DDR5 UDIMM Evolution To Clock Buffered DIMMs (CUDIMM)

DDR5 is the latest generation of PCDDR memory that is used in a wide range of application like data centers, Laptops and personal computers, autonomous driving systems, servers, cloud computing, and gaming are now increasingly being used for AI applications with advances in memory bandwidth and density to allow DDR5 DIMMs (Dual Inline Memory Modules) to support densities higher then 256 GB per DIMM card. The highest speed DDR5 SDRAM devices can support data rates of up to 8800 MTps.

DDR5 SO-DIMMs and UDIMMs

One of the most recognized uses of PCDDR is with client devices like laptops and personal computers. These client devices mostly use two types of DDR5 DIMMs called SO-DIMM (Small Outline Dual Inline Memory Module) and UDIMM (Unbuffered Dual Inline Memory Module).

These types of DIMMs have no signal regeneration or buffering (which, for example, the Registering Clock Driver or the RCD does for clocks/command/control signals for a registered DIMMs). A typical 2-Rank UDIMM with x8 DDR5 SDRAM components has 8 or 10 components per rank depending on the system ECC (Error Correction Code) memory being part of the DIMM.

Why DDR5 Clock Buffer and CUDIMM?

Clocks are one of the most important signals for synchronous devices, and DDR5 SDRAMs are no exception. The host is responsible for the fanout to all the DRAM input ports, such as clocks for UDIMMs. Driving of all these DRAM clocks can put quite a bit of load on the host output drivers, thus affecting the signal quality, which can result in unexpected memory errors. This issue gets amplified when operating at the higher clock and data rates where the clock signals transition from one logic value to the next over a very short time.

Fig.1: Source: Cadence

To solve these signal integrity issues with DRAM clocks, JEDEC has come up with a new type of DDR5 DIMM component that is called DDR5 clock buffer. Clock buffers can be used for both DDR5 SO-DIMMs and DDR5 UDIMMs. DDR5 UDIMMs that include a clock buffer component as part of the DIMM card are called DDR5 CUDIMMs (Clock Buffered UDIMMs).

DDR5 Clock Buffer Overview

DDR5 Clock Buffer is a simple logic device that takes in two sets of input clock pins and drives two sets of clock pins as output per channel. The clock buffer device can operate in three types of clock modes: –

• PLL bypass mode: In this mode, the clock buffer just passes on the input clocks to output without any kind of signal buffering. The PLL bypass mode enabled CUDIMM devices behave like traditional UDIMMs without any buffering of the clocks. This is why it’s also referred to as legacy mode. Recommended CUDIMM operating speeds in PLL bypass mode are typically limited to 3000 MHz.

• Single PLL mode: In the single PLL Mode, the clock buffer device will use a Phase Lock Loop (PLL) for the regeneration of the incoming host clock to create a better-quality clock that is sent to the DRAMs. However, since there is only one PLL that is used in this mode, both sub channel output clocks will be driven based on only one set of input clocks with the other set of input clocks remaining unused.

• Dual PLL mode: In this mode, the clock buffer will use two PLLs to independently generate each sub channel output clock based on each set of incoming host clocks. The second set of PLL can be turned on or off on the fly if needed to save power.

Beyond the clock modes, clock buffers provide additional flexibility to the system designers with register-controlled additional signal delays, optional output clock enable/disable per bit feature, drive strength and termination choices, etc. All DDR5 clock buffer device control word registers are accessible via DDR5 DIMM sideband.

Cadence VIPs offers a compressive memory subsystem solution that includes memory models for DDR5 SDRAM, DDR5 RCD, DDR5 DB, DDR5 clock buffer, all types of DDR5 DIMMs, including the DDR5 CUDIMMs, DFI Memory Controller/PHY VIPs, and a system VIP compliant to JEDEC specifications defined for each of those devices along with latest DFI Specification.

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