[poll; updated] eMMC vs. uSD; modular eMMC

[Grench]

I don't understand why we would want to have wifi/BT over SDIO instead of one of the USB 2.0 ports on the internal hub. Wifi over SDIO is simply slower - and it's an old kludge. We have plenty of USB ports on the SoC (Internal USB 2.0 hub). We're only exposing 1. Why not use one or more of the others for wifi/BT?

SDIO is a much more power efficient bus vs USB.

I suspect that is a myth.

 

[TrashyMG]

I suspect that is a myth.

USB requires a bit of power level translations to work properly, this alone makes it less efficient.. Then there is the fact that the USB would always in a mode where it was polling the status of the connections.

 

[Exophase]

I suspect that is a myth.

So why is it that you think SDIO became and remained the popular standard for WLAN communication for all of these years? You say it's an old kludge but USB on SoCs is nothing new, GP32 had one and its SoC is over 13 years old. What it did not have is SD or SDIO.

 

[hns]

Could you clarify if it would be possible to have two full SD slots, one µSD, one eMMC and wifi/BT over SDIO? Would that switching component you talked about allow that (at the cost of lower bandwidth), or would it merely be a way to have 4 potential storage options with only 3 of them usable at the same time?

Without having tested it in practice it is difficult to give a clear answer.From pin count and PCB space it looks good.

I think it could work to have the 8 bit eMMC and the 4 bit uSD on one SD port and switch 4 data wires through a GPIO. It could be possible to modify Linux to stop the clock, shut down the current choice and switch on the other one *and* switch between 4 and 8 bit mode. And repeat that on demand so that in the worst case it will switch back and forth between both devices after each block transfer. So if you use only one of them the speed should be the same as without switch. If both are actively accessed (mounted, syncing, reading/writing) it will be significantly slower.

One issue I have not yet thought about is if and how we boot. Since the Boot ROM does not know about that new GPIO we have to hard-wire some default boot source.

 

[_wb_]

Could you clarify if it would be possible to have two full SD slots, one µSD, one eMMC and wifi/BT over SDIO? Would that switching component you talked about allow that (at the cost of lower bandwidth), or would it merely be a way to have 4 potential storage options with only 3 of them usable at the same time?

Without having tested it in practice it is difficult to give a clear answer.
From pin count and PCB space it looks good.


I think it could work to have the 8 bit eMMC and the 4 bit uSD on one SD port and switch 4 data wires through a GPIO. It could be possible to modify Linux to stop the clock, shut down the current choice and switch on the other one *and* switch between 4 and 8 bit mode. And repeat that on demand so that in the worst case it will switch back and forth between both devices after each block transfer. So if you use only one of them the speed should be the same as without switch. If both are actively accessed (mounted, syncing, reading/writing) it will be significantly slower.


One issue I have not yet thought about is if and how we boot. Since the Boot ROM does not know about that new GPIO we have to hard-wire some default boot source.

Wouldn't it be a better idea then to put the µSD and the right SD slot together on one SD port? That way the eMMC and left SD still have dedicated ports (so you can boot from them and their bandwidth is guaranteed), and you can use both the eMMC and µSD together without any penalty (e.g. in principle you could do a RAID1 on them and get 200+100 MB/s theoretical read speed).

 

[Natsu]

I would personally like to have a relatively small µSSD (8 or 16GB) at good speeds, e.g. one of these:

If this happens, I think it should be 16GB.

 

[Granitehead]

I think putting two things on the same port is a bad idea.

What about connecting BT/WLAN over SPI instead of SDIO?

 

[Grench]

Not to overstate the obvious...

Why not JUST have a microSD internal, no eMMC and two full size SDXC ports external and call it good?

Frankly, the additional speed from the eMMC isn't going to really change system performance. It would boot faster - once every week to 3 months when the user chooses to do so. Otherwise, the difference is less than fractional in 'normal' use.

I guess I just don't see enough added benefit from doing both to justify the flips & twists required to get there.

