VHDL Programming

124 Chapter Five

Z0, Z1, ZX, R0, R1, RX, F0, F1, FX

weakest-----------------------------strongest

The system consists of three strengths and three logic values. The three

strengths represent the following:

Z—High impedance strength, few hundred k of resistance

R—Resistive, few k of resistance

F—Forcing, few ohms of resistance

The three logic levels are represented as follows:

 0 —Logical 0 or false

 1 —Logical 1 or true

X—Logical unknown

The nine states are described as follows:

Z0—High-impedance 0

Z1—High-impedance 1

ZX—High-impedance unknown

R0—Resistive 0

R1—Resistive 1

RX—Resistive unknown

F0—Forcing 0

F1—Forcing 1

FX—Forcing unknown

A few simple rules can be used to define how the resolution function

should work:

—Strongest strength always wins.

—If strengths are the same and values are different, return same

strength but Xvalue.

Following are the type declarations needed for the value system:

PACKAGE ninepack IS

TYPE strength IS (Z, R, F);

TYPE nineval IS ( Z0, Z1, ZX,

TYPE nineval IS ( R0, R1, RX,

TYPE nineval IS ( F0, F1, FX );

TYPE ninevalvec IS ARRAY(natural RANGE <>) OF nineval;