Does the Raspberry Pi 3 support UHS-II microSDHC cards? I guess not because those have two rows of connectors and the microSDHC slot on the Pi 3 seems to have a single pin row.
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There appear to be no UHS-II/2 cards listed here. Considering how expensive those cards are (like twice as much as the pi itself?), even if it does work, you are just wasting it for nothing: the pi's card reader has a max speed of 25 MB/s which many much cheaper "class 10" cards will do anyway.– goldilocks ♦Commented Apr 20, 2016 at 13:30
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Have a look at the SD cards performance chart at the bottom of that page and click the "Write" header twice to sort in terms of write speed; the cards that actually hit the 25 MB/s limit are all UHS-I cards you can probably hunt down for $20-25 somewhere.– goldilocks ♦Commented Apr 20, 2016 at 13:35
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Am I correct that this 25MB/s limitation is because SD card controller in Broadcom BCM2837 SoC is able to read and write data 4 bits at a time at 50MHz?– MartinCommented Apr 20, 2016 at 16:25
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Actually I don't know details or remember where I first read conclusively that it was due to a limitation of the controller. If it is part of the SoC that would explain why they've never upgraded this. I guess that is sort of what "SoC" means...and there's nothing obvious separate from it just looking at the board, it does seem to connect directly to the holder.– goldilocks ♦Commented Apr 20, 2016 at 16:29
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1 Answer
The Pi is limited to the 50MHz "high speed" mode because it's slot voltage is fixed at 3.3V and the higher speed modes require lower voltage.
I haven't personally tried a UHS-II card but it is supposed to be backwards compatible, so it should work (assuming the card manufacturer implemented the spec correctly).
answered Apr 20, 2016 at 12:32
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1Higher speed requires lower voltage? I'm not saying you're wrong... but that doesn't seem right. At least that's not how it works for CPUs... Commented Apr 20, 2016 at 16:06
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@AndroidDev I believe you are looking at it from the perspective of an overclocker. I think the reason it's possible to build lower voltage, higher speed components is that you can then use lower capacitance transistors which require less energy to change state. If you then "overvolt" them (as in overclocking), the state change will occur faster, but if they are built to change state at a lower voltage, then less voltage is required to make them go faster in the first place -- but you don't want to overvolt a transistor too much because it will fuse.– goldilocks ♦Commented Apr 20, 2016 at 16:40
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Also, if you have a limited supply of current, you could split it and do more in parallel -- but at a lower voltage. I'm guessing that's the extra pin here; you can transfer twice as much information with two lines using the same amount of energy.– goldilocks ♦Commented Apr 20, 2016 at 16:45
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Voltage is a double edged sword, higher voltage does mean faster switching but it also means higher power dissipation. Higher clock speeds also mean higher power dissipation. Presumablly the SD card association lowered the voltage for the higher speed modes to keep power dissipation within acceptable limits. It is of course possible to ignore the standards and try to operate in an overclocked mode but no sane manufacturer would do that by default. Commented Apr 20, 2016 at 16:47
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1Just because a card supports UHS-I doesn't mean it only operates in UHS-I mode. Commented Apr 20, 2016 at 16:49