Useful
Modeling
Techniques
Programmable Logic Design (40-493)
Fall 2001
Computer Engineering Department
Sharif University of Technology
Maziar Gudarzi
Outline
Procedural Continuous Assignment
 Overriding Parameters
 Conditional Compilation and Execution
 Useful System Tasks

Procedural
Continuous
Assignment
Useful
Modeling Techniques
Procedural Continuous
Assignment
Overrides, for a certain time, the effect
of regular assignments to a variable.
 Two types


assign/deassign


Works only on register data types
force/release

Works on both register and net data types
Procedural Continuous
Assignment (cont’d)

assign/deassign

Keywords
assign: overrides regular procedural
assignments
 deassign: re-enables regular procedural
assignments


After deassign:

Last value remains on the register until a new
procedural assignment changes it.
Procedural Continuous
Assignment (cont’d)

force/release

Keywords:
force: overrides all procedural/continuous/
procedural continuous assignments
 release: re-enables other assignments
 Hence, assignments in priority order:

1. force
2. assign (procedural continuous)
3. Procedural/continuous assignments
Overriding
Parameters
Useful
Modeling Techniques
Overriding Parameters

Two methods
defparam statement
 Module instance parameter values


defparam statement
Keyword: defparam
 Syntax:

defparam <parameter_hierarchical_name>=<value>;
Overriding
Parameters (cont’d)

Module instance parameter values
Parameters are overridden when the
module is instantiated
 Syntax:

<module_name> #(<param_vals>) <instance_name>;
Conditional
Compilation and
Execution
Useful
Modeling Techniques
Conditional Compilation

Usage:


To compile some part of code under certain
conditions
Keywords:

‘ifdef, `else, `endif
Conditional Execution

Usage:




Keywords:


To execute some part of code when a flag is set at
runtime
Used only in behavioral modeling
Not contained in Verilog IEEE Standard,
$test$plusargs
Syntax:

$test$plusargs( <argument_to_check> )
Useful
System Tasks
Useful
Modeling Techniques
Useful System Tasks
File Output

Opening a file

Syntax:
<file_handle> = $fopen( “<file_name>” );



File_handle is a 32 bit value, called multi-channel
descriptor
Only 1 bit is set in each descriptor
Standard output has a descriptor of 1 (Channel 0)
Useful System Tasks
File Output (cont’d)

Writing to files
$fdisplay, $fmonitor, $fwrite, $fstrobe
 Syntax:

$fdisplay(<handle>, p1, p2,…, pn);

Closing files

$fclose(<handle>);
Useful System Tasks
Random Number Generation

Syntax:
$random;
$random(<seed>);

Returns a 32 bit random value
Useful System Tasks
Initializing Memory from File

Keywords:


$readmemb, $readmemh
Syntax:
$readmemb(“<file_name>”, <memory_name>);
$readmemb(“<file_name>”, <memory_name>, <start_addr>);
$readmemb(“<file_name>”, <memory_name>,
<start_addr>,<finish_addr>);
Useful System Tasks
Value Change Dump (VCD) File

ASCII file containing information on




Simulation time
Scope and signal definitions
Signal value changes
Keywords





$dumpvars
$dumpfile
$dumpon
$dumpoff
$dumpall
Today Summary

Reviewed some modeling techniques
useful in various applications.
Complementary Notes

Assignment 7 (Optional, Extra Score)

Chapter 9:
All exercises with ModelSim, except for 3 and 5
 3 and 5 in paper and pencil
 Due date: Saturday, Day 22nd


Lab. Final Project

Announce your group members and tart
your work
Complementary Notes (cont’d)

Don’t forget to subscribe to course
mailing list:
ce493list