Chapter 5: Arcs
5-2: Constraints
5-2-1: Rigid and Fixed-Angle Arcs

The first constraint in Electric is the rigid constraint. When an arc is made rigid, it cannot change length. If a node on either end is moved, the other node and the arc move by the same amount. Besides keeping a constant length, rigid arcs attach in a fixed way to their nodes. This means that if the node rotates or mirrors, the arc spins about so that the overall configuration does not change. Without this rigidity constraint, arcs simply stretch and rotate to keep their connectivity.

The second constraint, which is used only if an arc is not rigid, is the fixed-angle constraint. This constraint forces a wire to remain at a constant angle (usually used to keep horizontal and vertical wires in their Manhattan orientations). For example, if a vertical fixed-angle arc connects two nodes, and the bottom node moves left, then the arc and the top node also move left by the same amount. If that bottom node moves down, the arc simply stretches without affecting the other node. If the bottom node moves down and to the left, the arc both moves and stretches. Rotation of nodes causes no change to fixed-angle arcs unless the arc is connected to an off-center port, in which case a slight translation and stretch may occur.

Figure 5.1

Most IC layout is done with Manhattan geometry. If you suspect that some of your wires have become skewed, use the Show Nonmanhattan command (in menu Edit / Cleanup Cell).

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