verilog FAQ

1. What is the race condition in verilog?

Ans :The situation when two expressions are allowed to execute at same instance of time without mentioning the order of execution.

2. List the levels of abstraction in verilog?

Ans : 1. Behavioral level 2. Register-Transfer level 3. Gate level 4. Switch levels

3. Which are the two types of design methodologies?

Ans : Top down and Bottom up

4. Num = 'bX; What will be the value of Num?

Ans : 8'bxxxx_xxxx

5. What are the differences between wire and register?

Ans : Wire just transfer input to output, Register stores the value as a variable until or unless another value replaces it.

6. What are the differences between logical AND and reduction AND?

Ans : Reduction and takes one operand and perform a bit by next bit operation.

7. What is the difference between swapping the contents of two registers with temp register and without temp register?

Ans : We use temp register in case of blocking statements to swap the registers contents. We wont use temp register for non blocking          statements.

8. How logical shift differs from arithmetic shift?

Ans : In logical shift the MSB will be accumulated by zero's In arithmetic shift MSB will be accumulated by sign bit.

9. What is the conditional operator and how it is used?

Ans : Operator which assigns any one variable out of two by evaluating the expression mentioned in that operator. Syntax : cond_expr?          true_expr:false_expr

10. What is the difference between “==” and “===”?

Ans : “==” it is logical equality.
“===” it is case equality.

11. What exactly expression reg[8*13:1] string_val; signifies?

Ans : Reg can hold up to 13 characters.

12. When output port is generated by a submodule what will be the type of output
a. register    b. wire

Ans : b

13. List the built in primitives?

Ans : gates, transmission gates and switches.

14. Name the two possible ways by which we can instantiate the modules?

Ans : Port order connection and Port name connection.

15. Which assignment statement we usually use in dataflow modeling?

Ans : A continuous assignment statement.

16. What are rise delay and turn off delays?

Ans :The rise delay is associated with a gate output transition to 1 from another value(0,x,z).
Turn off delay associated with a gate transition to Z from value (0,1,x).

17. What is inter delay? explain with an example

Ans : Delay where command waits for particular time steps before executing that command.
ex: #10 a=b+c;

18. What is intra delay? explain with an example

Ans : Here the evaluated expression waits for specified time to assign value to LHS.
ex : a = #10 b+c;

19. What are the difference between inter and intra delays?

Ans : Inter delay simply wait for appropriate no of time steps before executing the command.
ex : #10 q = x + y;
Intra delay wait for appropriate no of time steps before assignment of RHS to LHS.
ex : q = #10 x + y;

20. Which is the stable one in the following
a. #10 q=x+y;    b. q= #10 x+y;

Ans : b

21. What are the differences between continuous and procedural assignments?

Ans : Difference between continuous and procedural assignments.
Continuous assignments procedural assignments
1 Assigns primarily to net Assigns primarily to reg
2 Values continuously drive to out put Values will be stored into variables
3 Occurs in assignments to wire,port and net Occurs in constructs like always,initial,task,function.

22. What are the differences between assignment in initial and always blocks?

Ans : Assignment in initial Assignment in always
1. Execute from time 0 in simulation and proceed in the specified sequence. This also begin from time 0, and repeat forever as a function            of the changes on sensitivity list.
2. Execution stops when the end of the block is reached. Execution continuously repeats according to the change in values of sensitivity            list.
3. Non-synthasizable Synthasizable

23. What are the differences between blocking and non blocking statements?

Ans : Blocking Non blocking
1. The evaluation of the expression of the RHS is updated to LHS autonomously based on delay. But here RHS will not be updated to LHS            immediately
2. In case of multiple blocking assignments the trailing assignments are blocked. Multiple blocking assignments can be scheduled to occur            concurrently on next evaluation cycle.
3. There is a possibility of race condition in this. Race condition can be avoided.
4 Represented by “=” operator sign between LHS and RHS. Represented by “< =”operator sign between LHS and RHS.

24. What are the differences between Task and Functions?

Ans : Task Function
1. Can contain time control statements like @(posedge). Executes in zero simulation time.
2. Can call any number of function or tasks. Can call any number of functions but not tasks.
3. Cannot return any value when called instead the task can have output arguments. It can return any value when called.

