VHDL Programming

126 Chapter Five

implement. The basic algorithm of the function is the same as the fourval

resolution function; however, the operations with nine values are a little

more complex. Function resolve9still does a pairwise comparison of the

input values to determine the resultant value. With a nine-value system,

the comparison operation is more complicated, and therefore some constant

arrays were declared to make the job easier.

The constant get_strengthreturns the driving strength of the driver

value. The constant x_tabreturns the appropriate unknown nine-state

value, given the strength of the input. These constants could have been

implemented as IFstatements or CASEstatements, but constant arrays

are much more efficient.

In the nine-value system, there are three values at the lowest strength

level, so the variable result has to be initialized more carefully to predict

correct results. If there are no drivers, the range attribute of argument s

returns 0, and the default value (ZX) is returned.

Let’s look at a few examples of driver-input arguments and see what

the resolution function predicts. An example of two drivers is shown in

Figure 5-4.

This example contains two driver values,Z1and R0. Variable resultis

initialized to the first driver value, and the loop executes as many times

as there are drivers. The first time through the loop,resultequals Z1and

the first driver equals Z1. Variable resultremains at Z1because the

values are equal. The next time through the loop, variable resultcon-

tains Z1, and the second driver contains R0. The constant get_strength

returns strength R. The constant get_strengthfor variable resultreturns

strength Z. Strength Ris lexically greater than strength Z. This is because

value Rhas a higher position number than Z, because Ris listed after Z

in the type declaration for type strength. The fact that the new driver has

Driver Values

Initial

Value

Z1 Z1 R0

Z1

R0 Resultant Value

Figure 5-4
Nine State Resolution
with Two Values.