Reading VHDL BNF

Following is the BNF for the IFstatement: if_statement ::= IF condition THEN sequence_of_statements {ELSIF condition THEN sequence_of_statements} [ELSE sequence_of_statements] END IF; Th

Reading VHDL BNF

After the basic concepts of VHDL are understood, the designer might want to try to write VHDL in a more elegant manner To fully understand how to apply all of the syntactic constructs available in VHDL, it is helpful

to know how to read the VHDL Bachus-Naur format (BNF) of the

lan-guage This format is in Appendix A of the IEEE Std 1076-1987 VHDL Language Reference Manual (LRM), pages A1 to A17 This format specifies which constructs are necessary versus optional, or repeatable versus singular, and how constructs can be associated

BNF is basically a hierarchical description method, where complex constructs are made of successive specifications of lower-level constructs Our purpose for examining BNF is not to understand every nuance of the BNF but to put the basics to use to help build complex VHDL constructs

To this end, let us examine some BNF and discuss what it means

Following is the BNF for the IFstatement:

if_statement ::=

IF condition THEN

sequence_of_statements

{ELSIF condition THEN

sequence_of_statements}

[ELSE

sequence_of_statements]

END IF;

The first line of the BNF description specifies the name of the construct being described This line is read as follows: “The IF statement consists of,” or “The IFstatement is constructed from.” The rest of the description represents the rules for constructing an IFstatement

The second line of the description specifies that the IFstatement starts with the keyword IF, is followed by a condition construct, and ends the clause with the keyword THEN The next line contains the construct

SEQUENCE_OF_STATEMENTS (which is discussed later in this appendix) All of the constructs discussed so far are required for the IFstatement because the constructs are not enclosed in any kind of punctuation Statements enclosed in brackets [ ], as in lines 6 and 7, are optional constructs An optional construct can be specified or left out depending on the functionality required The ELSE clause of the IF statement is an example of an optional construct A legal IF statement may or may not have an ELSEclause

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 ELSIF clause, or have one or more ELSIFclauses, depending on the desired behavior

The last line of the IF_STATEMENT description 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_STATEMENTS construct 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 ASSIGNMENT state-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

boolean expression construct to describe the syntax required, none will be found The reason is that all expressions share the same syntax description For our purposes, the boolean type of the expression is ignored, and the construct description can be found under the following description:

expression ::=

relation {and relation}

|relation {or relation}

|relation {xor relation}

| relation [nand relation]

|relation [nor relation]

To summarize, curly braces { }are optional and repeatable constructs, square brackets [ ]are optional constructs, and italic pieces of a con-struct can be ignored for purposes of finding descriptions