SPI-4 Where is the Beef? Receiver Driver specifications

Richard Moore r_moore at qlc.com
Sat May 20 10:51:23 PDT 2000


* From the T10 Reflector (t10 at t10.org), posted by:
* Richard Moore <r_moore at qlc.com>
*
Paul,

There is an as-yet unconsidered question of what a 60-78% reduced drive
strength really means. At least I don't remember anyone addressing this
in the working group meetings.

If we pick some level for the strong drive between 320 mV and 800 mV,
this level includes the roughly 100 mV of bias. Asymmetry is built in
to the drivers to overcome this bias. Thus a 500 mV driver might be
400 mV negation and 600 mV assertion when the bias is removed. This might
correspond to 12 mA assertion current and 8 mA negation current (I forget
what resistance value was used in the Annex A test circuit so I picked 50
ohms for this example).

If we elect 75% reduced drive strength does this mean we reduce the
nominal signal voltage from 500 mV to 375 mV (475mV or 9.5 mA assertion
and 275 mV or 5.5 mA negation), or that we reduce the drive current from
12 mA and 8 mA to 9 mA (450 mV) and 6 mA (300 mV)? I suspect the former
since this maintains the same asymmetry with respect to the 100 mV offset.

 -- Richard Moore
    QLogic Corp.


-----Original Message-----
From: Aloisi, Paul
To: 'SCSI Reflector'
Sent: 5/18/00 3:44 AM
Subject: SPI-4 Where is the Beef? Receiver Driver specifications

* From the T10 Reflector (t10 at t10.org), posted by:
* "Aloisi, Paul" <paul_aloisi at ti.com>
*
The presentations on the SPI-4 driver and receiver specifications to
data
have not gone through the calculations to present the tolerances allowed
and
the cable loss allowed. A lot of the data has been imperial with Ideal
drivers. We have a lot of TBDs left in the SPI-4 document. We have a lot
of
designers well into designs that do no have all the right numbers.

00-227r2 uses the tolerances as they have been define to date for the
driver. The driver asymmetry has not been mentioned in any of the
presentations the current chart and equations are what we have to base
numbers on. V=(0.69*Vx)+50 and meet the minimum 320 mV and the maximum
800
mV, where V is the drive in one direction and Vx is the drive in the
other
direction.

The agreement on  drive system from Tuesday is a strong driver that does
not
exceed the 800 mV and a weak driver used after the first transition
reducing
the level to 60 - 78% of the strong driver that does not go below the
minimum 320 mV.

The cable loss has been defined by the cable working group for the
media,
backplanes are not well defines. Cables attenuation define with
backplanes
which are loaded give a signal that is 60% of the driving signal. 

The tolerance for the terminators and connectors are an additional 10%
loss.

The calculation of the tolerances using the driver asymmetry, Precomp
drive
as defined, the cable loss and the other system losses leaves a isolated
0
or 1 at the zero crossing. There is no eye opening. Receivers must
consider
a valid signal a transition from the fall back level to 0 mV as a valid
transition.


Document 00-239r0 defines the assumptions and the proposed table A1 that
should be used for design.

There is no meetings until July 11 for SPI-4 to make any changes, this
is
too late for most designs. Designs in progress should use the current
data
available based on 00-227r2 and 00-239r0 to insure interoperability for
SPI-4

Thank you,
Paul



Paul D. Aloisi                                  Phone: 603-429-8687
Texas Instruments   MS: 84            FAX: 603-429-8963 
7 Continental Blvd                          Mobile: 603-930-4104
Merrimack, NH 03054                   Email: paul_aloisi at ti.com
                                                      Web site:
www.ti.com

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