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In order to use standard models, there has to be a framework for terminals and variables, which is where the standard packages are used. There is a complete IEEE Std (1076.1.1) that defines the standard packages in their entirety; however, it is useful to look at a simplified package (electrical systems in this case) to see how the package is put together.
For example electrical systems models need to be able to handle several key aspects:
• electrical connection points;
• electrical through variables, that is, current;
• electrical across variables, that is, voltages.
The electrical systems package needs to encompass these elements.
First the basic subtypes need to be defined. In all the analog systems and types, the basic underlying VHDL type is always real and so the voltage and current must be defined as subtypes of real.
1 subtype voltage is real;
2 subtype current is real;
Notice that there is no automatic unit assignment for either, but this is handled separately by the unit and symbol attributes in IEEE Std 1076.1.1. For example, for voltage the unit is defined as Volt and the symbol is defined as V.
The remainder of the basic electrical type definition then links these subtypes to the through and across variable of the type, respectively:
1 package electrical_system is
2 subtype voltage is real;
3 subtype current is real;
4 nature electrical is
5 voltage across
6 current through
7 ground reference;
8 end package electrical_system;
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