Memory Interface Routing Guidelines
70 Design Guide
6.2 Source Synchronous Signal Group
The MCH source synchronous signals are divided into groups consisting of data bits (DQ) and
check bits (CB). An associated strobe (DQS) exists for each DQ and CB group, as shown in
Table 6-2. The MCH supports both x4 and x8 devices, and the number of signals in each data group
depends on the type of devices that are populated. For example, if x4 devices are populated, the
72-bit channel is divided into 18 data groups (16 groups consisting of 4 data bits each, and 2 groups
consisting of 4 check bits each). One DQS is associated with each of these groups (18 total).
Likewise, if x8 devices are populated, the 72-bit channel is divided into a total of nine data groups.
In this case, only 9 of the 18 strobes are used.
NOTE:
1
In x4 configurations, the high DQS is associated with the high nibble and the low DQS is associated
with the low nibble. In x8 configurations, only the low DQS is used.
Figure 6-4 shows the trace length requirements for the DQ, DQS and CB signals. All signals in a
data group must be length matched to the associated DQSs within ± 100 mils, as shown in
Figure 6-5. In addition, each DQS at a particular DIMM must be length matched to the CMDCLK/
CMDCLK# pair that is routed to that particular DIMM within ± 1.75", as shown in Figure 6-6.
Length matching past the last DIMM connector is not critical. Route all data signals and their
associated strobes on the same layer. Layer changes are only recommended at MCH ball breakout
and at the series resistor. The source synchronous signals require 10 Ω ± 2% series termination
resistors placed close to and before the first DIMM connector, and 22 Ω ± 2% parallel termination
resistors placed as close as possible and after the last DIMM connector (within 0.8").
Table 6-2. DQ/CB to DQS Mapping
Data Group Associated Strobe
1
DQ[7:0] DQS0, DQS9
DQ[15:8] DQS1, DQS10
DQ[23:16] DQS2, DQS11
DQ[31:24] DQS3, DQS12
DQ[39:32] DQS4, DQS13
DQ[47:40] DQS5, DQS14
DQ[55:48] DQS6, DQS15
DQ[63:56] DQS7, DQS16
CB[7:0] DQS8, DQS17
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