25. How re-entrant task,function differs from static task,functions ?

Ans : Re-entrant task,functions have a key word automatic between task and name of the task which replicates and allocates the variables. But          it is not possible in static.

26. How we can convert static task and function to re-entrant task and function?

Ans : By adding key word automatic between task and name of the task.

27. What is an effect of keyword automatic for re-entrant task?

Ans : It replicates the variables and allocates the variables in task.

28. What is an fork--join exactly mean?

Ans : fork--join groups two or more statements together in parallel, so that all statements are evaluated concurrently.

29. Which assignment statement will be used in Behavioral modeling?

Ans : Procedural assignment statements.

30. Always and initial blocks are called ______ blocks.

Ans : Procedural

31. What is sensitivity list?

Ans : It is an event timing control that controls when all statements in the procedural block will start to be evaluated.

32. What are procedural assignment statements?

Ans : The statements within a procedure which executes sequentially.

33. How we can avoid race condition?

Ans : By using non blocking statements.

34. How we can represent a blocking and non blocking assignments?

Ans : Blocking can be represented by wire.
Non blocking can be represented by latch.

35. List process synchronization supported by verilog?

Ans : event, fork and join, disable.

36. What actually event does?

Ans : It suspends the process until specified event to occur.

37. What wait does actually?

Ans : It suspends the process until expression become true.

38. What forever loop does?

Ans : this loop executes continuously and never completes until you break it intentionally.

39. When generate statements are used?

Ans : Generate statements are used when the same operation or module instance is repeated for multiple bits of vector.

40. List out the different methods to create generate statement?

Ans : Generate loop, Generate conditional, Generate case.

41. Which are the two types of UDP's in verilog?

Ans : 1. Combinational UDP
2. Sequential UDP

42. Why RTL synthesis is important?

Ans : It is important to improve designers productivity to meet today's design complexity.

43. What is the difference between always with @ and always without @?

Ans : The only difference is always with @ can be synthesized but not other one.
The always without @ will be used only in test benches.

44. What is the difference between $display and $monitor?

Ans : $display display its parameter whenever that is executed. $monitor display its parameter whenever the value of parameter changes.

45. Write a code for 2:1 Mux?

Ans : assign mux_out = (sel)? din_1 : din_0;

46. Write a Verilog code which divides the clock by 2?

Ans : always @ (posedge clk_in)
if (reset)
clk_out <= 1'b0;
else
clk_out <= ! clk_out ;

47. Write a code which converts 4 bit binary to gray code?

Ans : assign out = { binary[3]),(binary[3] ^ binary[2]), (binary[2] ^
binary[1]),(binary[1] ^ binary[0]) };

48. Write a code for one hot counter?

Ans : always @ (posedge clk)
if (reset)
out <= 8'b0000_0001 ;
else if (enable)
out <= {out[6],out[5],out[4],out[3],
out[2],out[1],out[0],out[7]};

49. Write a code for 16 bit counter?

Ans : always @(posedge clk)
if (reset)
out <= 16'b0 ;
else if (enable)
out <= out + 1;

50. Write a code for 8 bit parity checker?

Ans : assign parity_out = ^data_in; // data_in is 8 bits

51. Write a code for 8 bit shift-left register?

Ans : always @(posedge Clock)
begin
register <= register << 1;
register[0] <= Input;
end
assign output = register[7];

52. Write a code for 8 bit Unsigned adder?

Ans : assign {Carry_Out,SUM} = input_A + input_B + Carry_In;

53. Write a code for comparator?

Ans : assign out = (in_1 >= in_2)? 1'b1:1'b0;

54. When we use FSMs?

Ans : FSMs are widely used in applications that require prescribed sequential activity.

55. Difference between `define and parameter?

Ans : Define Parameter only one constant with the same name can exist in the whole scope multiple modules can have same parameter name cannot be overridden can be overridden used to specify macro used to specify constants

56. What value is inferred when multiple procedural assignments made to the same reg variable in an always block?

Ans : When there are multiple nonblocking assignments made to the same reg variable in a sequential always block, then the last assignment is picked up for logic synthesis.

57. What are “snake” paths?

Ans : A snake path, as the name suggests is a path that traverses through a number of hierarchies, and may eventually return back to the same hierarchy from which it originated.

58. What is “constant propagation”? How can I use constant propagation to minimize area?

Ans : Constant propagation is a very effective technique for area minimization, since it forces the synthesis tools to optimize the logic in both forward and backward directions. Since the area minimization is achieved using constants, this technique is called constant propagation.

59. What happens to the bits of a reg which are declared, but not assigned or used?

Ans : When any of the bits of a reg declaration is unused, the logic corresponding to those bits gets optimized

60. Why we use `ifdef and generate for in verilog?

Ans : Both `ifdef and generate constructs can be used for the purpose of area minimization.

61. What is the difference between using `ifdef and generate?

Ans : `ifdef generate we can use it inside the module as well as outside the module we can only use it inside the module works on the boolean presence or absence of `define of the `ifdef variable use the value of a variable using genvar, can be used inside loops used only in if-else and cannot perform any looping operation the genvar variable can be used inside for loop or case statements

62. What is “retime” logic between registers?

Ans : Retiming is the process of relocating registers across logic gates, without affecting the underlying combinatorial logic structure. This process is achieved by borrowing logic from one time frame and lending it to the other, while maintaining the design behavior.

63. Why one-hot encoding is preferred for FSMs?

Ans : Since there is one explicit FF per stage of a one-hot encoded state machine, there is no need of output state decoding. Hence, the only anticipated delay is the clock to q delay of the FF. This makes the one-hot encoding mechanism preferable for high-speed operation.

64. What are the main factors that affect testability of a design?

Ans : Reset of a FF driven by the output of another FF Presence of tri-state buses in the design Presence of derived clocks in the design Presence of gated clocks in the design Presence of latches in the design

65. What are the various methods to reduce power during RTL coding?

Ans : Reduce switching of the data input to the Flip-Flops. Reduce the clock switching of the Flip-Flops. Have area reduction techniques within the chip, since the number of gates/Flip-Flops that toggle can be reduced.

66. What is the advantage of using hierarchical names to refer to Verilog objects?

Ans : It is easy to debug the internal signals of a design, especially if they are not a part of the top level pinout.

67. What are the disadvantages of using hierarchical name to refer to Verilog objects?

Ans : During synthesis, these hierarchical names get ungrouped or dissolved or renamed, depending upon the synthesis strategy and switches used, and hence, will cease to exist. In that case, special switches need to be added to the synthesis compiler commands, in order to maintain the hierarchical naming. If the Verilog code needs to be translated into VHDL, the hierarchical names are not translatable.

68. What is the effect of specifying delays in assignment’s during synthesis?

Ans : Specifying any kind of delay before an assignment, or within an assignment, in a blocking or non-blocking procedural assignment is ignored by synthesis tools. If the functionality depends upon the presence of the delay, then a mismatch in functional simulation will be seen between the model and the synthesized netlist. Ex: reg1 = #3 reg2; // #3 will be ignored #6 reg3 <= reg4; // #6 will be ignored Since the above construct is syntactically legal, the synthesis tools will issue a WARNING and not an ERROR.

69. What is the synthesized hardware for the verilog code below?
module generator_ex1(data, select, out);
input [0:7] data;
input [0:2] select;
out out;
assign out = data [select];
endmodule

Ans: 8:1 Mux

70. What is the synthesized hardware for the verilog code below?
module generate_ex2(out,in,select);
input En;
input [0:1] select;
output [0:3] out;
assign out = En? (1 << select) : 4’b0;
endmodule

Ans: 2:4 Decoder

71. How to avoid unintentional latches in the design?

Ans : Completely specify all branches for every case and if statement. Completely specify all outputs for every case and if statement. Use synopsys full-case if all desired cases have specified.

72. What is a "full" case statement?

Ans : 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 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.

73. What is a "parallel" case statement?

Ans : 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.

