Once the devices have been placed, device properties can be assigned. This is the method by which Xic knows the values, models, and other characteristics of the devices. Device properties are initially added with the Property Editor brought up by the Properties button in the Edit Menu. The Property Editor contains a text window showing the properties of a selected device, if any. The features and capabilities of the Property Editor are rather complicated, and are described fully in the section of this manual (10.10) describing the Properties command in the Edit Menu. This section will describe some of the basic operations.
At this point there are four properties of interest: devref, value, model, and param. The purpose of the devref property is to hold the name of a device whose current is to be referenced. This is used by the current-controlled sources and switch devices only. The value and model are just different names for the same underlying text field, thus a device should not be assigned both a value and a model property. The param property will hold text for initial condition and parameter assignment.
The string for a device, which will be generated in SPICE output, has the generic form
device_name node_list [dev_ref] model_or_value [parameters]
The current-controlled dependent sources and switch require a devref property. This should not be used in other devices. Every device should have a model or value assigned. The parameter (param property) is optional, but may be needed for certain devices for particular types of simulation. It is also used to provide parameter values, such as the width or length of a MOSFET. This is where knowledge of the SPICE syntax is necessary, in order to know what parameters are required for a given device.
For simple devices such as resistors, only a value property is generally required. To apply a value property, with the Property Editor visible, click on the device to receive the property. The editor will list any existing properties, and the selected device will be highlighted. From the Add menu of the Property Editor, press the Value button, and enter the value on the prompt line, followed by Enter. A label showing the new value will appear next to the selected device.
The ``value'' can be just about any string, so it is important that this input have relevance to SPICE. The format of the numerical entries is as recognized by SPICE, in MKS units. One common error is to leave off the units, e.g., entering ``50'' for the value of a capacitor when the correct entry should be ``50fF''. Of course, ``50e-15'' would suffice as well in this case.
The Global button on the Property Editor can be used to set the properties of several devices at once. The Edit button can be used to edit an existing property. Once a property has been assigned to a device, copies of the device will contain the same property, thus it may be preferable to assign properties in part early in the placement step, and generate copies of similar devices rather than placing new instances.
Once a property has been assigned, it can be changed with the label editor, thus the Property Editor needs to be invoked only for the initial assignment. To change the value of any editable property, select the label displaying that value (you can select properties in multiple devices). Then, press the label button in the side menu. This will prompt for a new value, and when given, all of the selected labels will be updated with the new value, and the underlying properties will have been changed.