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E
4.3.
Package Specification
This information is also included for designers of
components for a GTL+ bus. The package that the
I/O transceiver will be placed into must adhere to two
critical parameters. They are package trace length,
(the electrical distance from the pin to the die), and
package capacitance. The specifications for package
trace length and package capacitance are not
explicit, but are implied by the system and I/O buffer
specifications.
4.3.1.
Package Trace Length
The System specification requires that all signals be
routed in a daisy chain fashion, and that no stub in
the network exceed 250 ps in electrical length. The
stub includes any printed circuit board (PCB) routing
to the pin of the package from the "Daisy Chain" net,
as well as a socket if necessary, and the trace length
of the package interconnect (i.e. the electrical length
from the pin, through the package, across a bond
wire if necessary, and to the die). For example, for a
PGA package, which allows PCB routing both to and
from a pin and is soldered to the PCB, the maximum
package trace length cannot exceed 250 ps. If the
PGA package is socketed, the maximum package
trace length would be ~225 ps since a typical PGA
PENTIUM® PRO PROCESSOR AT 150, 166, 180, and 200 MHz
socket is around 25 ps in electrical 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.
4.3.2.
The maximum package pin capacitance is a function
of
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.
4.4.
The Ref8N network shown in Figure 36, 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.
Package Capacitance
the
Input/Output
capacitance
Ref8N Network
of
the
I/O
55
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