Electric VLSI Design System User's Manual

Chapter 5: Arcs

5-2: Constraints
5-2-2: Slidable Arcs

Another constraining factor, available only for nonrigid arcs, is slidability. When an arc is slidable, its endpoints may move about within their ports. To understand this fully, you should know exactly where the arc endpoint is located. Most arcs are defined to extend past the endpoint by one-half of their width. This means that the arc endpoint is centered in the end of the arc rectangle. If the arc is 2 wide, then the endpoint is indented 1 from the edge of its rectangle. All arc endpoints must be inside of the port to which they connect. If the port is a single point, then there is no question of where the arc may attach. If, however, the port has a larger area, as in the case of contacts or stretched nodes, then the arc can actually connect in any number of locations.

When moving nodes or arcs, slidability allows the connection to adjust without moving the other component. Moving a slidable arc will not adjust the node position if the arc's endpoint is still inside of the node's port. Similarly, moving a node with slidable arcs will not adjust the arc positions if their endpoints are still inside of the node's port (this is illustrated above).

When an arc is constrained to be not-slidable, any motion of a node or arc causes everything connected to also move.

Note that slidability occurs only for complete motions and not for parts of a motion. If a node or arc moves by 10 and there is room in the port to slide by 1, then no sliding occurs, and the connected components also move by 10.

Because ports have area, and because arcs end somewhere inside of that area, the actual ending point can vary considerably. If the arc is at the far side of the port, it may protrude out of the far side of the node, causing unwanted extra geometry. You can shorten an arc so that its endpoint is at the very end and is not indented. This may help eliminate notches in the layout. To do this, use the Shorten Selected Arcs command (in menu Edit / Cleanup Cell).

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