Digital IP protection using threshold voltage control

Joseph Davis, Niranjan Kulkarni, Jinghua Yang, Enis Dengi, Sarma Vrudhula

Research output: Chapter in Book/Report/Conference proceedingConference contribution

7 Scopus citations


This paper proposes a method to completely hide the functionality of a digital standard cell. This is accomplished by a differential threshold logic gate (TLG). A TLG with n inputs implements a subset of Boolean functions of n variables that are linear threshold functions. The output of such a gate is one if and only if an integer weighted linear arithmetic sum of the inputs equals or exceeds a given integer threshold. We present a novel architecture of a TLG that not only allows a single TLG to implement a large number of complex logic functions, which would require multiple levels of logic when implemented using conventional logic primitives, but also allows the selection of that subset of functions by assignment of the transistor threshold voltages to the input transistors. To obfuscate the functionality of the TLG, weights of some inputs are set to zero by setting their device threshold to be a high Vt. The threshold voltage of the remaining transistors is set to low Vt to increase their transconductance. The number of low Vt transistors whose gates are driven by a given input xi determines the weight of that input. The function of a TLG is not determined by the cell itself but rather the signals that are connected to its inputs. This makes it possible to hide the support set of the function by essentially removing some variable from the support set of the function. This is done by selective assignment of high and low Vt to the input transistors. We describe how a standard cell library of TLGs can be mixed with conventional standard cells to realize complex logic circuits, whose function can never be discovered by reverse engineering. A 32-bit Wallace tree multiplier and a 28-bit 4-tap filter were synthesized on an ST 65nm process, placed and routed, then simulated including extracted parastics with and without obfuscation. By obfuscating the cells the delay was shown to increase by approximately 5% at the cell level. Both obfuscated designs had much lower area (25%) lower area and much lower dynamic power (30%) than their nonobfuscated CMOS counterparts, operating at the same frequency.

Original languageEnglish (US)
Title of host publicationProceedings of the 17th International Symposium on Quality Electronic Design, ISQED 2016
PublisherIEEE Computer Society
Number of pages6
ISBN (Electronic)9781509012138
StatePublished - May 25 2016
Event17th International Symposium on Quality Electronic Design, ISQED 2016 - Santa Clara, United States
Duration: Mar 15 2016Mar 16 2016

Publication series

NameProceedings - International Symposium on Quality Electronic Design, ISQED
ISSN (Print)1948-3287
ISSN (Electronic)1948-3295


Other17th International Symposium on Quality Electronic Design, ISQED 2016
Country/TerritoryUnited States
CitySanta Clara

ASJC Scopus subject areas

  • Hardware and Architecture
  • Electrical and Electronic Engineering
  • Safety, Risk, Reliability and Quality


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