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6.5.5.2. Two DIMMs per Channel (2DPC) for UDIMM, RDIMM, and LRDIMM DDR4 Topologies
The following figure illustrates the signal connection topology for a PTH type of connector for UDIMM, RDIMM, and LRDIMM topologies.
![](/content/dam/docs/us/en/683216/22-3-2-6-1/wyi1571938635453.png)
![](/content/dam/docs/us/en/683216/22-3-2-6-1/lbh1571938728390.png)
The following figure illustrates the signal connection topology for an SMT type of connector for UDIMM, RDIMM, and LRDIMM topologies.
![](/content/dam/docs/us/en/683216/22-3-2-6-1/zvh1571938807998.png)
![](/content/dam/docs/us/en/683216/22-3-2-6-1/ybe1571938878239.png)
The following table provides specific routing guidelines for two DIMMs per channel in UDIMM, RDIMM, LRDIMM, and SODIMM topologies for all supported signals in the interface.
Signal Group | Segment | Routing Layer | Max Length (mil) | Target Zse (ohm) | Trace Width, W (mil) | Trace Spacing, S1 (mil): Within Group | Trace Spacing, S2 (mil): CMD/CTRL/CLK to DQ/DQS | Trace Spacing, S3 (mil):DQ Nibble to Nibble | Trace Spacing (mil), Within DIFF Pair | Trace Spacing (mil), DQS pair to DQ | Trace Spacing (mil), CLK pair to CMD/CTRL/CKE | Channel to Channel Spacing (DQ to DQ between two channels) | |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Segment | Total MB | ||||||||||||
CLK | BO1 | US | 50 | 5000 | 4 | 5, 17 | 5, 17 | 4 | 17 | ||||
BO2 | SL | 1000 | 4 | 5, 17 | 5, 17 | 4 | 17 | ||||||
M | SL | 45 | 4.5 | 12 (3h) | 4 | 12 (3h) | |||||||
BI | US | 50 | 4 | 12 (3h) | 4 | 12 (3h) | |||||||
CMD, ALERT | BO1 | US | 50 | 5000 | 4 | 5, 17 | 5, 17 | ||||||
BO2 | SL | 1000 | 4 | 5, 17 | 5, 17 | ||||||||
M | SL | 40 | 5.5 | 8 (2h) | 12 (3h) | ||||||||
BI | US | 100 | 4 | 8 (2h) | 12 (3h) | ||||||||
D2D | SL | PTH: 340 SMT: 400 | 5.5 | 8 (2h) | 12 (3h) | ||||||||
CTRL | BO1 | US | 50 | 5000 | 4 | 5, 17 | 5, 17 | ||||||
BO2 | SL | 100 | 4 | 5, 17 | 5, 17 | ||||||||
M | SL | 45 | 4.5 | 8 (2h) | 12 (3h) | ||||||||
BI | US | 100 | 4 | 12 (3h) | |||||||||
DQ | BO1 | US | 50 | 5000 | 3 | 5, 17 | 17 | 17 | |||||
BO2 | SL | 1000 | 3 | 5, 17 | 17 | 17 | |||||||
M | SL | 40 | 5.5 | 8 (2h) | 12 (3h) | 16 (4h) | |||||||
BI | US | 50 | 4 | 8 (2h) | 12 (3h) | 16 (4h) | |||||||
D2D | SL | PTH: 500 SMT: 400 | 4 | 8 (2h) | 12 (3h) | 16 (4h) | |||||||
DQS | BO1 | US | 50 | 5000 | 3 | 5, 17 | 4 | 17 | |||||
BO2 | SL | 1000 | 3 | 5, 17 | 4 | 17 | |||||||
M | SL | 40 | 5.5 | 4 | 12 (3h) | ||||||||
BI | US | 50 | 4 | ||||||||||
D2D | SL | PTH: 500 SMT: 400 | 4 | ||||||||||
For an explanation of the guidelines represented in this table, refer to the bullet points immediately following Figure 126. |
The following figure shows the RESET signal scheme and routing guideline for two DIMMs per channel topologies.
![](/content/dam/docs/us/en/683216/22-3-2-6-1/zry1571940985811.png)
The target impedance for the RESET signal is 50 ohms. The RESET signal shall have at least 3×h (where h stands for trace to nearest reference plane height or distance) spacing to other nearby signals on the same layer. The end-to-end RESET trace length is not limited but shall not exceed 5 inches.