VHDL Programming

120 Chapter Five

ZL HX

Z

L

H

X

Z

L

H

X

L

L

X

X

H

X

H

X

X

X

X

X

Figure 5-1
Four State Truth
Table.

Let’s examine a resolution function for the type fourvalthat was used

in the conversion function examples. The type declaration for fourvalis

shown here:

TYPE fourval IS (X, L, H, Z);

Four distinct values are declared that represent all of the possible

values that the signal can obtain. The value Lrepresents a logical 0, the

value Hrepresents a logical 1, the value Zrepresents a high-impedance

or open-collector condition, and, finally, the value Xrepresents an unknown

condition in which the value can represent an Lor an H, but we’re not sure

which. This condition can occur when two drivers are driving a signal, one

driver driving with an H, and the other driving with an L.

Listed by order of strength, with the weakest at the top, the values are

as follows:

Z—Weakest,H,L, or Xcan override

H,L—Medium strength, only Xcan override

X—Strong, no override

Using this information, a truth table for two inputs can be developed,

as shown in Figure 5-1.

This truth table is for two input values. It can be expanded to more

inputs by successively applying it to two values at a time. This can be done

because the table is commutative and associative. An Land a Z, or a Zand

an L, gives the same results. An (L,Z) with Hgives the same results as an

(H,Z) with an L. These principles are very important, because the order of

driver values within the input argument to the resolution function is non-

deterministic from the designer’s point of view. Any dependence on order

can cause nondeterministic results from the resolution function.