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Method of Estimating Capacitance in RC Constraint Generation

IP.com Disclosure Number: IPCOM000121355D
Original Publication Date: 1991-Aug-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 3 page(s) / 87K

Publishing Venue

IBM

Related People

Luk, WK: AUTHOR [+2]

Abstract

Disclosed is a method to estimate the capacitance of a net and to incorporate it in the generation of RC constraints used for timing- driven layout. In the process of RC constraint generation described in [1] and [2], in order to set up a resistance constraint for each source-sink or sink-sink segment of a net, it is necessary to know the total capacitance of the net in terms of the equivalent resistance of the segment, or the ratio kR where C is the capacitance of a net and Rsegment is the resistance of a segment (source-sink or sink-sink) of the net. If a net has a fanout of N, it requires N segments to connect the net. The N-segment (or wiring configuration) can be determined by connecting the net using normal Steiner tree approximation procedures.

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Method of Estimating Capacitance in RC Constraint Generation

      Disclosed is a method to estimate the capacitance of a
net and to incorporate it in the generation of RC constraints used
for timing- driven layout.  In the process of RC constraint
generation described in [1] and [2], in order to set up a resistance
constraint for each source-sink or sink-sink segment of a net, it is
necessary to know the total capacitance of the net in terms of the
equivalent resistance of the segment, or the ratio kR
where C is the capacitance of a net and Rsegment is the resistance of
a segment (source-sink or sink-sink) of the net.  If a net has a
fanout of N, it requires N segments to connect the net.  The
N-segment (or wiring configuration) can be determined by connecting
the net using normal Steiner tree approximation procedures.

      Since total capacitance can only be accurately determined after
placement and wiring, capacitance is estimated at the time of
constraint generation, before and during placement, and even during
wiring of a net before its completion (wiring configuration partially
known).

      Let r be the average resistance per unit length, and c be the
average capacitance per unit length, then kR can be expressed as
where LSteiner is the length of a given Steiner tree interconnection
of a net, and Lsegment is the length (Manhattan) of a segment.  Let

      In general, for a net with fanout N, the value of kL ranges
between 1/N and 1.  The extreme case kL = 1/N occurs when all the N
sinks use the same identical path to connect to the source, (Figure
1a), maximal sharing.  The other extr...