Strings

Formal Definition

The strings are sequences of 8-bit ASCII characters enclosed within quotation marks.

Simplified Syntax

"This is a string"

reg [8*number_of_characters:1] string_variable;

Description

The string should be given in one line. Strings can contain special characters (Example 1).

Character	Meaning
\n	New line character
\t	Tab character
\\	\ character
\”	" character
\ddd	A character specified by octal digit

Table 24: Summary of special characters

String variables should be declared as reg type vectors (Example 2). Each character needs 8 bits.

If a string variable is used in an expression, it should be treated as an unsigned value. If the size of a string assigned to a string variable is smaller than the declared size of the variable, then it will be left-padded with zeros.

The null string "” should be treated same as "\0”.

Concatenations of string variables preserve left-padded zeros of these variables (Example 3).

Examples

Example 1

"\n This is the first line\n This is the second line"
"\t Line\t with\t tab\t characters"

Example 2

reg [8*12:1] message;

The message variable can contain 12 characters.

Example 3

reg [10*8:1] s1, s2;
s1 = "Verilog";
s2 = "-HDL";
{s1, s2} <> {"Verilog", "-HDL"}

These expressions are not equal because {s1, s2} has 0s between "Verilog” and "-HDL” and {"Verilog”, "-HDL”} does not have any 0s between words.

Important Notes

• Concatenation of string variables preserves left-padded zeros of these variables.

Strengths

Formal Definition

The strength declaration construct is used for modeling net type variables for a close correspondence with physical wires.

Simplified Syntax

(Strength1, Strength0)
(Strength0, Strength1)
Strength1:
supply1, strong1, pull1, large1, weak1, medium1, small1, highz1
Strength0:
supply0, strong0, pull0, large0, weak0, medium0, small0, highz0

Description

Strengths can be used to resolve which value should appear on a net or gate output.

There are two types of strengths: drive strengths (Example 1) and charge strengths (Example 2). The drive strengths can be used for nets (except trireg net), gates, and UDPs. The charge strengths can be used only for trireg nets. The drive strength types are supply, strong, pull, weak, and highz strengths. The charge strength types are large, mediumand small strengths.

All strengths can be ordered by their value. The supply strength is the strongest and the highz strength is the weakest strength level. Strength value can be displayed by system tasks ($display, $monitor - by using of the %v characters - see Display tasks for more explanation).

Strength	Value	Value displayed by display tasks
supply	7	Su
strong	6	St
pull	5	Pu
large	4	La
weak	3	We
medium	2	Me
small	1	Sm
highz	0	HiZ

Table 23 Strengths ordered by value

If two or more drivers drive a signal then it will have the value of the strongest driver (Example 3).

If two drivers of a net have the same strength and value, then the net result will have the same value and strength (Example 4).

If two drivers of a net have the same strength but different values then signal value will be unknown and it will have the same strength as both drivers (Example 5).

If one of the drivers of a net has an H or L value, then signal value will be n1n2X, where n1 is the strength value of the driver that has the smaller strength, and n2 is strength value of driver that has the larger strength (Example 6).

The combinations (highz0, highz1) and (highz1, highz0) are illegal.

Examples

Example 1

and (strong1, weak0) b(o, i1, i2);

Instance of and gate with strong1 strength and weak0 strength specified.

Example 2

trireg (medium) t;

The charge strength declaration for trireg net.

Example 3

buf (strong1, weak0) g1 (y, a);
buf (pull1, supply0) g2 (y, b);

If a = 0 and b = 0 then y will be 0 with supply strength because both gates will set y to 0 and supply (7) strength has bigger value than weak (3) strength.

If a = 0 and b = 1 then y will be 1 with pull strength because g1 will set y to 0 with weak (3) strength and g2 will set y to 1 with pull (5) strength (pull strength is stronger than the weak strength).

If a = 1 and b = 0 then y will be 0 with supply strength because g1 will set y to 1 with strong (6) strength and g2 will set y to 0 with supply (7) strength (supply strength is stronger than the strong strength).

