VHDL Programming

Sequential Processing 63

statement following the WAIT statement. A couple of examples are

shown here:

WAIT FOR 10 ns;

WAIT FOR ( a * ( b + c ));

In the first example, the time expression is a simple constant value.

The WAITstatement suspends execution for 10 nanoseconds. After 10

nanoseconds has elapsed, execution continues with the statement following

the WAITstatement.

In the second example, the time expression is an expression that first

must be evaluated to return a time value. After this value is calculated,

the WAITstatement uses this value as the time value to wait for.

Multiple WAIT Conditions

The WAITstatement examples we have examined so far have shown the dif-

ferent options of the WAITstatement used separately. The different options

can be used together. A single statement can include an ONsignal,UNTIL

expression, and FOR time_expressionclauses. Following is an example:

WAIT ON nmi,interrupt UNTIL ((nmi = TRUE) or

(interrupt = TRUE)) FOR 5 usec;

This statement waits for an event on signals nmiand interruptand

continues only if interruptor nmiis true at the time of the event, or until

5 microseconds of time has elapsed. Only when one or more of these

conditions are true does execution continue.

When using a statement such as this:

WAIT UNTIL (interrupt = TRUE) OR ( old_clk = ‘ 1 ’);

be sure to have at least one of the values in the expression contain a signal.

This is necessary to ensure that the WAITstatement does not wait forever.

If both interruptand old_clkare variables, the WAITstatement does not

reevaluate when these two variables change value. (In fact, the variables

cannot change value because they are declared in the suspended process.)

Only signals have events on them, and only signals can cause a WAIT

statement or concurrent signal assignment to reevaluate.