Subject: BIRD 6.1

From: [email-masked] ( Bob Ross)
Date: Sat, 29 Jan 94 19:56:44 PST

Hello IBIS members:

Here is an updated BIRD6.1 to reflect the discussed and emailed changes.
I would appreciate any comments or corrections from Kellee, Syed, Bob Ward,
Jon, and anyone else as to whether this version meets the simulator needs
and also contains information that is easily available from data sheets and
from vendors.  I am particularly concerned that "tskew_min" is meaningful.
I am also concerned whether the normal test setup for tskew in data books
which include 50pF/100pF loads gives overly pessimistic values.  If you wish,
you can reach me at (503) 684-6641, fax (503) 639-3469 or send email.

Changes: vpp renamed to vdiff, and polarity specification modified.
         tskew_typ, tskew_max, tskew_min columns added.
         Note regarding vinl and vinh removed since it was not relevant.
         Analysis Path text revised and expanded.

Note on vdiff notation:
    Motorola uses Vpp in ECLinPS family
    TI uses VT+ and VT- in Data Transmission Circuits Family
    National uses VTH in Interface Databook 

Bob Ross, Interconnectix, Inc.

***************************************************************************** 
*****************************************************************************
                 Buffer Issue Resolution Document  (BIRD)

BIRD ID#:                6.1
ISSUE TITLE:             Differential Pin Specification 
REQUESTOR:               Bob Ross, Interconnectix, Inc. 
DATE SUBMITTED:          12 January 1994
DATE REVISED:            29 January 1994
DATE ACCEPTED BY IBIS OPEN FORUM:     {will be filled in when accepted}

******************************************************************************
******************************************************************************
STATEMENT OF THE ISSUE:

  Several components contain pins which operate in a differential mode with
respect to other pins.  Since this operation in inherent to a component, 
the IBIS document should be extended to specify this functionality.

******************************************************************************
STATEMENT OF THE RESOLVED SPECIFICATIONS:

  An additional optional keyword, [Diff_Pin], is added to the specification.

|========================================================================== 
|    Keyword:  [Diff_Pin]
|   Required:  Optional
|Description:  Used to associate differential pins, their peak-to-peak 
|              differential voltages and timing skews.
| Sub-Params:  inv_pin, vdiff, tskew_typ, tskew_min, tskew_max
|Usage Rules:  Entries follow these rules: Only differential pin pairs
|              are entered.  The [Diff_pin] column contains a non-inverting 
|              pin number and the inv_pin column always contains the
|              corresponding inverting pin number for output and I/O output. 
|              The vdiff column contains the specified peak-to-peak voltage
|              between pins if the pins are Input or I/O Model_types.
|              For Output only differential pins, the vdiff entry is 0V.
|Other Notes:  The output pin polarity specification in the table overrides 
|              the [Model] Polarity specification such that the pin in the
|              [Diff_pin] column will be Non-Inverting and the pin in the 
|              inv_pin column will be Inverting.  This convention allows
|              one [Model] to be used for both pins. 
|
|              Column length limits are:
|                      [Diff_Pin]        5 characters max 
|                      inv_pin           5 characters max 
|                      vdiff             9 characters max 
|                      tskew_typ         9 characters max
|                      tskew_min         9 characters max
|                      tskew_max         9 characters max
|
|              Each line must contain either four or six columns.  If "NA" is
|              entered in the vdiff, tskew_typ or tskew_min columns, its
|              entry will be interpreted as 0V or 0ns.  If "NA" appears in
|              the tskew_max column, its value will be interpreted as the
|              tskew_typ value.  When using six columns, the headers 
|              tskew_min and tskew_max must be listed.
|--------------------------------------------------------------------------- 
[Diff_Pin]  inv_pin  vdiff   tskew_typ tskew_min tskew_max
|
 3           4       150mV     1ns       0ns       2ns  | Input or I/O pair
 7           8         0V      1ns        NA        NA  | Output* pin pair
 9          10         NA       NA        NA        NA  | Output* pin pair
16          15       200mV     1ns    | Input or I/O pin pair 
20          19         0V       NA    | Output* pin pair, tskew = 0ns
22          21         NA       NA    | Output*, tskew = 0ns
                                      | * Could be Input or I/O with vdiff = 0V

******************************************************************************
ANALYSIS PATH/DATA THAT LED TO SPECIFICATION

  Since only a small percentage of components contain differential pins,
[Diff_Pin] is optional.  The component itself may be required to convey the
associations between pins for differential inputs and/or outputs.  Such cases
may occur in practice when pairs of pins are connected using closely-spaced,
coupled nets or twisted-pair cabling.

