Minutes of X3T10 SPI-2 LVDS Study Group Meeting - 8/14/95

Lohmeyer, John JLOHMEYE at cosmpdaero.ftcollinsco.ncr.com
Tue Aug 15 11:14:00 PDT 1995


Minutes of X3T10 SPI-2 LVDS Study Group Meeting 
                X3T10/95-309r0


Accredited Standards Committee*
X3, Information Technology
                                                   Doc. No.: X3T10/95-309r0
                                                       Date: August 15, 1995
                                                    Project:
                                                  Ref. Doc.:
                                                   Reply to: John Lohmeyer

To:         Membership of X3T10

From:       John Lohmeyer, Chair X3T10

Subject:    Minutes of X3T10 SPI-2 LVDS Study Group Meeting
            Denver, CO -- August 14, 1995


                                       Agenda

1. Opening Remarks

2. Approval of Agenda

3. Attendance and Membership

4. Proposed Agreements [Aloisi]

5. Proposal for SPI-2 Electrical Interface (95-307r0) [Ham]

6. The Case for 50 Ohm Cables (95-308r0) [McCall]

7. LVDS (95-269r3) [Aloisi]

8. Single-ended Reset Signal [Gardner]

9. Meeting Schedule

10. Adjournment



                              Results of Meeting

1.    Opening Remarks

John Lohmeyer, the X3T10 Chair, called the meeting to order at 9:00 a.m.,
Monday August 14, 1995.  He failed to thank the host (himself).

As is customary, the people attending introduced themselves and a copy of 
the
attendance list was circulated.

2.    Approval of Agenda

An agenda was prepared and approved.

3.    Attendance and Membership

Attendance at working group meetings does not count toward minimum 
attendance
requirements for X3T10 membership.  Working group meetings are open to any
person or organization directly and materially affected by X3T10's scope of
work.  The following people attended the meeting:

Name             Organization        Email
John Lohmeyer    Symbios Logic       john.lohmeyer at symbios.com
Ed Suder         Texas Instruments   ROBO at msg.ti.com
Kevin Gingerick  Texas Instruments   4307725 at mcimail.com
Todd Nelson      National            ctwnsc at tevm2.nsc.com
                 Semionductor
Dennis Haynes    Burr-Brown          haynes_dennis at bbrown.com
Paul Aloisi      Unitrode            Aloisi at uicc.com
Brian Day        Symbios Logic       brian.day at symbios.com
Tracy Spitler    Symbios Logic       tracy.spitler at symbios.com
Frank Gasparik   Symbios Logic       farnk.gasparik at symbios.com
Mark Jander      Symbios Logic       mark.jander at symbios.com
Brian Davis      Seagate             davis at cdg.seagate.com
Wayne Werner     AT&T                aluxpo!wew at att.com
Chris Millsaps   Buslogic            chrism at buslogic.com
Michael Smith    Dallas              msmith at dalsemi.com
                 Semiconductor
Louis Grantham   Dallas              grantham at dalsemi.com
                 Semiconductor
Richard Uber     Quantum             duber at tdh.qntm.com
David McCall     Quantum             dmccall at tdh.qntm.com
Steve Robalino   DPT                 robalino at dpt.com
Wally            Adaptec             wally at eng.adaptec.com
Bridgewater
Bill Ham         Digital             ham at subsys.enet.dec.com
Edward A.        Quantum             gardner at acm.org
Gardner
Vit Novak        Sun Microsystems    vit.novak at sun.com

4.    Proposed Agreements [Aloisi]

John Lohmeyer presented Paul Aloisi's foil outlining the proposed agreements
to date on Low Voltage Differential Signaling.  The foil is paraphrased 
here:

*     A single chip can work for both single-ended (Async and  Fast-5 
through
      Fast-20) or LVDS (Async and Fast-5 through Fast-80)
*     Fast-40, -80 will not be defined for single-ended
*     Single-ended and LVDS signals will not be mixed (that is, hybrid buses
      are not defined)
*     The Single-ended/LVDS/RS-485 differential sensing circuit defined in 
95-
      269r3 is acceptable
*     The Fast-40 and Fast-80 skew budget defined in 95-307r0 is acceptable
*     The Single-ended/LVDS/RS-485 differential sensing circuit defined in 
95-
      269r3 permits automatic switching between single-ended and LVDS
      drivers/receivers and terminators
*     When operating in single-ended mode (the sense line is low), a device
      capable of both single-ended and LVDS shall enable its `ground 
drivers'
      continuously.
*     The pin-outs developed at previous meetings are acceptable (see 95-
      307r0)
*     Maximum node capacitance = 15 pF
*     Use current mode drivers, no driver output impedance specification
*     Terminators bias signal lines with 100 mV, limited to 100 uA maximum
      current
*     Receivers not required to have hysteresis

5.    Proposal for SPI-2 Electrical Interface (95-307r0) [Ham]

Bill Ham reviewed the subject document.  As expected, it provoked discussion
on several topics -- not all of which are captured here.  See 95-307r1, when
available, for additional changes that resulted from the discussion.