 

[vcoleiro1]

^ I think people have answered that several dozen times without having to reopen the same arguments that have gone on now for 17 pages - needless to say its more than just the speed, A level playing field as Exophase mentioned , robustness and importantly power consumption need to be considered.

SD is very power hungry compared to eMMC - SD uses about 4 times or so more power.  That's why eMMC is used in low power mobile devices for the primary storage

Anyway, for what it's worth eMMC is my strong preference for the reasons I gave in this post : http://boards.openpandora.org/topic/15651-poll-updated-emmc-vs-usd-modular-emmc/?p=316156

 

[Granitehead]

Why not JUST have a microSD internal, no eMMC and two full size SDXC ports external and call it good?

Frankly, the additional speed from the eMMC isn't going to really change system performance. It would boot faster - once every week to 3 months when the user chooses to do so. Otherwise, the difference is less than fractional in 'normal' use.

I think this would be best, but as people somehow think they need more speed...

 

[Natsu]

Right now, I'm either µSD+µSSD or µSD only.

 

[thatgui]

I still don't see any good argument that justifies to include another point of failure > MicroSD-Slot. eMMC (size >= 16GB) looks like a fine option for me. But if a MicroSD is a must (for whatever reasons) then skip the eMMC. Such switching solutions, just cry out for problems and OS adjustments.

 

[_wb_]

^ I think people have answered that several dozen times without having to reopen the same arguments that have gone on now for 17 pages - needless to say its more than just the speed, A level playing field as Exophase mentioned , robustness and importantly power consumption need to be considered. SD is very power hungry compared to eMMC - SD uses about 4 times or so more power.  That's why eMMC is used in low power mobile devices for the primary storage

Anyway, for what it's worth eMMC is my strong preference for the reasons I gave in this post : http://boards.openpandora.org/topic/15651-poll-updated-emmc-vs-usd-modular-emmc/?p=316156

Speed only makes a difference if we take a sufficiently expensive eMMC that actually reaches higher speeds than SD. Both are fast enough for our purposes.

"Level playing field" is not really an argument unless you also force people to only use full-size SD cards and USB drives that have the exact same latencies and speeds as the internal storage.

Robustness: all NAND-based storage degrades over time, but it does not seem to be a big issue anymore. In any case, µSD is a more robust option because it can be replaced. Unless you mean robustness of the mechanical port. I don't think this is an issue at all, since the number of expected insert/eject cycles is low.

Power consumption: this argument is new to me. Do you have references on that? I would expect power consumption to be similar (it's the same underlying technology), and if anything, eMMC to be more power consuming if it is higher speed and has more advanced controllers and caches to get faster random access. Just like SSD is even faster and consumes even more power. Why would SD use more power than eMMC, and what kind of power are we talking about? We have two SD slots already, so even if eMMC really does use 4 times less power, the total power consumption ratio for storage would not be 4:1 (µSD vs eMMC) but only 12:9  (µSD+SD+SD vs eMMC+SD+SD). So even if true (which I doubt), this is not a very compelling argument to me.

µSD only still seems a really good choice to me. Adding a SATA µSSD also makes sense, but the money / power budget is probably better spent on RAM like comradekingu suggested.

 

[vcoleiro1]

eMMC isn't expensive, there are lots of eMMC 4.5 options at 150MB/s read or higher for relatively cheap, Exophase pointed this out.

Remember 4.5 is the old standard, we are now at 5.0 which is superceding 4.5

Level playing field is an issue  as Exophase detailed - you disagree with Exophase - fair enough, I on the other hand agree with him.

