SCSI to SD/IDE

Charlie Carothers csquared3 at tx.rr.com
Mon Dec 6 10:29:30 CST 2010


On 12/5/2010 1:10 PM, Chuck Guzis wrote:
> Okay, a faintly related question to this thing with SD cards.
>
> Are USB pen drives any better at leveling wear than SDHCs?  The
> reason I ask is that there's a USB 2.0 flash driver available for the
> inexpensive AVR90USB162 chip that might also work as a SCSI-emulated
> device.  Any sort of buffering isn't possible, as the chip only has
> 512 bytes of SRAM.  But there are many other inexpensive USB-capable
> microcontrollers as well.
>
> --Chuck
>
>
I had been wondering the same thing.  I did a bit of internet searching 
and found that it appears that the controller inside the USB stick does 
do some degree of wear leveling.  I thought the following URLs were 
pretty interesting.
http://www.corsair.com/_faq/FAQ_flash_drive_wear_leveling.pdf
http://www.pcreview.co.uk/forums/thread-1941484.php
http://blog.ironkey.com/?p=595

I had not thought of the fact that wear leveling has security 
implications as indicated here.  Surely this is not an issue with 
vintage systems though.
http://www.truecrypt.org/docs/?s=wear-leveling

Then I wondered about CF and SD, and found that they appear to be wear 
leveled internally as well.
http://superuser.com/questions/17350/whats-the-life-expectancy-of-an-sd-card

I must say that all this makes me feel much better about their potential 
use as hard drive substitutes in vintage systems.  I also learned that 
some of the USB drives have a RAM buffer (512 bytes or so), so one might 
not even need to worry much about buffering data in a bridge device.  I 
do wonder what algorithm they use to decide when to write the RAM to 
flash - probably varies some by manufacturer.  I do think the bridge 
should provide a "flash drive busy" indicator to discourage removal of 
the flash device while a write is in progress.  The built in LED in most 
USB drives might suffice for them, but I don't believe I've ever seen 
that in a CF or SD card.

Later,
Charlie C.





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