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I've already asked the question about plausibility of socketed LPDDR here and got an answer LPDDR on separate replaceable modules / boards - possible? drawbacks?

You could probably extend LPDDR onto a longer, socketed bus, but extending the length of the bus is going to make low voltage operation harder.

Recently I've read about possible additional difficulty:

.6v, running that across a physical connector would cause irrecoverable losses.

It was a comment on reddit, the argument IMO might be correct, however it lacked substantiating proof. And that issue was not mentioned in the answer. How big of an issue is pluggable connection vs. soldered for low voltage?

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    \$\begingroup\$ There are other solutions, sometimes known as "interposer boards" that are installed for test, so the electrical situation is not a blocking issue, as long as the connector is well specified and designs modified to accommodate the additional impedance , likely the answer is that there isn't any pull from device manufacturers for such a more expensive solution. Even socketed CPUs, today, arguably are something of an expensive anachronism. \$\endgroup\$
    – crasic
    Commented Jan 6, 2022 at 19:04
  • \$\begingroup\$ @crasic I understand to develop and implement a standard, like LPDDR5 takes time and effort and resources. Consumers vote with their money to support current way of progress, they buy best of what is available, not wait and demand for better (at least most of them AFAIK). I for myself is inclined to give up, buy decent device with 32GB of LPDDR5 and relax for several years. \$\endgroup\$ Commented Jan 7, 2022 at 1:50

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It’s not about the low voltage per se, it’s about the signal integrity. Any kind of transition introduces a possible impedance discontinuity which distorts signals and reduces margins.

This loss or margin is sometimes referred to as 'closing the eye', meaning that there isn't enough margin left to allow for reliable clocking of the signal given all the other variations in timing arising from other causes, like push-out, clock jitter and signal-to-signal coupling.

Modules have several issues that work against signal integrity (eye opening):

  • Connector discontinuity (signal distortion)
  • Reference plane discontinuity (signal distortion)
  • Routing skew / longer traces (delay uncertainty)
  • Power stability (push-out / delay uncertainty)

Longer traces also add to latency. This can be compensated for in the controller, but nonetheless impacts performance (read latency especially.)

Given LPDDR4x small I/O swing and very high per-bit throughput, it may very well be that LPDDR4x lacks tolerance to allow for signal distortion inserted by connectors and reference plane changes, at least if it is to run at its full clock rate.

The SI impact something that a designer would need to model as a system when designing the module and the motherboard.

This all said, there's been moves on the data center side of things to address the limitations of DDRx. One is High Bandwidth Memory (HBM), which uses stacked die placed nearby the processor. A competing technology, Hybrid Memory Cube (HMC) is similar in that it also uses stacked die, but adopts a serdes interface and thus is somewhat more tolerant of layout and possible modularization.

More about HBM and HMC here: https://www.eetimes.com/hbm-flourishes-but-hmc-lives/

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  • \$\begingroup\$ how check that for myself, where in specs sheets to look? \$\endgroup\$ Commented Jan 6, 2022 at 5:12
  • \$\begingroup\$ This is something you’d have to model as a system. There’s lots of literature about modeling DIMMs for DDRx that will give a sense of the issue. \$\endgroup\$ Commented Jan 6, 2022 at 5:50
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Connectors do not necessarily have more voltage loss than traces of similar length, so I didn't mention it in your previous question. I think the person you asked is confusing voltage losses with the general signal integrity issues associated with running signal through connectors and long buses discussed previously.

You seem really concerned about technical limitations, but a more important reason no one sockets LPDDR, GDDR, etc is that there is no reason to do so. You use conventional DDR if you need to exchange memory chips.

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  • \$\begingroup\$ "there is no reason to do so" As I understand initially in consumer market LP was used in smartphones, now it gets widely used in laptops. many laptop users want to be able to upgrade RAM (also right to repair movement is at play) and there is space for sockets in a laptop. I personally want LP for more idle time on battery and also want to be able to upgrade later. \$\endgroup\$ Commented Jan 6, 2022 at 11:19
  • \$\begingroup\$ "the person you asked" It was a discussion about new line of laptops, somebody complained (as I read often) that RAM is not ugradable, so there were reasons given as part of the discussion. Thank you for the answer again! \$\endgroup\$ Commented Jan 6, 2022 at 11:22
  • \$\begingroup\$ @Martian2020 You can't eat your cake and still have it too. If you want slightly lower power, put the memory really close so you can lower your signal voltages a little. If you want the flexibility to run longer memory, you need to use slightly more power every time you transmit a bit. Like we discussed before, memory power consumption is low regardless, so there is little reason to use lpddr if you value socketed memory. \$\endgroup\$ Commented Jan 6, 2022 at 14:36
  • \$\begingroup\$ "Like we discussed before, memory power consumption is low regardless," - I don't recall that discussion btw. At work, yes, at idle - no, times less, see: powersystemsdesign.com/articles/…. And the reason is not AFAIK lower voltage per ser, it is different inside "forgoing the on-chip DLL (delay-locked loop), ..." I cannot leave my DDR based laptop for several days in suspend and find it ready to work, I could one with LP, so difference matters. \$\endgroup\$ Commented Jan 7, 2022 at 1:44
  • \$\begingroup\$ @Martian2020 That is an article about a 15 year old memory standard. Unless you have a laptop from that era, very little will apply to you. If you do have a 15 year old laptop, you should upgrade. I can leave my DDR4 laptop in suspend for months at a time since Windows will power down the memory, making the memory draw irrelevant. \$\endgroup\$ Commented Jan 7, 2022 at 3:30

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