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GTL+ INTERFACE SPECIFICATION
maximum package trace length cannot exceed 250 ps. If the PGA package is socketed, the max-
imum package trace length would be ~225 ps since a typical PGA socket is around 25 ps in elec-
trical length. For a QFP package, which typically requires a short stub on the PCB from the pad
landing to a via (~50 ps), the package lead frame length should be less than ~200 ps.
12.3.2. Package Capacitance
The maximum package pin capacitance is a function of the Input/Output capacitance of the I/O
transceiver. The I/O Buffer specification requires the total of the package capacitance, output
driver, input receiver and ESD structures, as seen from the pin, to be less than 10 pF. Thus, the
larger the I/O transceiver capacitance, the smaller the allowable package capacitance.
12.4.
REF8N NETWORK
The Ref8N network shown in Figure 12-15, which represents an eight-node reference network
(hence the name Ref8N), is used to characterize I/O drivers' behavior into a known environment.
This network is not a worst case, but a representative sample of a typical system environment.
A SPICE deck of the network is also given.
12-24
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