3D Timing for Sequential Cells with Dependent Outputs

027_cell : 3D Timing for Sequential Cells with Dependent Outputs

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

This example explains the use of AccuCell's characterization and modeling options for sequential cells with dependent output timing that requires the use of 3D timing characterization and model generation methods to account for the extra effects of multiple loading conditions. As an example a basic D Flip-Flop with rising edge clock and Dependent Outputs using the embedded SmartSpice SPICE engine to perform characterization.

Sequential cells often have a pair of complimentary inverted and non-inverted outputs. For area and speed reasons some sequential cells are designed with dependent outputs that may or may not be buffered from each other. The output of one polarity is also used unbuffered as the input to the other output. As a result any load on the unbuffered output impacts the timing of the other output pin, thereby setting up a dual output cap loading behavior between the dependent and independent output pins. AccuCell can characterize such cells using and extra output pin load index to form a 3D table instead of just the standard 2D slope and load table.

AccuCell has two commands to address this:

DELAYS_3D controls enabling the use of BOTH output pins as timing indexes in addition to clock pin input slope. This results in a 3D timing table.

UNBUF_OUT_LIST specifies the relationship the dependent and independent output pins and their unateness relationship to each other.