fallback speed of MMC flash memory?

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Here's what I think I know.
MMC flash has a one-bit data interface.
SD flash has a 4-bit data interface.
There's a new MMC flash that has an 8-bit interface.
The new MMC flash is backward compatible.

So, here's the question...backward compatible with what?
If I stick the new mmc flash card into a slot that supports
SD and old MMC and doesn't have all the pins required for the new
8-bit flash...

At what speed does the new flash card run in the old slot?
Does it fall back to 4-bit SD speed or 1-bit MMC speed?

Doesn't make much sense to buy a fast card if it turns out to
be much slower in all the applications I have for it.

Second question.  There exist small thumb-size USB2 flash readers that read
MMC and SD.  They are virtually unspecified.  How does one tell which
ones support the 52MB/S new MMC speeds?
Thanks, mike

Re: fallback speed of MMC flash memory?

mike wrote:
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Have a look at this doc. At least this one is free.


The interface has three modes. One bit, four bit, and eight bit mode.
The minimum mandated transfer rate is 2.4MB/sec, so no matter what,
you are supposed to get that much at the pins of the MMC. Since
I don't know the history of this technology, I have no idea what
percentage of devices support the three mentioned widths.

This pinout page on the web, shows only 7 pins on the edge of the
device. The implication, is *perhaps* MMC started life as a 1 bit
device, with a 7 pin interface. When they wanted higher performance,
they added another 6 pins, in the form of a second row of pins.


If the current device you are using with the MMC card, only had
7 pins, that implies 1 bit mode, and a definite cap on performance.

This doc will fill in the gaps when looking at the MMCA document.
Section 6.2.4 on PDF page 45, shows how the host probes for the
bus width of the MMC card.


As for the performance of USB, one USENET posting a while back,
claimed the theoretical max of USB2 was ~57MB/sec. Which is
just enough for the 52MB/sec number. But in the real world,
I somehow doubt you'll see exactly 52MB/sec when you benchmark


Re: fallback speed of MMC flash memory?

Paul wrote:
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Quoted text here. Click to load it
Thanks for the excellent links.

Nope, I really don't expect 52MB/S...my old slow hard drive won't even
keep up with that.  If I pay premium bucks, I expect to get the speed
when the rest of the world catches up.  Looks like 4-bit support is
there, so shouldn't lose speed.
Interesting that 52MHz is optional.  And the default 26MHz won't be much
more than twice the speed of SD, even in a perfect world.  Hmmmm.
Getting speed R/W speed specs out of SD card vendors has been a real


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