VHDL Programming

122 Chapter Five

Driver Values

Initial

Value

Z ZH

Z

H Resultant Value

Figure 5-2
Four State Resolution
with Two Values.

NULL;

END CASE;

WHEN X =>

result := X;

END CASE;

END LOOP;

RETURN result;

END resolve;

END fourpack;

The input argument is an unconstrained array of the driver-base

type,fourval. The resolution function examines all of the values of the

drivers passed in argument sone at a time and returns a single value

of fourvaltype to be scheduled as the signal value.

Variable resultis initialized to a Zvalue to take care of the case of zero

drivers for the signal. In this case, the loop is never executed, and the

result value returned is the initialization value. It is also a good idea to

initialize the result value to the weakest value of the value system to allow

overwriting by stronger values.

If a nonzero number of drivers exists for the signal being resolved, then

the loop is executed once for each driver value passed in argument s. Each

driver value is compared with the current value stored in variable result.

If the new value is stronger according to the rules outlined earlier, then

the current result is updated with the new value.

Let’s look at some example driver values to see how this works. Assuming

that argument scontained the driver values shown in Figure 5-2, what

would the result be?