SPI Tolerance 2

Bill=Galloway%HW=Stor%Sys=Hou at bangate.compaq.com Bill=Galloway%HW=Stor%Sys=Hou at bangate.compaq.com
Wed Jun 1 14:12:14 PDT 1994


1)  In my previous mail message I suggested a tolerance of +/- .25%
    as a tolerance for synchronous negotiation period.  I did not
    intend to put an upper limit on the period.  More correctly I
    should have suggested +infinity / -0.25%.  I arrived at 0.25%
    (2500ppm) because this tolerance is achievable by crystals,
    ceramic resonators, and phase locked loops.  It probably is not
    achievable by RC circuits.

2)  I am not opposed to a fixed tolerance such as 1ns (as Bruce Adler
    suggested) if that is what the committee wants.

3)  I agree that the correct reading of SPI today means a minimum of
    100.000000...ns not 99.99999999...ns.  I cannot negotiate for a
    period of 96ns because that is not defined in SCSI-2, and I doubt
    that many targets would agree to a period of 96ns.

4)  I believe that most target and initiator implementations today
    use a crystal oscillator that is some small integer multiple of
    the synchronous period.  With these designs, the synchronous
    period can only be changed in gross steps.  For example changing
    the divide from four to five would increase the period by 20%.
    I do not believe that it is acceptable to slow down by 20% to say
    that I will NEVER be less than 100ns.

5)  Because of 3) and 4) above I believe that most synchronous SCSI
    devices violate the SCSI-2 specification today.  I am trying to
    change the specification so that the devices will not be broken.
    If we get the specification changed, I can not use that to go
    beat up my drive vendor but maybe I can prevent the problem from
    happening again.

    Gerry,  If you believe that 100ns is the absolute lower limit then
    your Seagate 2Gb drive is broken unless you have found a 25Mhz
    oscillator with 0ppm tolerance.


Bill Galloway                           P.O. Box 692000
Compaq Computer Corp.                   MS 090504
(713) 374-6732                          Houston, TX 77269-2000

billg at bangate.compaq.com

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