74. Pros and cons of latch and Flip-Flop?

Ans : Latch takes less area, consume less power, facilitate time borrowing or cycle stealing, not friendly with DFT tools
always @(clk) begin
IF (clk == 1’b1)
q <= d;
end
Flip-flop takes more area, consumes more power, allow synchronous logic, friendly with DFT tools.
D-Flip-flop:
always @(posedge clk)
begin
if(reset)
q<=1'b0;
else
q <= d;
end

75. Write a verilog code for D flip flop with Synchronous reset?

Ans : D type flip flop with synchronous reset
reg q;
always @ (posedge clk)
if (reset)
q <= 1'b0;
else
q <= d;

76. Write a verilog code for D flip flop with asynchronous Reset?

Ans : D type flip flop with asynchronous reset
reg q;
always @ (posedge clk or posedge reset)
if (reset)
q <= 1'b0;
else
q <= d;

77. Write a verilog code for D flip flop with gated clock?

Ans : D type flip flop with gated clock
reg q;
wire gtd_clk = enable && clk;
always @ (posedge gtd_clk)
q <= d;

78. What are the guidelines for coding priority encoders?

Ans : Non-parallel case statements infer priority encoders. It is a poor coding practice to code priority encoders using case statements. It is better to code priority encoders using if-else-if statements.
Code all intentional priority encoders using if-else-if statements. It is easier for a typical design engineer to recognize a priority encoder when it is coded as an if-else-if statement.
Case statements can be used to create tabular coded parallel logic. Coding with case statements is recommended when a truth-table-like structure makes the Verilog code more concise and readable.
Examine all synthesis tool case-statement reports.
Change the case statement code, as outlined in the above coding guidelines, whenever the synthesis tool reports that the case statement is not parallel (whenever the synthesis tool reports "no" for "parallel_case").

79. What are the limitations of using tri-state logic?

Ans : The presence of internal tri-state logic is a critical consideration for power sensitive products. Normally a multiplexer should be used in place of tri-state logic. However, if the tri-state logic remains in the RTL, it is not an error for compilation. Synthesis tools sometimes warn the users. The linting tools also detect this condition, and report this to the user.

80. What is the effect of specifying a function without a range?

Ans : If a range is not specified, Verilog will assume a 1 bit return value. If a multi-bit return value was calculated in the function, only the least significant bit is returned.

81.How to selectively enable or disable monitoring?

Ans : $monitor can be selectively enabled or disabled by the $monitoron and the $monitoroff system calls, respectively. The $monitoron and $monitoroff system calls affect only the most recent call to $monitor.

82. What is the main limitation of fork-join in Verilog,

Ans : The main limitation of fork-join construct in Verilog is that it is static, that is, the execution of the code beyond the join is suspended until all the processes within the fork-join are completed.

83. What are the differences and similarities between the logical (<<, >>) and the arithmetic (<<<, >>>) shift operators?

Ans : The logical shift operators are (<< and >>). The logical shift operator has been present from Verilog-1995. The arithmetic shift operators are (<<< and >>>), which were introduced with Verilog-2001. Three of them, that is, logical left shift (<<), arithmetic left shift (<<<) and logical shift right(>>) operators, shift the bits left/right by the number of bit positions specified by the right operand, and the vacated bits are filled with zeros. The arithmetic right shift operator (>>>) will fill the vacated bits with 0 if the left operand is unsigned, and the most significant bit if the left operand is signed.

84. What is the difference between the logical (==) and the case (===) equality operators?

Ans : The “==” are synthesizable while “===” operators are not synthesizable.

85. What is the difference between assign-deassign and force-release?

Ans : Force - release can be applicable to nets and variables, whereas assign - deassign is applicable only to variables.

86. What is a critical path in a design?

Ans : There can be more than one critical path in a design. A critical path is the path through a circuit that has the least slack. Need not necessarily the longest path in the design.

87. If there are only inputs and no output what it will be synthesized?

Ans : A module with only inputs and no outputs will synthesize into a module with no logic, since there is nothing to be synthesized as an output.

88. what is the difference between casex and casez?

Ans : Casex – treats x,z both as don’t cares.

89. Write a Verilog code for ring counter?

Ans : module ring_counter(clk, initial_count, count);
input clk, initial_count;
output reg [7:0] count;
always @ (posedge clk)
begin
if (initial_count)
count = 8’b00000000;
else
count = {count (6:0), count[7]};
endmodule

90. What is `timescale?

Ans : It is compiler directive which indicates the time unit to be used for delays specified in the testbench.

