SPI-3 single-ended AC test circuit

Walter Bridgewater wally at eng.adaptec.com
Fri Jun 25 13:01:02 PDT 1999


* From the T10 Reflector (t10 at symbios.com), posted by:
* wally at eng.adaptec.com (Walter Bridgewater)
*
John,

I have always found that the spec requirement with both 15 & 200pf
was easy to meet.  We have gone way beyond what was in the spec,
for designing our SE SCSI I/O's.

By having your timing numbers for the assertion & negation period measured
with 200pf, you are insuring adequate drive strength and skew control
for low to high and high to low delays. (Of course the 15pf makes you
have moderate slew rates.)  I think the 15pf & 200pf limits give you a
good window to meet for driver strength, skew and slew rate.

It seems like the 200pf number was proposed because of the low bus 
impedance with 15 loads,and/or in some backplane applications 
the impedance is low.  Using 200pf was the easiest way to 
change the spec for these kind of cases, without 'changing' the 
spec very much.  Nobody wanted to tackle the problem of how
to better declare the loading situations, and if they did, would they
expect to get enough consenses? everybody else might have a different idea.
So, having 200pf as a load was agreed to.  That's what I remember.

Regards,

Wally



> 
> * From the T10 Reflector (t10 at symbios.com), posted by:
> * John Lohmeyer <lohmeyer at ix.netcom.com>
> *
> For quite some time we have specified an AC test circuit for single-ended
> that is rather unrealistic of actual bus loading.  Figure 27 in SPI-3 (PDF
> page number 76) shows a 200 pF capacitor to ground with a 47 Ohm resistor
> to a 2.5 volt source for measuring everything but slew rates, which use the
> same circuit with a 15 pF capacitor.  
> 
> Does anyone remember why we used 200 pF for these measurements?  I would
> think that it would be better if the test circuit load was closer to that
> of an actual bus, say 30 pF.
> 
> John
> 
> 
> --
> John Lohmeyer                  Email: lohmeyer at ix.netcom.com
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