Cells with Unbuffered Passgate Inputs

014_cell : Cells with Unbuffered Passgate Inputs

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

This example explains the characterization and modeling options for a MUltipleXor(MUX) cell with unbuffered passgate data inputs using the embedded SmartSpice SPICE engine to perform characterization.

The use of passgate cell design methods can be very advantagous in achieving specific design objectives however, this cell architecture can also present difficulty with characterization if the same methods for standard logic functions are employed for these cell types without suitable modification in characterization method.

The most common use of passgate design methods is in the design of MUX or XOR cells but the design method and AccuCell do NOT limit the use of such methods of design and characterization to only these cell functions. Some designs utilize unbuffered input pins which result in driver source dependent cell delay times and significant state dependent input load variation effects.

AccuCell permits the user to define a specific driver for characterization to minimize these effects by the use of driver normalization. The delay of the user defined driver is compensated for in deriving the final timing model for the cell. AccuCell has two cell level .cfg file commands that can be used to control and specify the characterization conditions.

UNPROTECT_BUF 1 controls enabling this feature on a cell basis.

UNPROT_BUFFER_TABLE { <p_length> <p_width> <n_length> <n_width> } specifies the unique drive parameters of the buffer cell used to drive the unbuffered cell input pins of this specific cell.

For maximum results accuracy the use of SILVACO's AccuCore is recommended for circuits with critical paths utilizing unbuffered input pin cells. However, this special processing option in AccuCell usually allows adequate accuracy for cell based flows without the need for complex block level custom timing characterization methods in most cases.