Ram CAS Latency Question

Jumpin' Jack Flash

Jumpin' Jack Flash
Hi Folks :)

I'm contemplating buying some DDR5 ram sticks (32GB (2x16) at 6000Mhz, but I'm a bit confused with the CAS specifications.
What I've gathered (I think) is the lower the numbers the 'better' the ram? For example, cas latency 36-44-44-96 and for another example, cas latency 36-36-36-76.
Any clarification on this question would be appreciated.

Jack :)
 
Last edited:

Morbad

Morbad
Jumpin' Jack Flash said:
I'm contemplating buying some DDR5 ram sticks (32GB (2x16) at 6000Mhz, but I'm a bit confused with the CAS specifications.
What I've gathered (I think) is the lower the numbers the 'better' the ram? For example, cas latency 36-44-44-96 and for another example, cas latency 36-36-36-76.
Any clarification on this question would be appreciated.
Click to expand...

CAS itself is one single timing parameter of many, and not always the most impactful. Tighter (lower) timings, at a given clock speed, generally imply lower latency and better performance, especially where primary timings (CAS-tRCD-tRP) are concerned.

There are a few exceptions to this. tREFI, for example, is a refresh interval (DRAM refreshes cost performance because they interrupt other operations) which is faster when set higher. A few other timings are aren't hard limits, but are minimum timing windows within which other operations must fit; these tend to have a range of ideal settings that may not always be the lowest possible. Still more timings are dependent upon others in ways that are not at all obvious and setting these too low can cost performance and/or stability, for no gains. Since essentially none of these timings are advertised and few of them are even set by EXPO/XMP, there is usually no sense in fussing over them unless one is intent on manually tuning their memory.

In general, when choosing memory you want to be aware of:
  • What the platform--which is the CPU socket and CPU's integrated memory controller--can reliably support, without requiring dangerous voltages.
  • What the motherboard can support. This will depend on things like number of memory slots, number of PCB layers, trace routing/topology, voltage regulation, firmware, etc...but is rarely a major limiting factor. Boards that only have one DIMM per channel almost always have an edge over boards with more memory slots (shorter traces, and hundreds of fewer overlapping traces makes for much better electrical properties, other things being equal). Going from a 4 DIMM to a 2 DIMM board is worth losing two PCB layers; a $125 mATX board with two DIMM slots will handle memory better than most $350 full size boards with four.
  • The memory itself. Rated specs (clock speed, timings, and the voltage required) are important, but so are the ICs actually used to build the DIMMs, the PCB the DIMMs use, how they are cooled, and their layout. Most of this doesn't mean much if your goal is to just plug in memory and go, but it's still worth keeping in mind that two-DIMMs have much fewer compatability issues than four; that dual-rank DIMMs (usually synonymous with double-sided) are faster at the same settings than single rank DIMMs (due to better interleaving), but at the cost of lower peak clocks; and that memory advertised to require much higher than stock voltage is a demerit (it directly implies more heat, which can make memory cooling an issue, and indirectly implies ICs that are pushed closer to their limits, which may in turn imply profiles that are less compatable).

More specifically, the two examples you've provided are both obviously kits of single rank DIMMs and the one rated at 36-44-44-96 is very likely either using different ICs, or is rated at a lower voltage, than the kit rated at 36-36-36-76. The latter kit will be faster, but depending on various factors could be more likely to have compatability issues.
 

Jumpin' Jack Flash

Jumpin' Jack Flash
Hi All :)

Thanks for all the replies! (y)

Relevant Hardware I'll be using just for reference.

Asus Rog Strix B650E-F Gaming WiFi motherboard (8 layer pcb?) (AMD Expo).( 4 Memory slots). Rated Ram. Memory speeds = DDR5 4800Mhz -up to- 6400Mhz + (OC).
AMD Ryzen 7 7800X3D Cpu. Max Memory Speed DDR5 5200Mhz.

Regarding the cpu, would it be wise to choose ram that's no faster than 5200Mhz?
Would it also be wise to choose AMD Expo compatible ram?...(I would have thought so but it doesn't hurt to ask. :)).
@Morbad Hmm, there's a lot more thought needed when choosing ram so it seems! :D...I think I'd be wanting a stable system rather than speed. 🤔

Jack :)
 

Morbad

Morbad
Jumpin' Jack Flash said:
Asus Rog Strix B650E-F Gaming WiFi motherboard (8 layer pcb?) (AMD Expo).( 4 Memory slots). Rated Ram. Memory speeds = DDR5 4800Mhz -up to- 6400Mhz + (OC).
AMD Ryzen 7 7800X3D Cpu. Max Memory Speed DDR5 5200Mhz.

Regarding the cpu, would it be wise to choose ram that's no faster than 5200Mhz?
Would it also be wise to choose AMD Expo compatible ram?...(I would have thought so but it doesn't hurt to ask. :)).
@Morbad Hmm, there's a lot more thought needed when choosing ram so it seems! :D...I think I'd be wanting a stable system rather than speed. 🤔
Click to expand...

Virtually every 7800X3D and AM5 board (even quad slot ones) can handle DDR5-6000 with two DIMMs. Past that gets rather hit or miss, however.

My advice would be to get the tightest timings you can find at 6000MT/s and 1.4v or less. Then make sure your board is flashed to the newest firmware before enabling EXPO.
 
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