Verilog HDLVerilog HDL(Behavioral Modeling)(Behavioral Modeling)

Bilal Saqib

Behavioral ModelingBehavioral Modeling

Structured ProceduresStructured Procedures

In One ModuleIn One Module

A module may contain multiple always statements and multiple initial statements. Each statement starts a separate control flow and starts execution at time 0.

Initial statementInitial statement

An initial statement executes only once and begins its execution at start of simulation which is at time 0.

Syntax : initial[timing_control] procedural_statement

Always statementAlways statement

An always statement executes repeatedly and also begins its execution at start of simulation which is at time 0.

Syntax : always [timing_control]

procedural_statement

where a procedural_statement is one of :procedural_assignment ( blocking or

non_blocking)procedural_continuous _assignment conditional_statementcase_statementloop_statementwait_statementdisable_statementevent_triggersequential_blockparallel_blocktask_enable (user

Procedural BlocksProcedural Blocks

Procedural Blocks are constructed from the following components.◦Procedural Assignment Statements◦High-Level Constructs

Procedural AssignmentsProcedural Assignments

Procedural Execution ControlProcedural Execution Control

Execution of Procedural Blocks can be specified in different ways◦Simple Delays: #<delay>

Specify delay before and after execution for a number of time steps.

◦Edge-Sensitive Controls: always @ (<edge><signal>) Execution occurs only at a signal edge. Optional

keywords “posedge” or “negedge” can be used to specify signal edge for execution.

NonBlocking v Blocking NonBlocking v Blocking AssignmentsAssignments

NonBlocking v Blocking NonBlocking v Blocking AssignmentsAssignments

Continuous assignment vs Continuous assignment vs Procedural assignmentProcedural assignment

Procedural assignment

◦ Occurs inside an always statement or an initial statement.

◦ Execution is with respect to other statements surrounding it.

◦ Drives registers.◦ Uses “ = “ or “ < = “

assignment symbol.◦ No assign keyword

Continuous assignment

◦ Occurs within a module.◦ Executes concurrently

with other statements ; executes whenever there is a exchange of value in an operand on its right-hand side.

◦ Drives nets.◦ Uses “ = “ assignment

symbol.◦ Uses assign keyword

Block statementsBlock statements

A block statement provides a mechanism to group two or more statements to act syntactically like a single statement. There are two kinds of blocks in Verilog HDL. These are :

◦ Sequential block ( begin…end ) : Statements are executed sequentially in the given order.

◦ Parallel block ( fork … join ) : Statements in this block execute concurrently.

Sequential blockSequential block

Statements in a sequential block execute in sequence.

Syntax : begin [ : block_id { declarations} ] procedural_statement (s)end

Parallel blockParallel block

Statements in a parallel block execute in concurrently.

Syntax : fork [ : block_id { declarations} ] procedural_statement (s)join

Conditional Statements: if elseConditional Statements: if else

Conditional Statements: caseConditional Statements: case

casex and casezcasex and casez

Looping Statements: repeat

Looping Statements: while

Looping Statements: forever

Looping Statements: for

ExampleExample

module FA_Seq (A , B , Cin , Sum, Cout) ; input A , B, Cin ; output Sum, Cout ; reg Sum, Cout ; reg T1, T2, T3 ; always

@ ( A or B or Cin ) begin

Sum = ( A ^ B ) ^ Cin ;T1 = A & Cin ;T2 = B & Cin ;T3 = A & B;Cout = ( T1 | T2 ) | T3 ;

endendmodule