VHDL Programming

62 Chapter Three

WAIT ON Signal

We have already seen an example of the first type in the previous process

example. The WAIT ONsignal clause specifies a list of one or more signals

that the WAITstatement will wait for events upon. If any signal in the

signal list has an event occur on it, execution continues with the statement

following the WAITstatement. Here is an example:

WAIT ON a, b;

When an event occurs on either aor b, the process resumes with the

statement following the WAITstatement.

WAIT UNTIL Expression

The WAIT UNTIL boolean_expressionclause suspends execution of the

process until the expression returns a value of true. This statement effec-

tively creates an implicit sensitivity list of the signals used in the expres-

sion. When any of the signals in the expression have events occur upon

them, the expression is evaluated. The expression must return a boolean

type or the compiler complains. When the expression returns a true

value, execution continues with the statement following the WAITstate-

ment. Otherwise, the process continues to be suspended. For example:

WAIT UNTIL (( x * 10 ) < 100 );

In this example, as long as the value of signal xis greater than or equal

to 10, the WAITstatement suspends the process or subprogram. When the

value of xis less than 10, execution continues with the statement following

the WAITstatement.

WAIT FOR time_expression

The WAIT FOR time_expression clause suspends execution of the

process for the time specified by the time expression. After the time

specified in the time expression has elapsed, execution continues on the