input decoder and persent state r combine and output decoder
seperated in other process
4) using 3 process where all three, input decoder, present state and output decoder r
separated in 3 process.

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34)what is verilog case (1) ?

wire [3:0] x;
always @(...) begin
case (1'b1)
x[0]: SOMETHING1;
x[1]: SOMETHING2;
x[2]: SOMETHING3;
x[3]: SOMETHING4;
endcase
end
The case statement walks down the list of cases and executes the first one that matches.
So here, if the lowest 1-bit of x is bit 2, then something3 is the statement that will get
executed (or selected by the logic).

35) Why is it that "if (2'b01 & 2'b10)..." doesn't run the true case?

This is a popular coding error. You used the bit wise AND operator (&) where you meant
to use the logical AND operator (&&).

36)What are Different types of Verilog Simulators ?

There are mainly two types of simulators available.

Event Driven
Cycle Based
Event-based Simulator:

This Digital Logic Simulation method sacrifices performance for rich functionality: every
active signal is calculated for every device it propagates through during a clock cycle.
Full Event-based simulators support 4-28 states; simulation of Behavioral HDL, RTL
HDL, gate, and transistor representations; full timing calculations for all devices; and the
full HDL standard. Event-based simulators are like a Swiss Army knife with many
different features but none are particularly fast.

Cycle Based Simulator:

This is a Digital Logic Simulation method that eliminates unnecessary calculations to

achieve huge performance gains in verifying Boolean logic:

1.) Results are only examined at the end of every clock cycle; and
2.) The digital logic is the only part of the design simulated (no timing calculations). By
limiting the calculations, Cycle based Simulators can provide huge increases in
performance over conventional Event-based simulators.
Cycle based simulators are more like a high speed electric carving knife in comparison
because they focus on a subset of the biggest problem: logic verification.
Cycle based simulators are almost invariably used along with Static Timing verifier to
compensate for the lost timing information coverage.

37)What is Constrained-Random Verification ?

Introduction

As ASIC and system-on-chip (SoC) designs continue to increase in size and complexity,
there is an equal or greater increase in the size of the verification effort required to
achieve functional coverage goals. This has created a trend in RTL verification
techniques to employ constrained-random verification, which shifts the emphasis from
hand-authored tests to utilization of compute resources. With the corresponding
emergence of faster, more complex bus standards to handle the massive volume of data
traffic there has also been a renewed significance for verification IP to speed the time
taken to develop advanced testbench environments that include randomization of bus
traffic.

Directed-Test Methodology

Building a directed verification environment with a comprehensive set of directed tests is

extremely time-consuming and difficult. Since directed tests only cover conditions that
have been anticipated by the verification team, they do a poor job of covering corner
cases. This can lead to costly re-spins or, worse still, missed market windows.

Traditionally verification IP works in a directed-test environment by acting on specific

testbench commands such as read, write or burst to generate transactions for whichever
protocol is being tested. This directed traffic is used to verify that an interface behaves as
expected in response to valid transactions and error conditions. The drawback is that, in
this directed methodology, the task of writing the command code and checking the
responses across the full breadth of a protocol is an overwhelming task. The verification
team frequently runs out of time before a mandated tape-out date, leading to poorly tested
interfaces. However, the bigger issue is that directed tests only test for predicted behavior
and it is typically the unforeseen that trips up design teams and leads to extremely costly
bugs found in silicon.

Constrained-Random Verification Methodology

The advent of constrained-random verification gives verification engineers an effective

method to achieve coverage goals faster and also help find corner-case problems. It shifts
the emphasis from writing an enormous number of directed tests to writing a smaller set
of constrained-random scenarios that let the compute resources do the work. Coverage
goals are achieved not by the sheer weight of manual labor required to hand-write
directed tests but by the number of processors that can be utilized to run random seeds.
This significantly reduces the time required to achieve the coverage goals.

Scoreboards are used to verify that data has successfully reached its destination, while
monitors snoop the interfaces to provide coverage information. New or revised
constraints focus verification on the uncovered parts of the design under test. As
verification progresses, the simulation tool identifies the best seeds, which are then
retained as regression tests to create a set of scenarios, constraints, and seeds that provide
high coverage of the design.