Is the latency of stock 2019-20 iMac RAM CL18 or CL19?

[theorist9]

[And, relatedly, is there anyway to check this if you have the stock RAM?]

This thread (https://forums.macrumors.com/threads/memory-upgrade-compatibility-list-for-imac-2019-2020.2248911/) says the 2019 & 2020 iMacs need CL19, citing Apple as a source:

• CL19
• Source : https://support.apple.com/en-mide/HT201191#27inch2020

Consistent with this, Crucial's Mac-specific aftermarket RAM sticks for the 19-20 iMac are CL19: https://www.crucial.com/memory/ddr4/ct2k32g4s266m

Kingston also recommends CL19 for the 2019 and 2020 iMacs; 2019 link shown here:

Memory for a Apple - iMac Retina 5K, 27-inch (2019) 3.0GHz - Kingston Technology

Kingston has compatible memory for Apple - iMac Retina 5K, 27-inch (2019) 3.0GHz

www.kingston.com

Indeed, that seems to be the case for nearly all RAM vendors.

BUT:

1) The link to Apple Support doesn't give latency requirements. So there is no CL19 specification there.

2) OWC's RAM for the 19-20 iMacs is CL18. I called OWC Tech Support, and they say that's what the Apple OEM RAM is as well:

64GB Memory upgrade for 21.5-inch iMac 2019 (2 x 32GB)

Get everything you need to upgrade your 2019 21.5-inch iMac to 64GB, and save money by trading in the original memory chips.

eshop.macsales.com

So what's going on? What is the latency of stock Apple RAM on 19-20 iMacs, and why do most aftermarket RAM vendors recommend CL19, even though OWC says it's CL18 (and thus spec'd out its own RAM to be CL18 as well)?

Pure speculation, but here's a guess: Apple's stock 19-20 iMac RAM actually is CL18. However, getting CL18 to run on those iMacs requires the specs fall within a very narrow window, which is costly to achieve. Thus, to avoid failures without having to manufacture to Apple's requirements, most vendors recommend their CL19 RAM instead.

 

[wilberforce]

This is the stock Apple RAM from my 2020 iMac, and according to the Micron part number decoder it is CL19. I understand that 2019/2020 iMac only support CL19, so using lower CL RAM will be of no benefit.

https://media-www.micron.com/-/media/client/global/documents/products/part-numbering-guide/numsdrammod.pdf?la=en&rev=79e3864cbeac4be190fa3c619a12bddb

 

[theorist9]

Thanks! So you just have to find the company's part decoder. Given this, it appears Apple is showing the wrong picture for this 2666 MHz RAM on its website, since it says "UHC", which (according to Hynix's part decoder) indicates it's 2400 MHz:

 

[Nermal]

My 2020 came with SK Hynix, and once again CL19 (HMA81GS6DJR8N-VK as listed in System Profiler).

 

[theorist9]

I wonder, then, why OWC's RAM for the 19-20 iMac's is CL18.

 

[wilberforce]

I wonder, then, why OWC's RAM for the 19-20 iMac's is CL18.

Not to be cynical, but maybe just because it sells better, because it sounds more premium?

 

[theorist9]

Not to be cynical, but maybe just because it sells better, because it sounds more premium?

Except that info. is buried fairly deeply in their specs, so I wouldn't expect it's part of their marketing.

If you look at the pic OWC displays, you can read off the part no. (K4A4G085WB-BCTD), and an intenet search indicates it's made by Samsung. According to Samsung's part decoder (available as a download here: https://semiconductor.samsung.com/dram/ddr/ddr4/k4a8g085wb-bctd/), the "TD" at the end indicates it is indeed CL19. So I may drop OWC an email about this. I suspect it's just human error.

 

[wilberforce]

According to Crucial/ Micron, most RAM latency is around 14 nanoseconds. Because 2667 MT/s DDR ram has a clock frequency of 1333 MHz, CL = 1333 MHz x 14 ns = 19 clock cycles

 

[theorist9]

But:

There are only three RAM mfrs.: Samsung, Micron, and Hynix. And we've already seen from their data sheets that their 2666 MHz memory is always listed as CL18, CL19, or CL22. Yet one can purchase 2666 MHz memory with CL15 and CL16 timings, e.g., this CL15 RAM from Kingston (which must have been made by one of the three). Plus I recall seeing a Hynix document saying the timing on its 2666 MHz DDR4 RAM was programmable.

E.g., this screenshot, from the PassMark site (https://www.memorybenchmark.net/ram.php?ram=SK+Hynix+HMA81GS6CJR8N-VK+8GB&id=11662 ), which is specifically for the "VK" CL19 RAM shows that, in spite of that, multiple CAS timings are allowed:

So could it be the case that the CL value we infer from the label is merely the nominal timing for that module, such that we can't tell the actual CL value from the label code? If so, might it be possible that OWC's (and Apple's) RAM is actually CL18 (due to custom programming), the label code notwithstanding?

z

 

[theorist9]

And just to add this to the mix: Corsair's 2666 MHz "Mac Memory" is CL18: