Cell Organization and Libraries

Xic provides several methods by which collections of cells can be organized.

• Xic makes use of a search path for file names given to Xic which do not have a directory path prepended. A search path is a list of directories where Xic searches for a named file. If the file name contains a full path, that path will be used to obtain the file. If a file name has a relative path, Xic will look for the file relative to each of the directories in the search path. The search path can be set in the technology file, or by setting the Path variable with the !set command. The current path can be examined by entering ``!set'', which pops up a list of the currently defined variables, including Path. The directories are searched in left-to-right order.

• Xic accepts library files. These are text-mode files which contain references to cells and other libraries, and may contain cell definitions. If a library file is ``open'', cell names referenced or defined in the library will be resolved through the library, before resolving through the search path. The name of a cell reference in a library is the name by which the cell will be added to Xic memory, which can be different from the name by which the cell is stored on disk. The fact that a library can reference other libraries allows a hierarchy to be established for accessing cells, independent of the search path.

  The Libraries List button in the File Menu brings up a panel which lists the currently open libraries, and provides command buttons for performing basic manipulations on libraries, including opening/closing, viewing content, and opening cells.

• Cells contained in archive files can be randomly accessed from the file, thus these files can be used for archival purposes. The Contents button in the panel brought up by the Files List button in the Files Menu will display the cells contained in these files. The Contents button will also list the contents of library files. Individual cells (and their subcells) can be opened for editing or placement through this panel. Also, when giving a name to the Open command, or the place command in the side menu, one can give two names: the name of an archive file and a space-separated name of a cell in the archive. That cell will be opened. If the cell name is not given, the top-level cell in the archive is opened.

The strategy used to organize cells is highly dependent upon the user's needs and preferences. Below are some recommendations which are probably suitable for most applications.

• Keep the search path short. This can usually consist of two directories: the current directory (``.'') listed first, and a root directory for the user's design files. The search path is most conveniently defined in the technology file, with the Path keyword. The search path has the disadvantage that all components are visible at all times. If a cell name appears more than once in the search path, only the first instance will be found, unless the full path is given. Libraries, on the other hand, can be opened and closed easily, changing the accessibility of the contents.
• Use hierarchies of libraries rooted in the search path to organize cells. One can open only the libraries in use, preventing loading of cells unexpectedly.
• Place collections of cells to be referenced through libraries in separate directories not in the search path. Alternatively, the Xic cell definitions can be incorporated directly into the library file. The cells can otherwise be kept as individual cells of any compatible format, or combined into a single archive file.

Library files have a simple format which allows the user to easily create and customize them with a text editor. There is a !mklib command in Xic which can create a new library or append to an existing library references to cells in the current editing hierarchy or cells in a given archive file.

If one clicks on a reference in a library content listing which points to another library, without a resolving ``cellname'', a second content window appears providing a listing of the second library's references. Thus, when constructing library files, one should use an easily recognizable name for browsable references to other libraries. This is natural, if the file name is used as the reference name, and the filename has a ``.lib'' extension as is recommended.