CUDIMM in Action: Case Studies and Practical Applications
The CUDIMM Standard Will Make Desktop Memory Much Robust and a Little Smarter
While the new CAMM and LPCAMM memory modules for laptops have attracted notice in recent months, the PC memory sector is experiencing changes beyond mobile. The desktop memory market will get a new DIMM type dubbed the Clocked Unbuffered DIMM to increase DIMM performance. At this year’s Computex trade expo, numerous memory makers displayed their first CUDIMM products, offering a glimpse into desktop memory’s future.
Clocked UDIMMs and SODIMMs are another answer to DDR5 memory‘s signal integrity issues. DDR5 allows for fast transfer rates with removable (and easily installed) DIMMs, but further performance increases are running into the laws of physics when it comes to the electrical challenges of supporting memory on a stick, especially with so many capacity/performance combinations today. While these issues aren’t insurmountable, DDR5 (and eventually DDR6) will need more electrically resistant DIMMs to maintain growing speed, which is why the CUDIMM was created.
CUDIMMs, standardised by JEDEC earlier this year as JESD323, add a clock driver (CKD) to the unbuffered DIMM to drive the memory chips. CUDIMMs can improve stability and reliability at high memory speeds by generating a clean clock locally on the DIMM rather than using the CPU clock, as is the case today. This prevents electrical issues from causing reliability issues. A clock driver is needed to keep DDR5 running reliably at high clockspeeds.
JEDEC recommends CUDIMMs for DDR5-6400 speeds and above, with the initial version covering DDR5-7200. On paper, the new DIMMs will be drop-in compatible with existing systems, using the same 288-pin socket as the regular DDR5 UDIMM and allowing a smooth transition to higher DDR5 clockspeeds.
CUDIMMs = Faster DDR5
High-clocked memory subsystems struggle to preserve signal integrity, especially over long distances and with many interconnections (e.g., multiple DIMMs per channel). Since UDIMMs are dumb devices, the memory controller/CPU and motherboard have traditionally carried this load. However, CUDIMMs will make DIMMs intelligent and assist maintain signal integrity.
Clock drivers (CKDs) accept base clock signals and regenerate them for module memory components, which is the main change. CKDs buffer and amplify the clock signal before driving it to DIMM memory chips. CKDs use signal conditioning characteristics including duty cycle correction to reduce jitter and clock signal timing fluctuations.
Another important CKD function is minimising clock skew, or the difference in clock signal arrival times at various components. The CKD synchronises memory chips and DIMMs by matching propagation delays for each clock path.
Phase adjustment lets CKD match the clock signal to component timing needs, which requires more work from the memory module builder. Many memory module makers have yet to demonstrate their CKD-enabled devices since they are still learning the technology. As a scaled-down version of the Registered DIMM (RDIMM), which has been used in servers for years and is the only DDR5 DIMM supported by Intel and AMD’s server (and workstation) CPUs, embedding clock drivers in DIMMs isn’t a novel notion.
RDIMMs buffer the command and address buses along with the clock signal, but CUDIMMs just buffer the clock signal. In that sense, CUDIMMs are half-RDIMMs.
While some CPU designers would be thrilled if all systems employed RDIMMs (and ECC), consumer PC economics favour cheaper and simpler solutions where available. A JEDEC-standard CKD has 35 pins, about half of which are voltage/ground pins. CKDs increase to DIMM construction costs, but they are purposefully cheaper than RDIMMs.
CKDs will be available in all JEDEC DDR5 memory form factors. Clocked SODIMMs, CUDIMMs, and DDR5 CAMM2 memory modules will need clock drivers.Since memory frequency determines the need for clocked DIMMs, CUDIMMs and their other variants are backwards compatible with DDR5 systems and memory controllers. So CUDIMMs use the same 288-pin DIMM slot as DDR5 DIMMs.
In PLL Bypass mode, a CUDIMM can run a clock signal via its CKD buffers or bypass them. Bypass mode is only supported up to DDR5 6000 (3000MHz), hence JEDEC complaint DIMMs will use CKD mode (Single or Dual PLL) at DDR5-6400 and higher. A CUDIMM can bypass a slower/older DDR5 memory controller, while DIMMs without a CKD can’t reach higher speeds (at JEDEC-standard voltages and timings).
CUDIMMs, CSODIMMs at Computex
G.Skill, TeamGroup, and V-Color displayed CUDIMMs and CSODIMMs at Computex. Memory vendors aren’t revealing details because these new DIMMs accompany new systems. However, because they displayed the hardware, don’t be shocked if they enter production systems (and retail shelves) shortly.
While Biwin still sells high-performance devices under the Acer Predator brand, enthusiast-grade memory modules are a newer product. However, its 16 GB and 32 GB modules may operate at 6400–8800 MT/s, which is faster than’regular’ enthusiast-grade DIMMs. These devices will launch in September.
G.Skill, a longtime enthusiast memory vendor, showed their Trident Z5 CK CUDIMMs at Computex. The company did not highlight their results, maybe because it is still perfecting its CKD-enabled goods and not establishing records. Finally, G.Skill has shown a substantially overclocked system running at DDR5-10600 using ordinary DDR5 modules, thus early CUDIMMs are less impressive.
At the trade event, V-Color displayed CUDIMMs and CSODIMMs, showing that it is taking advantage of CKD chips for high-performance memory. The business plans to sell 16GB and 24 GB CUDIMMs with performance bins between 6400 MT/s and 9000 MT/s at 1.1 V–1.45 V. This is supposed to demonstrate the benefits of clock unbuffered memory modules, as 9000 MT/s is faster than any widely available enthusiast-class memory kit.
Four of many high-performance memory module providers showed CUDIMMs at Computex. Only two talked about CUDIMM performance (TeamGroup’s demonstration seemed incomplete). Since the JEDEC standard has been in place for about half a year, they will soon be joined by the many PC memory providers.
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