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Properties are one of the schematic editor's most important concepts. They are actually used for a number of purposes but the most importantly they are used to determine how a schematic device behaves during simulation. A property tells the simulator what type of device it is (resistor, BJT, sub-circuit etc.), another property specifies a device's value or model name and, for a hierarchical block, a property specifies the file location of the underlying schematic.

For many applications, you only need to understand the meaning of ref, value and model properties. These are explained below but also in Adding Standard Properties. It is also useful, but not essential, to understand the schematic_path property used in hierarchical blocks.

What is a Property?

A Property is an item of text that is attached to a schematic part to specify some circuit parameter such as a part reference (e.g. R23), value (e.g. 2.2K) or model name (e.g. BC547).

All properties have a name, a value and a number of attributes. A property's value may be displayed on the schematic. Most attributes determine how the value is displayed; an exception is the protected attribute which determines whether a property is allowed to be modified.

A property can have any name (as long as it does not have spaces in it) and any value. However, certain property names have a special meaning and impart a particular functionality on the part that owns it. These special properties are described in the following table. Note, however, that this is not an exhaustive list as many properties are used for special parts and the behaviour they impart is defined in the script that is used to edit those parts.

Property name Function
Ref Part reference. (E.g. R23) All circuit devices must have this property and its value must be unique.
Value Part value or model name. (E.g. BC547). All circuit devices must have this property. (This may be confusing. What is described here is a property of name Value, not the property's value.)
Model Single letter to signify type of device. For list of signifying letters for each device supported by simulator see Summary of Simulator Devices. If absent the first letter of the part reference will be used instead (as for SPICE) For example, a device with a Model property of value Q will always be a BJT regardless of its part reference. Model properties of X, H and F have a special significance as follows:
X Subcircuit instance. pinnames specifier will be added to inform simulator of the devices pin names. The simulator will then choose names for device current vectors which will allow cross-probing of currents from the schematic.
F Current controlled current source. The standard SPICE CCCS is a two terminal device which uses a separate voltage source for the controlling current. SIMetrix provides the facility to use a single four terminal device with pins 3 and 4 for the controlling current and pins 1 and 2 for the output. Any symbol with four terminals and a Model property of F will be treated as a such a device. An additional voltage source will be created by the netlist generator and connected to pins 3 and 4 to be used as the controlling current.
H Current controlled voltage source. As F above but has a voltage output.

For a list of valid device types and their signifying letters see Summary of Simulator Devices.

(In some respects, the special behaviour of Model property values X, F and H is legacy from the past. The recommended method of customising netlist output is to use the Template property, but this was not supported in very early versions of SIMetrix.)
Netname If this property is present on a symbol, all nets connected to any of its pins will be named according to the Value property. The Netname property is used by the Terminal part from menus Place > Connectors > Terminal and Place > Connectors > Small Terminal. The Terminal parts force the net to which it is attached to have a user specified name. (The value of the Netname property will be used in the absence of a Value property).
scterm Identifies the part as a Module Port. These identify connections in hierarchical blocks.
tol, lot, match These are used for Monte Carlo analysis to specify tolerances. See page Monte Carlo Analysis.
schematic_path Path of schematic in hierarchical designs
mapping Rearranges pin order. This is a sequence of numbers each representing a symbol pin order. The order of the numbers in the mapping is the order in which the schematic symbol pins placed on the netlist.

For example the LMC6762B comparator in the library is assigned a mapping of 1,2,5,3,4. The output on the comparator symbol is pin 5 but the model requires this to be the third node in the netlist entry.
params Additional parameters for device appended to value. If Model property is X, the keyword params: prefixes the Params property value.
template Specifies a customised netlist entry for the device. See Template Property for full details.
valuescript Specifies a script to be called when F7 or equivalent menu is selected.
incscript Script to be called when the shift-up key is pressed. This is to increment a part's value. Currently used for potentiometers and some passive devices.
decscript As incscript but for shift-down to decrement a device.
handle This property is automatically allocated to every instance and always has a unique value. Because it is automatically added, it is the only property that every schematic part is guaranteed to possess. This property is protected and therefore cannot be edited
simulator Determines simulator compatibility. See Adding Standard Properties.

Template Property

This is the subject of its own section. See below.

Editing Properties in a Schematic

Unprotected properties of a symbol placed on a schematic may be edited using the popup menu Edit Properties.... This first opens a dialog listing all properties owned by the device. After selecting the property to edit a dialog box similar to the box described in Defining Properties. If the property you select is protected, the dialog box will still open but you will not be able to change any of the settings.

Restoring Properties

This is a method of restoring an instance's properties to the values and attributes of the original symbol. This is especially useful in situations where a symbol has been edited to, for example, add a new property and you wish that new property to be included on existing instances of that symbol.

To restore instances properties follow the instructions below.

  1. Select the instances whose properties you wish to restore.
  2. Select popup menu Restore Properties...
  3. There are two options:

    New Properties Only will only add new properties to the selected instances. That is any property that is present on the symbol definition but not on the schematic instance of it will be added. All other properties will remain intact.

    All Properties will restore all properties to that of the symbol definition. This includes deleting any instance properties that are not in the symbol definition. In effect this will restore the symbol as if it had just been placed using the Place > From Symbol Library menu. Note that REF properties will be automatically annotated to make them unique. This option must be used with care. Don't use it unless you are very clear about what it will do.

This function will restore properties according to the local symbol definition stored in the schematic. This won't necessarily be the same as the global definition in the symbol library. For more information see How Symbols are Stored.