15.2 Format of the value change dump file

The dump file is structured in a free format. White space is used to separate commands and to make the file easily readable by a text editor.

Syntax of the value change dump file

The syntax of the VCD file is as follows:

Syntax 15-2: Syntax of the output VCD file

The VCD file starts with header information giving the date, the simulator's version number used for the simulation, and the timescale used. Next, the file contains definitions of the scope and type of variables being dumped, followed by the actual value changes at each simulation time increment. Only the variables that change value during a time increment are listed.

The simulation time recorded in VCD file is the absolute value of the simulation time for the changes in variable values that follow.

Value changes for real variables are specified by real numbers.Value changes for all other variables are specified in binary format by 0 , 1 , x , or z values. Strength information and memories are not dumped.

A real number is dumped using a %.16g printf() format. This preserves the precision of that number by outputting all 53 bits in the mantissa of a 64-bit IEEE STD-754 double precision number. Application programs can read a real number using a %g format to scanf() .

The value change dumper generates character identifier codes to represent variables. The identifier code is a code composed of the printable characters that are in the ASCII character set from ! to ~ (decimal 33 to 126).

NOTES

1) --The VCD format does not support a mechanism to dump part of a vector. For example, only bits 8 to 15 (8:15) of a 16-bit vector cannot be dumped in VCD file, instead, the entire vector (0:15) must be dumped. In addition, expressions, such as a + b, cannot be dumped in the VCD file.

2) -- Data in the VCD file is case sensitive.

Formats of variable values

Variables may be either scalars or vectors. Each type is dumped in its own format. Dumps of value changes to scalar variables shall not have any white space between the value and the identifier code.

Dumps of value changes to vectors shall not have any white space between the base letter and the value digits, but must have one white space between the value digits and the identifier code.

The output format for each value is right-justified. Vector values appear in the shortest form possible: redundant bit values that result from left-extending values to fill a particular vector size are eliminated.

The rules for left-extending vector values are as follows:

The following table shows how the VCD may shorten values:

Events are dumped in the same format as scalars: for example, 1*%. For events, however, the value (1 in this example) is irrelevant. Only the identifier code (*% in this example) is significant. It appears in the VCD file as a marker to indicate that the event was triggered during the time step.

Examples:

 
1*@	     No space between the value 1 and the identifier code *@

 
b1100x01z (k		 					No space between the b and 1100x01z, 							but a space between b1100x01z and (k  

Description of keyword commands

The general information in the VCD file is presented as a series of sections surrounded by keywords. Keyword commands provide a means of inserting information in the VCD file. Keyword commands can be inserted either by the dumper or manually.

This section deals with the following keyword commands:

$comment

The $comment section provides a means of inserting a comment in the VCD file.

Syntax:

 
	 
$comment
 comment_text
 $end

Examples:

 
$comment
 	This is a single-line comment	      $end
 

 
$comment
 	This is a 		 multiple-line comment 	   $end
	

$date

The $date section indicates the date on which the VCD file was generated.

Syntax:

 
	$date
 
 date_text 
$end 

Example:

 
$date 
     		June 25, 1989 09:24:35$end

$enddefinitions

The $enddefinitions section marks the end of the header information and definitions.

Syntax:

 
	$enddefinitions $end
 

$scope

The $scope section defines the scope of the variables being dumped.

Syntax:

 
	$scope
 
 scope_type scope
_identifier 	$end

 
	scope_type ::= module
 | task
 | function
 | begin
 | fork
 

The scope type indicates one of the following scopes:

module top-level module and module instances
task tasks
function functions
begin named sequential blocks
fork named parallel blocks

Example:

 
$scope
     		module
 top$end

$timescale

The $timescale keyword specifies what timescale was used for the simulation.

Syntax:

 
	$timescale
 
 number time_unit $end
number ::= 1
 | 10
 | 100
time_unit ::= s
 | ms
 | us
 | ns
 | ps
 | fs

Example:

 
$timescale
  10  ns  $end

$upscope

The $upscope section indicates a change of scope to the next higher level in the design hierarchy.

Syntax:

 
	$upscope $end
 

$version

The $version section indicates which version of the VCD writer was used to produce the VCD file.

Syntax:

 
	$version
 
 version_text 
$end

Example:

 
$version
      VERILOG-SIMULATOR 1.0a $end

$var

The $var section prints the names and identifier codes of the variables being dumped.

Syntax:

 
$var 
 
var_type size identifier_code reference $end

 
var_type ::= event
 | integer
 | parameter 
| real
 | reg
 | supply0
 | supply1
 | time
 | tri
 | triand
 | trior
 | trireg
 | tri0
 | tri1
 | wand
 | wire
 | wor

 
size ::= decimal_number

 
reference ::= identifier| identifier [ bit_select
_index ]| identifier [ msb
_index : lsb
_index ]

 
index ::= decimal_number

Size specifies how many bits are in the variable.

The identifier code specifies the name of the variable using printable ASCII characters, as previously described.

a) The msb index indicates the most significant index; lsb index indicates the least significant index.

b) More than one reference name may be mapped to the same identifier code. For example, net10 and net15 may be interconnected in the circuit, and therefore may have the same identifier code.

c) The individual bits of vector nets may be dumped individually.

d) The identifier is the name of the variable being dumped in the model .

Example:

 
$var 
		     integer
 32 (2 index 	 $end

$dumpall

The $dumpall keyword specifies current values of all variables dumped.

Syntax:

 
	$dumpall 
 
{ value_changes } $end

Example:

 
$dumpall
   1*@  x*#   0*$   bx   (k   $end 
		

$dumpoff

The $dumpoff keyword indicates all variables dumped with X values.

Syntax:

 
	$dumpoff 
 
 
 
{ value_changes } $end

Example:

 
$dumpoff  x*@  x*#   x*$   bx   (k   $end 		

$dumpon

The $dumpon keyword indicates resuming of dumping and lists current values of all variables dumped.

Syntax:

 
	$dumpon 
 
 
{ value_changes } $end

Example:

 
$dumpon
   x*@  0*#   x*$   b1   (k   $end
 		

$dumpvars

 
The section beginning with $dumpvars
 keyword lists initial values of all variables dumped.

Syntax:

 
	$dumpvars
 
 
  { value_changes } $end

Example:

 
$dumpvars
   x*@   z*$   b0   (k   $end

Value change dump file format example

The following example illustrates the format of the value change dump file.