I've had 2 SD cards fail - you can say since you can replace them that's not an issue, well it was for me at the time , particularly if the OS is on it. Anyway, I was only pointing out in that post you quoted that  eMMC is robust, made no mention that uSD wasn't

SD uses considerably more power than eMMC. Someone asked why eMMC is used by all current tablets and phones.  I wondered that also, So I did some research.  I kept seeing that eMMC was low power as the main reason its used along with speed etc.  But until yesterday, I over looked the low power reason,  as I also thought SD was low power. Turns out low power is not low power if get my drift.   It turns out that when people say eMMC is used for primary storage in mobile devices because its low power, they really meant that.   

eMMC uses about 1/4 or less  the power used by SD cards.   Have a look at the link below,  just to match the Nand speed of the current Pandora ~ 50-60 MB/s according to Wizardstan , an SD draws 1.44Watts to 2.88Watts when reading - that's up there with modern SOCs under load.   AS for ratio's, if you were to use just the eMMC the ratio would be 4:1 .   It's only lower than that, if decide to use other storage options.

http://en.wikipedia.org/wiki/Secure_Digital#Power_consumption

Comparitavely , a rather pedestrian similar speed eMMC 4.5 at 60MB/s uses between 300mW to 650mW during read or write.  I'll post the example when I find it again in my links.

uSD is not designed to be the primary storage for mobile devices for a reason - power consumption and speed/cost. When you think about it , it makes sense, eMMC is designed specifically to be the primary storage for low power devices - Manufacturers are pushed hard to deliver that requirement and they compete on that requirement. 

As I said before , to me , eMMC with the 2 SD card slots is the best overall solution .  It provides the speed, the low power consumption, robustness and flexibility to add more storage via the SD's if you want to in the future. SD cards after all, will always provide better size options then uSD anyway.  It's the best all round solution.

 

[Askarus]

Wouldn't it be a better idea then to put the µSD and the right SD slot together on one SD port?

No.

Really bad idea.

If I boot from Micro SD my second SD will be slower or unusable.

I want the 2 Full SD slots to be untouched.

 

[comradekingu]

Did you read that source link? 2.88W into a uSD isnt anywhere close to sane. http://www.dtt8.com/images/micro-sd%20specification.pdf

0.08 times 3.6 is 0.228

 

[vcoleiro1]

^ did you read the link.  The link you posted has the power usage for default and high speed, the bottom 2 rungs in the table I linked highlighting the slowest speeds .   Different speed SD use different power.   That's why I said look at the power consumption required for a SD running at 50-64MB/s - which is the equivalent speed of the Pandora Nand according to Wizardstan

http://en.wikipedia.org/wiki/Secure_Digital#Power_consumption

 

[comradekingu]

The samsung plus, or any other cards we have looked at, havent been fullsize uhs2 cards. Am i missing something?

I dont think UHS-1 can do 2.8W based on how uSD cards tend to have less performance when doing simultaneous read/write.

Also UHS-2 has more pins, i would think thats done to accept more power?

50MB/s is SDR50, which is UHS-1, i dont understand how its 4 times less efficient. Its half if it can even be compared.

I understand that latency and general speed is sub par on uSD compared to eMMC, but then again it sits idle where numbers even out, most the time, even moreso if more ram is employed.

People dont want it for those reasons, but because it can fail, which sadly is how the comparison to emmc comes in, only that its less generic in modular form.

emmc storage fails, so its a mistake in the long run, if soldered in than performance also scales off, maybe it hasnt failed so far for you, but thats no reason to say it doesn't. It will. _When_ it does thats exactly the same, at which point modular emmc makes just as much modular sense.

How about, both modular emmc and uSD connectors? This is possible on omap5 if a (usb combo bt/wifi) rtl8723au solution is employed, freeing up the sdio.

Ive seen omap5 boards with minipcie connector, but i dont find any pci-e in the specs, nor any http://en.wikipedia.org/wiki/AMBA_High-performance_Bus  (AHB)

 

[Askarus]

I don't think UHS-2 is possible.

For normal SD cards it requires different connectors:

I doubt that the OMAP (if we take it) does support that standard.

(Don't know if micro SD-cards also need different connectors)

 

[vcoleiro1]

The table  shows the power consumption, ie up to 50MB/s using 1.44Watts . Greater speeds at even higher power usage.

UHS-ii is the top rungs, were not talking about the top rungs on that table, just as were not talking about the bottom rungs.  Look at the 50MB/s and what power it uses.  Remember the Nand in the Pandora supposedly reads at up to 64MB/s.