New Features in Expert Node Probing

 

Introduction

Expert’s Node Probing is powerful tool for inspection of netlist components on layout. It is designed for highlighting layout objects electrically connected with each others.

The additional features of Node Probing have been added in the new QT version of Expert. Because the layout extraction is being performed by hierarchical Guardian LPE, the user has possibility to get hierarchical node names troughout node tracing. The original net name in cell instance is keeping and is displaying aditionally to net name from top cell.

Short Locator is another new feature in Node Probing. This feature simplifies connectivity investigation and shorts searching on layout.

 

Hierarchical Net Names

The Node Probing in preivious versions of Expert was designed to trace electricallly connected nodes throughout the hierarchy of a cell[1]. But, because Expert used the Maverick netlist extractor, which works on flatten layout only, the names of nets can be taken from flatten top cell. The original net names in low levels of hierarchy became unavailable.

New version of Expert uses hierarchical netlist extractor Guardian LPE. For hierarchical extraction the original net names in cells instances are stored and are displayed when net is traced in Node Probing. This feature helps to check the hierarchical connectivity and to find possible wrong connection.

Below there is an example which demonstrates this feature. The part of hierarchical netlist is shown Figure 1 a. Subcircuit “TRA48” contains instances of “DFFC” and “OR2” subcircuits. Net with name “D1” in top subcircuit “TRA48” is connected to pin “D” of XI15 instance of “DFFC” subcircuit and to pin “Y” of XI5 instance of “OR2” subcircuit. So during net tracing in Node Probing we should see these net names for objects from different instances. Figures 1 b,c,d show the highlighted net, which has name “D1” in top cell “TRA48” and names “D” and “Y” in instances of “DFFC” and “OR2”.

Figure 1a. The part of hierarchical netlist. Netlist corresponds to layout on Figure 1 b,c,d.

 

Figure 1b. Highlighted net obtained name “D1” from top cell TRA48.

 

Figure 1c. Highlighted net “D1” has name “D” in the instance XI15 of cell DFFC.

 

Figure 1d. Highlighted net “D1” has name “Y” in the instance XI5 of cell OR2.

 

Short Locator

Short Locator is a new feature of the Node Probing in Expert. It permits to simplify search of shorts in layout. Short Locator is based on well known Dijkstra’s algorithm for shortest-paths problem [2].

Short Locator works with the net selected by Node Probing. To build a graph for Dijkstra’s algorithm the geometrical objects from highlighted net are splitted into set of trapezoids. The trapezoids from one layer touch each other but don’t overlap. The middle points of common parts of edges of these trapezoids became graph vertexes. Each set of connected objects from one layer produces own subgraph. Subgraphs are combined to one graph using additional graph vertexes which are centers of overlapping area of two trapezoids from diffrent connected layers.

Figure 2 shows small example. There are three layers M1, M2 and Contact. Layer M1 has electrical connection with layer Contact and layer M2 has electrical connection with layer Contact too, but there is no direct connection between M1 and M2. Wire from layer M1 is splitted to 3 trapezoids and wire from layer M2 is splitted to 2 trapezoids. The orginal graph consists from 4 points A,B,C,D. Points 1 and 2 are added to graph when short path between these points are searched.

Figure 2. Path between point1 and 2. A, B, C, D are vertexes of graph which is used by Dijkstra’s algorithm.

 

Really Short Locator builds the shortest paths between sequentinal pairs of points on selected net. To run Short Locator user should at first select net in Node Probing. When selected net has been highlighted, Short Locator can be run trough menu “Verification”/”Node Probing”/”Short Locator”. Clicks on left mouse button set points for short path search. The point input is finished by click on right mouse button. When points are set, Short Locator attempts to build short paths between sequentinal pairs of points, like point1 - point 2, point 2 - point 3, and so on.

Some kinds of single shorts between two nets can be detected by building short paths between 3 points. If we know what objects should belong to different nets on shorted net, we can set first and third points on objects from one net, and second point on the object from onother net. Short locator will build two paths between first and second points, and between second and third points. Because it is single short, the both paths will go through this short and will have the common part along second net. If first and third points are placed in different ends of the first net, there is good probability that the paths will split in the place of short, and, so short will be detected very simple.

Figure 3 illustrates this feature. Figures 3a and 3b show two different nets in original layout. These two nets are shorted in modified layout, see Figure 3c. Figure 3d shows the shortest paths between 3 points. The short is located in place where shortest paths are splitted.

Figure 3a. First net “Q0” in original layout.

 

Figure 3b. Second net Q1 in original layout.

 

Figure 3c. Net in modified layout, which obtained from two original nets by shortening. Place of short is shown in circle.

 

Figure 3d. Shortest paths between points 1,2 and 2,3. Shortest paths are splitted in the place of the short.

 

Conclusion

Node Probing tool in Expert Layout Editor obtained new features, that make it more powerfull. Hierarchical net naming and Short Locator are intended to simplify and accelerate layout debugging.

 

References

  1. Expert: Recent Improvements in Hierarchical Layout Inspection”, Simulation Standard, Volume 11, Number 9, September 2000.
  2. Cormen, T. H.; Leiserson C. E.; & Rivest R. L. (1990) Introduction to Algorithms. MIT Press. ISBN 0-262-03141-8

 

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