What are the DDR5 design challenges?

11th August 2020
Alex Lynn

Changes in DDR5 have introduced a number of design considerations dealing with higher speeds and lower voltages – raising a new round of signal integrity challenges. Designers will need to ensure that motherboards and DIMMs can handle the higher signal speeds. When performing system-level simulations, signal integrity at all DRAM locations need to be checked.

For DDR4 designs, the primary signal integrity challenges were on the dual-data-rate DQ bus, with less attention paid to the lower-speed command address (CA) bus. For DDR5 designs, even the CA bus will require special attention for signal integrity. In DDR4, there was consideration for using differential feedback equalization (DFE) to improve the DQ data channel. But for DDR5, the RCD’s CA bus receivers will also require DFE options to ensure good signal reception.

The power delivery network (PDN) on the motherboard is another consideration, including up to the DIMM with the PMIC. Considering the higher clock and data rates, you will want to make sure that the PDN can handle the load of running at higher speed, with good signal integrity, and with good clean power supplies to the DIMMs.

The DIMM connectors from the motherboard to the DIMM will also have to handle the new clock and data rates. For the system designer, at the higher clock speeds and data rates around the printed circuit board (PCB), more emphasis must be placed on system design for electromagnetic interference and compatibility (EMI and EMC).

How do DDR5 memory interface chipsets harness the advantages of DDR5 for DIMMs?

The good news is that DDR5 memory interface chips improve signal integrity for the command and address signals sent from the host memory controller to the DIMMs. The bus for each of the two channels goes to the RCD and then fans out to the two halves of the DIMM. The RCD effectively reduces the loading on the CA bus that the host memory controller sees.

DDR5 data buffer chips will reduce the effective load on the data bus, enabling the higher-capacity DRAMs on the DIMM without degrading latency.

Rambus offers a DDR5 memory interface chipset that helps designers harness the full advantages of DDR5 while dealing with the signal integrity challenges of higher data, CA and clock speeds.

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