SPI Tolerance

[Roy Shenfield x2473]

   I'm not sure how negotating for a slightly lower speed as suggested by Jim McGrath
   would solve the crystal tolerance problem.  As I understand it negotation implies
   you are informing a device of the maximum transfer rate (minimum period) at which
   you can RECEIVE data (or conversely the maximim transfer rate (minimum period) at
   which a device can SEND you data).

   Thus, when I negotiate with another device do I take into account the tolerance of my
   crystal, the tolerance of the other device's crystal, or both?  If the answer is either
   the second or third choice, then there must be a tolerance stated in the spec
   
   Furthermore, since according to the SCSI Specification the negotiated transfer period
   is the transfer period factor times four (Section 6.6.21), I can not negotiate for only
   a slightly lower speed, I have to negotiate for a slightly times 4 lower speed.
   In other words the next transfer rate available below 10 MB/s (100 ns period)
   is ~9.6 MB/s (104 ns period).  Thus in the nominal case we have given up ~ 4% performance
   to solve an ~.250% crystal tolerance problem.  Is this reasonable?

   By the way does the definition of transfer period present a minor problem with Fast-20?
   Since 20 MB/s is a period of 50 ns, the transfer factor period would have to be 50(ns)/4
   which is 12.5, which is not an integer.  Thus the value of the transfer factor period
   would have to be set to 13, which gives a transfer period of 52 ns, which is a transfer
   rate of 19.23 MB/s.  Am I nit-picking?

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   The views stated above are my own and do not reflect any company position.


Roy Shenfield
Senior Staff Engineer
Adaptec

Email: roys at adaptec.com