External Memory Interfaces Intel® Agilex™ FPGA IP User Guide

ID 683216
Date 10/04/2021
Public

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6.5.5.3. Two DIMMs per Channel (2DPC) for SODIMM Topology

The interface covers data bytes (DQ/DQS), address signals, command signals (BA, BG, RAS, CAS, WE, ACT, PAR), control signals (CKE, CS, ODT) and clocks (CLK).

For DDR4 two DIMM per channel (2DPC) designs, Intel® recommends placing the two DIMM connectors for a channel on both the top and bottom sides of the board, for best performance.

Figure 86. Signal connections for DDR4 2DPC SODIMM configuration using SMT DIMM connector

(In the above figure, CLK, CTRL refers to the per-DIMM signals (mem_ck, mem_cke, mem_odt) and remain as point-to-point connections.)

The following table provides specific routing guidelines for two DIMMs per channel in SODIMM topologies for all supported signals in the interface.

Table 104.  Specific DDR4 2DPC routing guidelines for SODIMM configurations
Signal Group Segment Routing Layer Max Length (mil) Target Zse (ohms) 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 300   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 300   4 8 (2h) 12, (3h)          
CTRL BO1 US 50 5000   4 5, 17 5, 17          
BO2 SL 1000   4 5, 17 5, 17          
M SL   45 4.5 8 (2h) 12 (3h)          
BI US 300   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 300   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 300   4              
For an explanation of the guidelines represented in this table, refer to the bullet points immediately following .

The following figure shows the RESET signal scheme and routing guideline for two DIMMs per channel topologies.

Figure 87. Reset scheme for 2DPC DIMM topologies

The target impedance for 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.