ONE_HOT Reduced Vector Sets for Wide MUX Cells

013_cell : ONE_HOT Reduced Vector Sets for Wide MUX Cells

Minimum Required Versions: AccuCell 2.6.0.R, SmartSpice 4.6.2.R

This example explains the characterization and modeling options for a wide MUX cell with 16 data inputs and 16 pairs of select lines for a total of 48 inputs using the embedded SmartSpice SPICE engine to perform characterization.

Wide MUX cells can present difficulty with characterization using standard exhaustive expansion vector generation methods due to the very large number of vectors (2^N where N=# of inputs) that they would otherwise require if they were treated just like any standard logic function.

The use of ONE_HOT methods to define vector set generation restrictions is a key part in minimizing the vector set compared to the fully expanded set. Other special processing specific to MUltipleXor(MUX) characterization for optimizing the vector sequence also occurs by default when a MUX cell is detected by AccuCell. It is therefore important that the .class file or log file details be checked to affirm that the cell was recognized as a MUX cell.

MUX cells like XOR cells, have BOTH positive_unate and negative_unate vector dependent timing behavior in that the signal that "gates" the timing event may not directly determine the state of the output change. This results in the need for the vectors to be generated with this effect accounted for and requires more than one vector sequence for the SAME timing arc for full characterization. The result is a a need for a state dependent timing model process.