INTRODUCTION TO VHDL-AMS
105
6.4
Description of Nonconservative Systems
Nonconservative semantics describe parts of analog systems where
Kirchhoff's laws do not apply. The energy in nonconservative systems does
not remain constant, but it is added from or lost to the outside of the system.
In modeling we usually name those systems nonconservative, where
unidirectional signals instead of through and across quantities are present,
for instance when modeling control systems. Ports of nonconservative
terminals carry only analog waveforms. Control subsystems can be modeled
using nonconservative ports. These ports are characterized in the following
way:
Nonconservative ports are so-called quantity ports. They also carry a
direction mode in or out.
The general form of the interface description of nonconservative
input ports look like
quantity identifier_list : in real_type
The mode in can be omitted.
output ports look like
quantity identifier_list : out real_type
If a quantity interface element within an interface list includes a default
expression as for example
quantity port_identifier : in real_typ := expression;
if the port is unassociated the value of the input quantity equals the
expression.
The identifiers of the quantity ports can be used in the associated
architecture just like quantities.
Each quantity port of mode out increases the number of required
simultaneous statements in the associated architecture.
In a structural description
Quantity ports can be associated to quantities.
A quantity port of mode in may be unconnected or unassociated only if
its declaration includes a default expression.
A quantity port of mode out may be unconnected if its type is not an
unconstrained array.
If any quantity is associated as an actual with more than one formal of
mode out an error results.
It should be checked whether the simulation tool being used supports
unassociated ports.