Fast-20 transfer period factor.

Tak Asami asami at dt.wdc.com
Mon Nov 20 18:49:27 PST 1995


>    The transfer period factor times four is the value of the
>    transfer period.
> Value is ns.  For 20 mega-transfers/second the transfer period is 50ns.
> Which implies the transfer period factor is 12.5.  But transfer period
> factor is an int.

Yes, as I recall, that has been a major issue when drafting Fast-20.
But to make the long story short, here's what you do.

You negotiage for 48nsec cycles.  That's 12(dec) in SDTF message.
SPI states that you do the transfer "not exceeding the negotiated rate",
or something to that effect, so it is totally legal to negotiate for
48nsec, and do 50nsec.
Strictly speaking, there are two schols of thought here:
(a) Because minimum transfer cycle time is 50nsec, even though you 
    negotiated for 48nsec, 50nsec cycle is the fasted you can legally
    do.
(b) No, when you said 48, you really meant 50nsec.  48nsec=50nsec!

Now, it has a potential of becoming the problem when Fast-40 timing is
adopted, since if you go along with (a) school, your counterpart may
really do 48nsec!
This can ignite another round of debate in a good time, but for now,
you know what to do with the Fast-20.
=========================================================================
                           Tak Asami
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