VHDL Tutorial

Process Statements in VHDL
Introduction
Fundamental concepts
Modelling concepts
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Scalar type
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Assertion statements
Array types & array operations
Architecture bodies
Entity declarations
Behavioral descriptions 
Wait statements
Delta delays
Process statements

Conditional signal assignment 
Selected signal assigment
Structural descriptions
Library and library clauses
Procedures
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Unconstrained array parameter
Functions
Package declarations and bodies
Subprograms in package
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Resolved signals and subtypes
Resolved signals and ports
Parameterizing behavior
Parameterizing structure
 

Process Statements

 

We  have  been  using  processes  quite  extensively  in  examples  in  this  and  previous chapters, so we have seen most of the details of how they are written and used.   To summarize, let us now look at the formal syntax for a process statement and review process operation.   The syntax rule is

 

process_statement

process_label : process [ ( signal_name { , } ) ] [ is ]

{ process_declarative_item }

begin

{ sequential_statement }

end process [ process_label ] ;

 

The declarative items in a process statement may include constant, type and vari- able  declarations,  as  well  as  other  declarations  that  we  will  come  to  later.   The  se- quential  statements  that  form  the  process  body  may  include  any  of  those  that  we introduced earlier, plus signal assignment and wait statements.  When a process is ac- tivated  during  simulation,  it  starts  executing  from  the  first  sequential  statement  and continues until it reaches the last.   It then starts again from the first This would be an infinite loop, with no progress being made in the simulation, if it were not for the inclusion  of  wait  statements,  which  suspend  process  execution  until  some  relevant event occurs.   Wait statements are the only statements that take more than zero sim- ulation time to execute.   It is only through the execution of wait statements that sim- ulation time advances.

 

A process may include a sensitivity list in parentheses after the keyword process. The sensitivity list identifies a set of signals that the process monitors for events.   If the sensitivity list is omitted, the process should include one or more wait statements. On the other hand, if the sensitivity list is included, then the process body cannot in- clude any wait statements.  Instead, there is an implicit wait statement, just before the end process keywords, that includes the signals listed in the sensitivity list as signals

in an on clause.

 

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