91. What are different types of Verilog simulators?

Ans : Event-based Simulator:
Event base Simulation method sacrifices performance for rich functionality.
Every active signal is calculated for every device it propagates through during a clock cycle.
Full timing calculations for all devices and the full HDL standard.
Cycle Based Simulator:
Cycle based Simulation method eliminates unnecessary calculations to achieve huge performance gains in verifying Boolean logic.
At the end of every clock cycle results are examined.
The digital logic is the only part of the design simulated. These simulators are mainly meant for logic verification, no timing calculations. By limiting these calculations, cycle based Simulators can provide huge increases in performance over Event-based simulators.
Static Timing analysis is used to compensate for the lost timing information coverage.

92. How we can use Verilog function to define the width of a multi-bit port, wire, or reg type?

Ans : The width elements of ports, wire or reg declarations require a constant in both MSB and LSB. Before Verilog 2001, it is a syntax error to specify a function call to evaluate the value of these widths. For example, the following code is erroneous before Verilog 2001 version. reg [ port1(val1:vla2) : port2 (val3:val4)] reg1; Verilog-2001 allows the use of a function call to evaluate the MSB or LSB of a width declaration.

93. What is alias?

Ans : System Verilog has introduced a keyword alias, which can be used only on nets to have a two-way assignment.
module top_alias ();
wire rhs,lhs;
alias lhs=rhs;
In the above verilog code, if LHS changes it reflets to RHS and similarly if RHS changes it reflects to LHS as well.

94. What is the difference between: c = con ? a : b; and if (con) c = a; else c = b;

Ans :The operator (?) gives answers even if the condition is "x", so for example if con = 1'bx, a = 'b10, and b = 'b11, it will give c = 'b1x. Whereas “if” treats Xs or Zs as FALSE, so it will return always c = b.

95. What is scheduling semantics for the simulation time in Verilog?

Ans : Verilog basically imply a four-level deep queue for the current simulation time:
Active Events (blocking statements)
Inactive Events (#0 delays, etc)
Non-Blocking Assign Updates (non-blocking statements)
Monitor Events ($display, $monitor, etc).

96. What is PLI?

Ans : 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.

97. List all the system tasks and their purpose?

Ans : $display – Displays once every time when they are executed.
$strobe – Displays the parameters at the very end of the current simulation time rather than exactly where it is executed.
$monitor – Displays every time when one of its parameter changes.
$reset – Resets the simulation back to zero.
$stop – Halts the simulator and puts in the interactive mode where user can enter commands
$finish – Exits simulator back to operating system.
$time, $stime, $realtime – Currenmt simulation time as a 64-bit integer, 32-bit integer and real number respectively.
$scope (hierarchy_name) – sets the current scope to hierarchy scope to hierarchy name.
$showscopes – Lists all modules,tasks & block naems in current scope.
$fopen – Opens an ouput file and gives the open file a handle for use by the other commands.
fclose – Closes the file and lets the other programs access it.
$fdisply, $fwrite – Writes formated data to a file when ever they executed. Both are same except $fdisplay inserts a new line after every execution and $fwrite does not inserts.
$fstrobe – Writes a file when executed, but waits until all other operations in the time step are complete before writing.
initilal
# 1 a= 1; b=0;
#fstrobe (file, a,b );
b=1;
Will writes 11 for a and b.
$fmonitor – Writes to a file when ever any argument changes
$dumpfile, $dumpvar, $dumpon, $dumpoff, $dumpall – The dump files are capable of dumping all the variables in a simulation.
$dumpvar – dumps all the variables in the design.
$dumpvar(1,top) – Dumps all variables in the top and below but not sub modules.
$dumpvar(2,top) – Dumps all variables in the top and one level below.
$dumpvar(n,top) – Dumps all variables in the top and n-1 level below.
$dumpvar(0,top) – Dumps all variables in the top and all levels below.
$dumpon – Initiates the dump.
$dumpoff - Stops dumping.
$random – Generates radom integer every time when it is called.

98. What is a netlist?

Ans : Gate level representation of RTL.

99. How memory is declared in Verilog?

Ans : Memories are declared as two-dimensional arrays of registers.
syntax: reg [msb:lsb] identifier [first_addr:last_addr] ;
Where msb:lsb determine the width (word size) of the memory first_addr:last_addr determine the depth (address range) of the memory

100.What are the guidelines for using functions in Verilog? Ans : A function cannot advance simulation-time, using constructs like #, @. etc.
A function should not have nonblocking assignments.
A function without a range defaults to a one bit reg for the return value.
It is illegal to declare another object with the same name as the function in the scope where the function is declared