SPI-4 Where is the Beef? Receiver Driver specifications

Aloisi, Paul paul_aloisi at ti.com
Thu May 18 03:44:17 PDT 2000


* 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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