Visible to Intel only — GUID: ttn1544551463023
Ixiasoft
Visible to Intel only — GUID: ttn1544551463023
Ixiasoft
7.5.4.7. Clamshell Topology
The small footprint of the clamshell topology requires less board space than fly-by topology. However, the close proximity of the memory devices in clamshell topology increases the complexity of the required device routing to prevent signal integrity problems.
Clamshell topology uses Address Mirroring to minimize undesired effects such as cross-talk, by splitting the chip select signal for each rank:
- A chip select that accesses the top layer of components, which have not been mirrored.
- A chip select that accesses the bottom layer of components, which have been mirrored.
The total number of chip selects required is double the interface's rank — for example, a single-rank memory interface requires two chip selects. The two chip selects are required for proper calibration of the interface, as a way of accounting for address mirroring. Because the I/O columns have 4 chip-select pins, an external memory interface for a clamshell memory topology has a maximum of 2 ranks, in contrast with the fly-by topology which supports up to 4 ranks.
The JEDEC specification JESD21-C defines address mirroring for DDR4 as shown in the table below.
Memory Controller Pin | DRAM Pin (Non-Mirrored) | DRAM Pin (Mirrored) |
---|---|---|
A3 | A3 | A4 |
A4 | A4 | A3 |
A5 | A5 | A6 |
A6 | A6 | A5 |
A7 | A7 | A8 |
A8 | A8 | A7 |
A11 | A11 | A13 |
A13 | A13 | A11 |
BA0 | BA0 | BA1 |
BA1 | BA1 | BA0 |
BG0 (1) | BG0 | BG1 |
BG1 (1) | BG1 | BG0 |
(1) BG0 and BG1 can be mirrored only when pin BG1 is present on the memory device. |
Enabling Clamshell Topology in Your External Memory Interface
- Configure a single memory interface according to your requirements.
- Select Use clamshell layout on the General tab in the parameter editor.
- Set the number of chip-select pins equal to the number of ranks.
Mapping
Rank | Top/Bottom of Memory Device | CS Pin on Memory Device | CS Pin on FPGA |
---|---|---|---|
0 | Top | CS0 | CS0 |
0 | Bottom | CS0 | CS1 |
Rank | Top/Bottom of Memory Device | CS Pin on Memory Device | CS Pin on FPGA |
---|---|---|---|
0 | Top | CS0 | CS0 |
0 | Bottom | CS0 | CS2 |
1 | Top | CS1 | CS1 |
1 | Bottom | CS1 | CS3 |