Chapter 9: Tools
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9-2: Design Rule Checking (DRC)
9-2-1: Introduction to DRC |
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There are three built-in design-rule checkers: incremental, hierarchical,
and schematic.
After analysis of the circuit,
you can review the errors by typing ">" and "<" to step to the next and previous error that was found.
You can also see a list of errors in the cell explorer (see Section 4-5-2).
Incremental DRC
The incremental design-rule checker is always running, examining your layout,
and issuing error messages when an error is detected.
It checks only the current cell, and does not consider the contents of cell instances,
lower in the hierarchy.
It therefore offers an instant analysis, but not a complete one.
The incremental DRC also shows simple design-rules violations when a node or arc is being moved.
See Section 2-4-1 for more on this.
Hierarchical DRC
The hierarchical design-rule checker uses the same rules and techniques as the incremental checker,
but it checks all levels of hierarchy below the current cell.
To run it, use the Check Hierarchically command (in menu Tools / DRC).
To check only a selected subset of the current cell, use Check Selection Hierarchically.
When checking hierarchically, it may be the case that a cell is not designed to be checked in isolation,
but must have higher levels of the hierarchy considered.
For example, notches in the well areas may be covered at higher levels of hierarchy.
When this happens, tell the DRC to ignore the cell by using the command Add Skip Annotation to Cell.
Schematic DRC
The schematic design-rule checker looks for issues that make drawing or editing of the cell difficult.
These are the errors that is finds:
- Nodes:
- Nodes whose parameters don't match the cell definition (check export names, units, and visibility).
- "Stranded" pins: with no connections, exports, or attached text.
- "Inline" pins: those that sit in a line between two arcs (both of which could be replaced by a single straight arc).
- Nodes whose ports touch but are not connected.
- Invisible pins with text that is offset from the node center (this is an internal consistency check).
- Nodes whose names are the same as network names in the cell.
- Schematic exports whose characteristics are different from the equivalent export in the icon.
- Arcs:
- Unnamed arcs that "dangle": one end is unconnected and unexported (does not apply to busses).
- Arcs that end on another arc without connecting to them.
- Bus arcs whose width is inconsistent with its two nodes.
- Bus pins that "float": do not connect to bus arcs and are not exported.
- Bus taps that connect to a wire which is not part of the bus.
- Bus pins that connect to more than 1 wire.
- Network names that differ only by their case (i.e. networks "A" and "a" are actually different networks).
