XO-1.5 MicroSD replacement speed? External SD?

[James Cameron]

Summary: the XO-1.5 has two SDHC slots, with high speed bus, minimum 2
MByte/s, up to 10 MByte/s.

You can't predict how a combination will work, because mixing the new
and old can give inconsistent results.  You can only test and see.

Speed class rating is minimum data writing speed for making a
recording, so it will constrain Sugar or Gnome only if you are saving
a very large file.  It may have no effect on reading data, but in my
tests there is often some effect.

On Tue, Oct 27, 2015 at 09:52:59PM -0400, Adam Holt wrote:
> Does anybody have recommendations for minimum read/write speeds of
> MicroSD cards, inserted in/on the XO-1.5 motherboard?

The microSD card slot is 3.3V only.

Class 2 is minimum.

Class 4 was what we used in production.

Anything above 4 MB/s is enough, so Class 4 is the minimum.

> And external full-size SD card speed requirements similarly!

The SD card slot is 3.3V or 1.8V.

Class 10 will be acceptable.

UHS Speed Class 1 (U1) will be acceptable.

UHS Speed Class 3 (U3) won't work at the rated speed, because the bus
interface will be the bottleneck instead of the Flash cells.

At Class 10 and above, serving content will depend on the bus speed,
not the class.

Anything above 4 MB/s is enough, so Class 4 is the minimum.

> Disregarding SD cards' well-known reliability failures :). Many
> suggest "use Class 10 SD cards" and I get that the marketplace
> compels such sweeping broadbrushed rules-of-thumb (far better than
> nothing, heuristics most welcome if based on legit science?!)
> 
> But I was also looking for more -precise- numbers, if someone might
> know the stated (or approximate) I/O requirements of the XO-1.5's
> two SD interfaces?

The microSD slot is 3.3V only

The SD slot can be powered at either 3.3V or 1.8V.

A card forced to operate at higher than normal voltage will usually
reduce performance of the CPU on the card, so that it won't overheat,
and because the bus interface will be slower anyway.

So if it claims a certain performance on the packaging, that claim
won't be met in an XO-1.5.

Cards marked with I or II, for UHS-I or UHS-II, won't work any better
than the equivalent card without those markings.

SDXC cards are not obliged to work, but if they do they will fall back
to SDHC compatibility mode.

-- 
James Cameron
http://quozl.linux.org.au/