If a = 1 and b = 1 then y will be 1 with strong strength because g1 will set y to 1 with strong (6) strength and g2 will set y to 1 with pull (5) strength.

Example 4

buf (strong1, weak0) g1 (y, a);
buf (strong1, weak0) g1 (y, b);

If a = 0 and b = 0 then y will be 0 with weak strength.

If a = 1 and b = 1 then y will be 1 with strong strength.

Example 5

buf (strong1, weak0) g1 (y, a);
buf (weak1, strong0) g1 (y, b);

If a = 1 and b = 0 then y will be x with strong strength.

Example 6

bufif0 (strong1, weak0) g1 (y, i1, ctrl);
bufif0 (strong1, weak0) g2 (y, i2, ctrl);

If ctrl = x, i1 = 0, and i2 = 1 then y will be x with 36X strength, because g1 will set y to L with strong strength (StL - 6) and g2 will set y to H with weak strength (WeH - 3).

Important Notes

• If one of the drivers has an H or L value, then the output value will be X.

Module Instantiation

Stochastic Analysis Tasks

Formal Definition

Stochastic analysis tasks provide a means of managing queues.

Simplified Syntax

$q_initialize(identifier, type, length, status) ;
$q_add(identifier, job, information, status) ;
$q_remove(identifiers, job, information, status) ;
$q_full(identifier, status) ;
$q_exam(identifier, statistic_code, statistic_value, status) ;

Description

All statistic analysis tasks include identifier, and status arguments. The identifier is a unique integer value identifying the queue. The status argument is an output integer parameter giving information about the correctness of the specified task execution.

Status	Message
0	The queue generation was successful.
1	The queue cannot be increased; queue is full.
2	The identifier is undefined; please define an identifier.
3	Cannot remove a value; queue is empty.
4	The queue cannot be generated; this type is unsupported.
5	The length parameter is <= 0; the queue cannot be generated.
6	The identifier is duplicated; please define new identifier.
7	The queue cannot be generated; insufficient memory.

$q_initialize creates a queue. Parameter type determines the type of a queue. In case of 1, the created queue is FIFO and in case of 2 the queue is LIFO. Parameter length is an integer value specifying the maximum number of entries.

$q_add adds a new entry to the queue. Parameter job identifies the job. The special user-defined parameter, information, can maintain any information useful for the user.

$q_remove receives data from the queue. Parameters of this task are the same as those of the $q_add task.

$q_full checks to see of the queue identified by the identifier parameter is full.

$q_exam returns statistical information about the queue. Parameter statistic_code determines what you need to check and returns the result in statistic_value. The following table lists all possible values of statistic_code:

Statistic_code	Statistic_value
1	Length of queue.
2	Mean interarrival time.
3	Maximum length of queue.
4	Shortest wait time.
5	Longest wait time for jobs still in the queue.
6	Average wait time in the queue.

Examples

Example 1

always @(posedge clk)
begin
// check if queue1 is full
$q_full(queue1, status);
// if full then show message and remove one item
if (status) begin
  $display("Queue is full”);
  $q_remove(queue1, 1, info, status);
end
// add a new item to queue1
$q_add(queue1, 1, info, status);
// show message if there was an error
if (status)
  $display("Error %d”,status);
end
end

This example shows how to add a new element to the queues.

Important Notes

• The status parameter value should be checked after all operations on the queue.

Specify Block

Formal Definition

Allows a specific delay across a module.

Simplified Syntax

specify

  specparam declaration ;

  path declaration ;

  system timing check ;

endspecify

Description

The Specify block was designed to define a delay across a module. It starts with specify and ends with the endspecify keyword. Inside the block the user can specify: specparamdeclaration, path declaration or system timing check.

