Alexandre Souza - Listas pu1bzz.listas at
Sat Dec 4 14:50:26 CST 2010

> If reading speed was an issue, one could employ the faster 4-bit
> access to SD rather than the simpler and much slower SPI mode that's
> usually used for slow embedded applications.

    Hey people, c'mon?!

    SD Card speeds:
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Speed Class Rating
The Speed Class Rating is the official unit of speed measurement for SD 
Cards, defined by the SD Association. The Class number represents a multiple 
of 8 Mb/s (1 MB/s), and meets the least sustained write speeds for a card in 
a fragmented state.[12]

These are the ratings of some currently available cards:[12]

  a.. Class 0 cards do not specify performance, which includes all legacy 
cards prior to class specifications.
  b.. Class 2, 2 MB/s, slowest for SDHC cards.
  c.. Class 4, 4 MB/s.
  d.. Class 6, 6 MB/s.
  e.. Class 10, 10 MB/s.
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    Ok, now to SCSI specs

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SCSI-1: 5MB/s
Fast SCSI: 10MB/s
Fast-Wide SCSI: 10MB/s
Ultra SCSI: 20MB/s
Ultra-Wide SCSI: 40MB/s
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    - I hope I can develop a SCSI bridge that can move 10MB/s, because I 
doubt a normal Atmel XMega will be capable of that.
    - If I can design a fast enough circuit, I can use a Class 10 card and 
will have 10MB/s.
    - Since it will be a hard disk emulator, and not a SD Card reader, I can 
have TWO interleaved cards on the bridge, so making 20MB/s easy
    - We are talking old computers here. What do you use about CLASSIC 
computing that goes beyond Fast-Wide SCSI?
    - Not all people are capable of begin big.
    - Maybe with ARM microcontrollers, Ultra2Wide can be achived. Who knows?

    Time to define DOWN TO EARTH specs and begin working :)

    Greetings from Brazil,
    Alexandre SOuza 

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