Kevin Gingerick was concerned about the proposed terminator bias circuit. 
 It
includes a "magic" circuit to provided 100 mV bias at no more than 100 uA. 
 He
thought a simpler circuit, perhaps just using three resistors should be
permitted.  However, Bill and Paul Aloisi objected to the higher power 
levels
that would result.  The issue was deferred until later in the meeting.

Bill included several tables with first cuts at proposed electrical
parameters.  The group suggested several refinements to the specifications
that will be incorporated in revision 1.

After examining the leakage currents of CMOS driver/receivers, the group
agreed to increase the proposed leakage current specification from 10 uA to 
20
uA.  This also caused the terminator bias current specification to increase
|from 100 uA to 250 uA.

The minimum DIFFSENS low-pass filtering was changed from an RC value to a
minimum time constant.  An example circuit will be included.  The
specification on the input pull-down resistor was changed to be at least 80 
K
Ohms and small enough to overcome any receiver leakage current.  The idea is
to bias the circuit so that an open input will cause the receiver "to 
detect"
single-ended state.

Bill's 35 meter maximum cable length specification invoked considerable
discussion.  While SCSI timing specifications may work at  35 meters, it is
not clear that LVDS will support that long of a cable due to attenuation. A
related (and equally controversial) discussion ensued on how many devices 
can
be supported on a LVDS bus.  Ed Gardner wanted to permit more than 16 
devices,
while Bill felt that this capability was outside of the scope of SPI-2. 
 Bill
plans to do some lab testing to determine the effects of cable length and
number of devices on signal attenuation.

After discovering that earlier power calculations had included all receivers
powered on even when the drivers were enabled (not necessary and wasteful of
power), Bill re-opened Pandora's box and we re-visited the power budget
assuming that receivers would be powered off when the corresponding drivers
are enabled.  As a result of the re-negotiations, we ended up with:

      Driver current:        4 mA minimum (up from 3 mA)
                             5 mA nominal (up from 3.5 mA)
                             6 mA maximum (up from 4 mA)

      Receiver sensitivity:  100 mV minimum (up from 50 mV)

      Terminator bias:       150 mV maximum at 250 uA maximum (up from 100 
mV)

6.    The Case for 50 Ohm Cables (95-308r0) [McCall]

David McCall presented 95-308r0, "Fast 40 Proposal", which proposes that an
option for 50 Ohm cables be included in SPI-2.  He pointed out that many
customers want to configure systems with 50 Ohm backplanes with close stub
spacing.  A 50 Ohm configuration option would facilitate such systems.

Bill Ham spoke against this proposal as it would invalidate the power budget
just agreed to.  Furthermore, it would require about twice as much power
dissipation for the drivers/receivers.  He preferred that we push backplane
designers to use higher impedance design techniques.  No consensus was
reached.

7.    LVDS (95-269r3) [Aloisi]

Paul distributed copies of revision 3 of his proposal.  Most of his comments
had been covered under item 5, above.

8.    Single-ended Reset Signal [Gardner]

Ed Gardner suggested that it might be desirable to keep the RST signal 
single-
ended in a LVDS system.  This would facilitate using the trailing edge of 
the
RST signal to qualify whether the system is single-ended or LVDS.  Since the
RST signal is highly filtered, it should not be seriously affected by 
common-
mode noise.  This topic needs further thought and discussion -- no consensus
was reached.

9.    Meeting Schedule

The next meeting of SPI-2 Study Group will be September 11, 1995, in 
Bedford,
NH at the Sheraton Tara Wayfarer Inn (603-622-3766), hosted by Digital
Equipment Corp..

10.   Adjournment

The meeting was adjourned at 4:40 p.m. on Monday August 14, 1995.


 --
John Lohmeyer             E-Mail:  john.lohmeyer at symbios.com
Symbios Logic Inc.         Voice:  719-573-3362
1635 Aeroplaza Dr.           Fax:  719-573-3037
Colo Spgs, CO 80916     SCSI BBS:  719-574-0424 300--14400 baud




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