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VLSI Design
Volume 7 (1998), Issue 1, Pages 31-57

Automated Synthesis of Skew-Based Clock Distribution Networks

Department of Electrical Engineering, University of Rochester, Rochester, NY 14627, USA

Copyright © 1998 Hindawi Publishing Corporation. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


In this paper a top-down methodology is presented for synthesizing clock distribution networks based on application-dependent localized clock skew. The methodology is divided into four phases: 1) determination of an optimal clock skew schedule for improving circuit performance and reliability; 2) design of the topology of the clock tree based on the circuit hierarchy and minimum clock path delays; 3) design of circuit structures to implement the delay values associated with the branches of the clock tree; and 4) design of the geometric layout of the clock distribution network. Algorithms to determine an optimal clock skew schedule, the optimal clock delay to each register, the network topology, and the buffer circuit dimensions are presented.

The clock distribution network is implemented at the circuit level in CMOS technology and a design strategy based on this technology is presented to implement the individual branch delays. The minimum number of inverters required to implement the branch delays is determined, while preserving the polarity of the clock signal. The clock lines are transformed from distributed resistive-capacitive interconnect lines into purely capacitive interconnect lines by partitioning the RC interconnect lines with inverting repeaters. The inverters are specified by the geometric size of the transistors, the slope of the ramp shaped input/output waveform, and the output load capacitance. The branch delay model integrates an inverter delay model with an interconnect delay model. Maximum errors of less than 2.5% for the delay of the clock paths and 4% for the clock skew between any two registers belonging to the same global data path are obtained as compared with SPICE Level-3.