Optimal wire-sizing function with fringing capacitance consideration

  • Authors:
  • Chung-Ping Chen;D. F. Wong

  • Affiliations:
  • Semiconductor R & D Center, Samsung Electronics Co., Ltd., San #24 Nongseo-Ri, Kiheung-Eup, Yongin-Si, Kyungki-Do, Korea;Semiconductor R & D Center, Samsung Electronics Co., Ltd., San #24 Nongseo-Ri, Kiheung-Eup, Yongin-Si, Kyungki-Do, Korea

  • Venue:
  • DAC '97 Proceedings of the 34th annual Design Automation Conference
  • Year:
  • 1997

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Abstract

In this paper, we consider non-uniform wire-sizing under theElmore delay model.Given a wire segment of length L, letf(x) be the width of the wire at position x, 0 驴 x 驴 L.It was shown in [Optimal Wire-sizing formula under the Elmore delay model, Shaping a distributed-RC line to minimize Elmore delay] that the optimal wire-sizing functionwhich minimizes delay is an exponential tapering functionf(x) = ae{-bx}, where a 0 and b 0 are constants.Unfortunately, [Optimal Wire-sizing formula under the Elmore delay model, Shaping a distributed-RC line to minimize Elmore delay] did not consider fringing capacitancewhich is at least comparable in size to area capacitance indeep submicron designs.As a result, exponential taperingis no longer the optimal strategy.In this paper, we showthat the optimal wire-sizing function, taking fringing capacitanceinto consideration, is f(x) = \frac{{ - c_f }}{{2c_0 }}(\frac{1}{{W(\frac{{ - c_f }}{{ae^{ - bx} }})}} + 1) whereW(x) = \sum\nolimits_{n = 1}^\infty{\frac{{( - n)^{n - 1} }}{{n!}}} x^n is the Lambert's W function, c{f}and c{0} are the respective fringing capacitance and area capacitanceof wire per unit square, a 0 and b 0 are constants.The optimal wire-sizing function degenerates into an exponentialtapering function as c}{f} = 0, and degenerates into asquare-root tapering function (f(x)=\sqrt {b - ax}, where a 0and b 0) as c{f} 驴 驴.Our experimental results show thatthe optimal wire-sizing function can significantly reduce theinterconnection delay of exponentially tapered wires.In thecase where lower and upper bounds on the wire widths aregiven, the optimal wire-sizing function is a truncated versionof the above function.Finally, our optimal wire-sizing functioncan be iteratively applied to optimally size all the wiresegments in a routing tree for objectives such as minimizingweighted sink delay, minimizing maximum sink delay, orminimizing area subject to delay bounds at the sinks.