24 July 2013

Wire load models


1.    Wire load models

Extraction data from already routed designs are used to build a lookup table known as the wire load model (WLM). WLM is based on the statistical estimates of R and C based on “Net Fan-out”.

For fanouts greater than those specified in a wire load table, a “slope factor” is specified for linear extrapolation.
wire_load (“5KGATES”) {
resistance : 0.000271 -------------> R per unit length
capacitance : 0.00017 -------------> C per unit length
slope : 29.4005 ---------------------> Used for linear extrapolation
fanout_length (1, 18.38) ----------> (fanout = 1, length = 18.38)
fanout_length (2, 47.78)
fanout_length (3, 77.18)
fanout_length (4, 106.58)
fanout_length (5, 135.98)
}
Eg:
Fanout = 7
Net length = 135.98 + 2 x 29.4005 (slope) = 194.78 ----------> length of net with fanout of 7
Resistance = 194.78 x 0.000271 = 0.05279 units

Capacitance = 194.78 x 0.00017 = 0.03311 units
 

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