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CADENCE CONFIDENTIAL

1  CADENCE DESIGN SYSTEMS, INC. 

Liberty Requirements 

For Multiple Voltage Islands 

Mark Hahn

Senior Architect

2

CADENCE CONFIDENTIAL

Modeling Issues

Need examples of describing multiple voltage islands
in the same file

Show use of calc_mode to distinguish between different
characterizations of the same cell

Since multiple voltage island support requires SPDM,
need SPPM too
Need different delay and slew thresholds for each
island

Thresholds are defined as a percentage of the voltage
range

input/output_threshold_pct_*
slew_lower/upper_pct_*

Problem as voltage range changes between islands

3

CADENCE CONFIDENTIAL

Modeling Issues (cont.)

Need to model effect of IR drop on both power and
ground

Ambiguous how characterization was done

Using just power rail voltage?
Using difference between power and ground rail?

Ground voltage affects thresholds
Problem for level shifters

SPDM only allows two voltage variables, but there
can be four relevant voltages

VDD = 2.9V

OUT

VSS = 0.1V

4

CADENCE CONFIDENTIAL

Modeling Issues (cont.)

Need to distinguish between reference and driven voltage
ranges on inputs

Transistor source/drain connections determine reference
voltage
Driver voltage affects transistor gate