Design Guide
75
Memory Interface Routing Guidelines
6.4
Source Clocked Signal Group Routing
The MCH drives the command clock signals required by the DDR interface; therefore, no external
clock driver is required for the DDR interface. The source-clocked signals are “clocked” into the
DIMMs using the command clock signals. Because the MCH drives the command clock signals
and the source-clocked signals together, these signals can be source clocked. That is, the MCH
drives the command clock in the center of the valid window, and the source-clocked signals
propagate with the command clock signal. Therefore, the critical timing is the difference between
the command clock flight time and the source clocked signal flight time. The absolute flight time is
not as critical.
The source-clocked signals have a topology similar to the source synchronous signals. These
signals require parallel termination resistors (Rtt) to DDR VTERM. The MCH requires matching
the lengths of the source-clocked signals to the lengths of the command clocks for each DIMM
within 2.0 inches. For example, if CMDCLK0 and CMDCLK0# are 3 inches long, all source
clocked signals from MCH to the DIMM that CMDCLK0/CMDCLK0# is routed to should be 3
inches ± 2.0 inches.
f
NOTE:
Indicated lengths measure from the MCH pin to the DIMM connector pin.
Table 6-5. Source Clocked Signal Group Routing Guidelines
Parameter
Intel
®
E7500
Reference
Signal Group
RAS#, CAS#, WE#, MA[12:0], BA[1:0]
Topology
Daisy Chain
Reference Plane
Ground
Trace Impedance (Zo)
50
Ω
± 10%
Nominal Trace Width
5 mil
Nominal Trace Spacing
15 mil
Trace Length – MCH to DIMM1
1.8” to 6.0”
Trace Length – DIMM to DIMM
0.8” to 1.2”
Trace Length – DIMM to Rtt
< 0.8”
Termination Resistor (Rtt)
22
Ω
± 2%
MCH Breakout Guidelines
5/5, < 500 mil
Length Tuning Requirements
To CMDCLK pair: ± 2.0”
Figure 6-10. Source Clocked Signal Topology
DIMMs
MCH
RAS#, CAS#, WE#
MA[12:0], BA[1:0]
DDR VTERM
(1.25V)
MCH to DIMM1
DIMM to
DIMM
Rtt
Rtt
DIMM to
DIMM
DIMM to
DIMM
DIMM
to Rtt
Summary of Contents for Xeon
Page 24: ...Introduction 24 Design Guide This page is intentionally left blank ...
Page 30: ...Component Quadrant Layout 30 Design Guide This page is intentionally left blank ...
Page 52: ...Platform Clock Routing Guidelines 52 Design Guide This page is intentionally left blank ...
Page 66: ...System Bus Routing Guidelines 66 Design Guide This page is intentionally left blank ...
Page 118: ...Intel 82870P2 P64H2 118 Design Guide This page is intentionally left blank ...
Page 146: ...I O Controller Hub 146 Design Guide This page is intentionally left blank ...
Page 148: ...Debug Port 148 Design Guide This page is intentionally left blank ...
Page 210: ...Schematic Checklist 210 Design Guide This page is intentionally left blank ...
Page 220: ...Layout Checklist 220 Design Guide This page is intentionally left blank ...
Page 222: ...Schematics 222 Design Guide This page is intentionally left blank ...