Question

Explain how to do the left edge algorithm in vlsi design

Answer 1

The left edge is very simple algorithm and it has an assumption.This algorithm is the most common channel routing algorithm. This algorithm has various assumptions which are the first assumption is only twoterminal nets the second assumption is that no vertical constraints, the third one is HV two layer model and last is doglegs are not allowed.

Basic steps to solve a left edge algorithm are-

1. sort the nets according to the x-cordinate of the leftmost tterminal of the net.

2.Route the nets one by one according to the order.

3.For a net, scan the tracks from top to bottom and give this value to the first track that can accomodate it.

in the absence of vertical constraints, this algorithm produces a minimum track solution.

Extension to left edge algorithm-

1.Vertical constraints may exist, but there are no directed cycles in the VCG

2. Select the net for routing if both the following condition are true:-

       I. The x-cordinate of the leftmost terminal is the least.

       II.There is no edge incident on the vertex corresponding to that net in the VCG.

3. Afterrouting a net, the corresponding vertex and the incident edges are deleted from the VCG.

4.Other constraints are the same as the basic left edge algorithm.

Drawback of left edge algorithm-

1.The entire net is on the single track.

2. sometimes leads to routing with more tracks then necessary.

The steps wise solution of an example-