next up previous contents index
Next: Extraction System: Logging and Up: The Extract Menu: Extraction Previous: The Extract Menu: Extraction   Contents   Index

Extraction System: Methodology and Overview

To use the extraction capability, one typically first designs the circuit in electrical mode, producing a schematic, which operates correctly in simulation. One then produces a corresponding layout in physical mode. Generally, the objective and requirement is that the layout vs. schematic test initiated with the Dump LVS button in the Extract Menu will show no errors. This usually requires that the subcircuits, if any, individually pass LVS. Thus, one would build the cell hierarchy from the bottom up, enforcing LVS to pass at each level.

The Xic extraction system differs from others in that it does not (at least presently) provide fixed associations between electrical and physical devices, and subcells. The user has the ability to place cell contact terminals in the layout, and to create net name labels in the layout and set schematic net names by various means, and matching names will associate. Many cells, though, will associate with no user intervention. If the association initially fails, generally a placed cell terminal or two, or judicious use of net name labels, will allow correct association and passing of LVS testing.

There are three important diagnostic and setup tools available from the buttons at the top of the Extract Menu. The Setup button will bring up the Extraction Setup panel. This panel contains four tabbed pages. The Views and Operations page contains controls that make certain extraction-related features, such as terminals and group numbers, visible. It also allows terminal placement and parameter editing. It provides means to clear and run extraction, which is otherwise automatic. There is also provision to select devices and other objects that fail to associate. The other three pages of this panel provide controls which map to variables and flags which control extraction system behavior. Most of this will be set in the technology file, and it is unlikely that there will be a frequent need to change the parameters interactively. The Misc Config page does provide some display attribute choices which may be an exception.

The Path Selection Control panel is obtained from the Net Selections button in the Extract Menu. This allows visualizing conductor groups, and the corresponding nets in the schematic if there is an association. One can click to select the groups/nets, or enter a group number to highlight that group. The highlighting can follow the net as it descends through the cell hierarchy to any depth. The panel provides a very useful tool for diagnosing net connectivity problems.

The Show/Select Devices panel appears on pressing the Device Selections button in the Extract Menu. This will list the devices found in the current physical layout. These devices can be highlighted by index number, or selected by clicking on them. When selected, measured parameters may be printed and/or compared with the dual device in the schematic. The panel is a useful tool for addressing device recognition issues.

There are cases where one starts with a layout, and it is desirable to generate a schematic. There are also situations where the physical and electrical designs are generated in separate files, and it is desirable to merge these into a single file. Xic has provisions to assist in these cases.

Schematics can be generated in various ways. The schematics that are machine-generated by Xic have each device individually connected to gnd or tbar terminals, so there are no wires. These schematics are electrically correct, but lack human-readability and aesthetics. They serve, however, as a starting point if the user wishes to rearrange the devices and add wires as in a normal schematic.

A schematic can be generated from a SPICE file with the Source SPICE button in the Extract menu. This can create devices and subcircuits as needed. Existing devices will have properties updated with values from the SPICE file.

Similarly, the Source Physical button will create or update the schematic from an intermediate SPICE file extracted from the physical layout. Existing devices will have properties updated with values extracted from the physical layout, and missing devices and subcircuits are added.

The Import Control panel from the Convert Menu is used to copy either the electrical or physical part of another cell into the current cell. It is able to extract this information from cell definitions within an archive file. This can be used to combine separate electrical and physical designs into a single hierarchy.

Separate commands are available for generating netlist files from the physical and electrical data. The Dump LVS command in the Extract Menu performs the layout vs. schematic comparison, and prints errors in a file which may be displayed on-screen.

The Dump Phys Netlist command in the Extract Menu generates a connectivity listing extracted from the physical database. This includes a listing of extracted devices, in various formats. One format is SPICE, so that the Dump Phys Netlist command can be used to generate a SPICE listing extracted from the physical layout.

Commands in the Extract Menu also work with the node mapping facility for SPICE output. It is often necessary to know the name of specific circuit nodes in a SPICE file, which by default is not possible as Xic assigns then internally. The node mapping facility, controlled with the nodmp button in the electrical mode side menu, allows the node tokens to be preassigned.

next up previous contents index
Next: Extraction System: Logging and Up: The Extract Menu: Extraction Previous: The Extract Menu: Extraction   Contents   Index
Stephen R. Whiteley 2022-05-28