FSB Design Guidelines
R
44
Intel
®
855PM Chipset Platform Design Guide
Table 4. FSB Source Synchronous Data Signal Routing Guidelines Topology 1
Signal Names
Total Trace Length
CPU MCH
Transmission
Line Type
Min
(inches)
Max
(inches)
Nominal
Impedance
(
)
Width &
spacing (mils)
DINV[3:0]#
DBI[3:0]#
Strip-line
0.5
5.5
55 ± 15%
4 & 12
D[63:0]#
HD[63:0]#
Strip-line
0.5
5.5
55 ±15%
4 & 12
DSTBN[3:0]# HDSTBN[3:0]#
Strip-line
0.5
5.5
55 ± 15%
4 & 12
DSTBP[3:0]# HDSTBP[3:0]#
Strip-line
0.5
5.5
55 ±15%
4 & 12
If routing space constraints do not allow 1:3 spacing of the source synchronous data signals, Table 5
lists alternative routing requirements for some of these signals if 1:2 spacing is used. In both topologies,
the pin-to-pin trace length should be limited to a minimum of 0.5 inches and a maximum of 5.5 inches.
The adherence to tighter characteristic trace impedance tolerances for the alternative routing
requirements allows the closer spacing of the data and bus inversion signals to be achieved. The use of ±
10% tolerance for the trace impedance in the alternative topology allows designs to maintain the same
overall minimum and maximum trace lengths as the primary topology that utilizes a looser ± 15%
tolerance. Although the data and bus inversion signals for the FSB can be routed with 1:2 spacing when
using the tighter trace impedance tolerance, the data strobes
must
maintain 1:3 spacing. In this case, the
processor’s DSTBN[3:0]# and DSTBP[3:0]# strobe signals must be routed to the MCH’s
HDSTBN[3:0]# and HDSTBP[3:0]# strobe signals with 1:3 spacing from all signals even if ± 10% trace
impedance tolerance is used.
Table 5. FSB Source Synchronous Data Signal Routing Guidelines Topology 2
Signal Names
Total Trace Length
CPU MCH
Transmission
Line Type
Min
(inches)
Max
(inches)
Nominal
Impedance
(
)
Width & spacing
(mils)
DINV[3:0]#
DBI[3:0]#
Strip-line
0.5
5.5
55 ± 10%
4 & 8
D[63:0]#
HD[63:0]#
Strip-line
0.5
5.5
55 ± 10%
4 & 8
DSTBN[3:0]#
HDSTBN[3:0]#
Strip-line
0.5
5.5
55 ±10%
4 & 12
DSTBP[3:0]#
HDSTBP[3:0]#
Strip-line
0.5
5.5
55 ± 10%
4 & 12
4.1.3.3.
Source Synchronous – Address
Source synchronous address signals operate at 200 MHz. Thus, their routing requirements are very
similar to the data signals. Refer to Sections 4.1.3.1 and 4.1.3.2 for further details. Table 6 details the
partition of the address signals into matched length groups. Due to the lower operating frequency of the
address signals, pin-to-pin length matching is relaxed to ± 200 mils. Each group is associated with only
one strobe signal. To maximize setup/hold time margin, the address strobes should be trace length
matched to the average trace length of the address signals of their associated group. In addition, each
address signal should be trace length matched within ± 200 mils of its associated strobe signal.
