VHDL Programming

Unaffected

In VHDL87 it was sometimes difficult to describe exactly the behavior

required with a concurrent signal assignment statement. For instance,

there are cases where the modeler wants the value to remain unchanged

if certain conditions are met. In VHDL87 this can be accomplished with

the following statement:

new_state <= state5 WHEN current_state = state1 AND input

= 7 ELSE

state6 WHEN current_state = state5 AND input

= 8 ELSE

state7 WHEN current_state = state2 AND input

= 9 ELSE

new_state;

If none of the above conditions are met the designer has to assign the

current value,new_stateto new_stateto ensure no value change. This

produces the correct value but has a side effect that a transaction is gen-

erated on signal new_state. Any behaviors sensitive to transactions on

new_statewill be evaluated and may update their values causing further

activity when none is wanted.

VHDL93 has the new keyword UNAFFECTEDthat allows no change on a

signal.UNAFFECTEDcauses no value changes or transactions on the signal.

The same statement above rewritten to include this new feature looks

as follows:

new_state <= state5 WHEN current_state = state1 AND input

= 7 ELSE

state6 WHEN current_state = state5 AND input

= 8 ELSE

state7 WHEN current_state = state2 AND input

= 9 ELSE

UNAFFECTED;

Now if none of the conditions are met nothing will be assigned and no

transactions will be generated.

XNOR Operator

VHDL87 contained a number of operators such as OR,NOR,AND, etc. but

did not contain the XNORoperator. Therefore the XNORoperator could not

be overloaded. VHDL93 adds the XNORoperator to the list of operators

466 Appendix D: VHDL93 Updates