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Quality Metrics for Synthesised Netlist

The quality of the synthesised multiplier netlist is measured with four metrics.

Critical Path

The critical path is the path with the biggest delay from any input to any output. The delay includes the RC delay of the input assuming an input drive strength of 1ps/fF (roughly x3 drive strength) and the delay driving a 50fF load. Each cell's delay is the average of its rise and fall delay, and the total path is the sum of these averages.

The faster the critical path the better the netlist.

Gate Count

The gate count is is the area occupied by the netlist. One gate is 3 routing tracks wide, and the cell height is 9 tracks tall. Each track is 8λ where for this 0.13µm technology lambda = 0.055µm. Thus the area in microns2 is
area in gates × 3 × 9 × 8 × 8 × 0.0552. One gate occupies 5.2272µm2.

The smaller the gate count the better the netlist.

Cell Count

The cell count is the number of separate instances needed to make the function. If the cell library is well matched to the synthesised function, then there will be fewer instances needed.

The smaller the cell count the better the netlist

Porosity

The porosity is the inverse of the connector density. If there are lots of connectors, then a wire must be routed around all the connectors, or routed in a higher metal layer. This is known as routing congestion. Here, the porosity is measured as

1 - number_of_connectors/width_of_all_cells

where the cell width is measured in tracks.

The higher the porosity, the more routable the netlist.