The syntax of specparam declaration is the same as that of the parameter declaration. After the specparam keyword the user can specify any number of parameters but only constant expressions are allowed on the right-hand sides of parameter assignments. A comma can separate the assignments, and the last statement ends with a semicolon. A previously declared specparameter can be used to declare the new specparameters. Unlike parameters, specparams cannot be overwritten, nor can they be used outside of a specify block.

Examples

Example 1

module ...
...
specify
  (In => Out) = (10);
endspecify
...
endmodule

A specify block with only a path declaration. Delay between input In and output Out is 10 time units.

Example 2

module ...
...
specify
  specparam TRise = 10,
  TFall = 15;
  (In => Out) = (TRise, TFall) ;
endspecify
...
endmodule

Specparam declaration with two parameters TRise and TFall to specify delays on rising transition and falling transition.

Example 3

module ...
...
specify
  specparam TRise = 10,
  TFall = 15;
  (In => Out) = (TRise, TFall) ;
  $setup(Data_in, posedge Clock, TRise) ;
endspecify
...
endmodule

The full specify block with specparam declaration, path declaration and system timing check task.

Important Notes

· Specify blocks have to be inside the module declaration location.

· Specparams are not visible outside of the Specify blocks.

· The defparam keyword cannot be used to override a specparam value.

State Dependent Path

Formal Definition

State Dependent Path is a path that occurs only when the condition is met.

Simplified Syntax

if (condition) simple_module_path;

if (condition) edge_sensitive_path;

ifnone simple_module_path;

Description

Generally, state dependent path is comprised of three parts. A condition that enables the module path, a module path description and a delay that applies to the module path.

The condition is an expression using scalars or vectors of any type. It can also be part-selects or bit-selects of a vector. Constant numbers and specparams can be used in the condition expression. The result of the conditional expression can be one bit or multiple bits. If it is more than one bit, the least significant bit represents the result.

When no edge transition is specified for the inputs, it is called the simple state-dependent path. Example 1 shows the simple-dependent path.

If any edge transition is specified for the input, then it is called an edge-sensitive state-dependent path. Different delays can be used to the same path if the following rules are followed:

a. A declaration must be unique and should use an edge or a conditional expression or both.

b. The port for which the delay is specified must be referenced in exactly the same way. You cannot mix part select, bit-select and complete ports.

Examples

Example 1

module example1 (cond, in_1, in_2, out);
input in_1, in_2, cond ;
output out ;
and (out, in_1, in_2) ;
specify
  specparam TRise1 = 5,
  TFall1 = 5,
  TRise2 = 7,
  TFall2 = 7;
  if (cond) ( in_1, in_2 *> out ) = (TRise1, TFall1);
  if (~cond) ( in_1, in_2 *> out ) = (TRise2, TFall2);
endspecify
endmodule

If a conditional expression 'cond' is true, TRise1 and TFall1 will be assigned to a path as delays. When the conditional expression 'cond' is false, TRise2 and TFall2 will be used as the path delays.

Important Notes

· When a conditional expression evaluates to x or z, it should be treated as true.

Simulation Time Functions

Formal Definition

The simulation time function provides an access to current simulation time.

Simplified Syntax

$time ;

$stime ;

$realtime ;

Description

When the $time system function is called, it returns the current time as a 64-bit integer value. However, this value is scaled to the `timescale unit. (See Timescale chapter)

The $stime system function returns current time as a 32-bit unsigned integer value. If the current simulation time is too large and the value does not fit in 32 bits, the function only returns the 32 low order bits of the value. The returned value is also scaled to the `timescale.

The $realtime system function returns a value as a real number. As with the other time tasks, the returned value is scaled to the `timescale.

Examples

Example 1

integer cur_time ;
cur_time = $time ;

Example 2

integer cur_time ;
cur_time = $stime ;

Example 3

real cur_time ;
cur_time = $realtime ;

Example 4

$display($time, "is current time.");

Important Notes

· $time, $stime, $realtime are functions which return specific values. If you are using these functions you have to declare registers that can support the returned value.