VHDL Programming

Statements enclosed in curly braces { }, as in lines 4 and 5, are

optional and repeatable constructs. An optional and repeatable construct

can either be left out or have one or more of the construct exist. The

ELSIFclause is an example of an optional and repeatable construct. The IF

statement can be constructed without an ELSIFclause, or have one or

more ELSIFclauses, depending on the desired behavior.

The last line of the IF_STATEMENTdescription contains the END IF

clause. This is a required clause because it is not optional [ ]and is not

optional and repeatable { }.

The IF statement contains two other constructs that need more

description: the SEQUENCE_OF_STATEMENTS and the CONDITION. The

SEQUENCE_OF_STATEMENTSconstruct is described by the BNF shown here:

sequence_of_statements ::=

{sequential_statement}

The SEQUENCE_OF_STATEMENTSconstruct is described by one or more

sequential statements, where a sequential statement is described in the

following:

sequential_statement ::=

wait_statement

| assertion_statement

| signal_assignment_statement

| variable_assignment_statement

| procedure_call_statement

| if_statement

| case_statement

| loop_statement

| next_statement

| exit_statement

| return_statement

| null_statement

The |character means OR, such that a sequential statement can be a

WAITstatement, or an ASSERTstatement, or a SIGNAL ASSIGNMENTstate-

ment, and so on. From this description, we can see that the statement part

of the IFstatement can contain one or more sequential statements, such

as WAITstatements,ASSERTstatements, and so on.

The CONDITIONconstruct is specified with the BNF description shown

here:

condition ::= boolean_expression

Notice that the keyword booleanis italic. The italic indicates the type

of the expression required for the CONDITION. If a designer looks for a

446 Appendix C: Reading VHDL BNF