  Pins which provide complimentary outputs should not be associated with
each other when the analysis is normally done using only one pin at a
time.  However, there may be cases related to other pending extensions
(package models, power association, etc.) where association of complimentary
pins may be appropriate.

  [Diff_pin] should be used for pins designed for differential operation.
Inputs of such components have a differential input sensitivity specification 
such as "Vpp" or "VT+" and "VT-" which defines a peak-to-peak voltage between 
two input pins.  The vdiff column is introduced for such specification 
limits (as a positive value) which would trigger an output transition.  One
application is for timing analysis.

 |<-- OUTPUT MEASUREMENT POINT FROM ANOTHER COMPONENT
 |
 |
 |<-- tmax -->|
 |            |
 |<-- tmin ->||
 __________  ||  ____________ __
           \ || /             A0
          __\||/__             
             \/    vdiff       DIFFERENTIAL INPUT
          __ /\ __
            /  \
 __________/    \____________ A0

  For timing purposes, an output is referenced to an equal voltage cross-over
of output pins.  Setting the vdiff entry to 0V is thus chosen when the pins
are for differential outputs only.  Note, the cross-over does NOT mean that
the outputs are at 0V.

  The tskew value is the time difference between the mid-point of the two
output transitions.  It is equivalent to the time-delay of one pin relative 
to the other pin.  Although an absolute value is specified, either pin can
delayed relative to the other pin.  This specification assumes the outputs 
are reasonably identical and the rise and fall transitions are reasonably
similar. 

  Tskew can be shown per National Interface Databook, diagram on pg 1-121:

       3V _______________
         /               \  INPUT TO SAME COMPONENT
 1.5V__ /                 \
       /|                 |\
0V ___/ |                 | \______________
        | tPLH |          |tPHL|  
 ___________        __________    _________ __
 ^     ^    \      /          \  /     ^    D0
 |    Vo/2   \    /            \/     Vo/2
Vo     v______\  /__        __|/\|_____v        DIFFERENTIAL OUTPUT
              |\/|            /  \
 |            |/\|           /|  |\
 v____________/  \__________/ |  | \_______ D0
              |  |            |  | 
              |  | Tskew      |  | Tskew 
              
 Tskew = |tPLH - tPHL|             

  Conventions similar to those in [Pin] are followed with respect to required
and optional column entries and to column lengths.

******************************************************************************
ANY OTHER BACKGROUND INFORMATION:

  Introducing a [Tskew] keyword similar to [Package] (used for defaults) was
suggested and could be considered as another method for dealing with skew.

  An alternative polarity mechanism for differential outputs could have been
based on the Polarity sub-parameter within the [Model].  This was not done
because Polarity is optional and also because two [Model]s would be 
required: one for the "Non-inverting" pin and another for the "Inverting"
pin.  In BIRD6.1, vdiff is used as an absolute difference regardless of 
polarity for Input specfication, so "polarity" no longer is applicable for
inputs.

  However, it still may be reasonable to require two Output or I/O [Model]s 
for easier translation into simulator databases.  In this case, the "inv_pin"
name could be changed to "2nd_pin".  Also, with a Polarity sub-parameter, 
the possibility exists that the [Models] are in phase with each other.  This
may be useful for double strength buffer options.

******************************************************************************

---

From: [email-masked] (Robert Canright)
Date: Mon, 31 Jan 94 9:20:29 CST

> you can reach me at (503) 684-6641, fax (503) 639-3469 or send email.

Regarding your statement:
"I am also concerned whether the normal test setup for tskew in data books
 which include 50pF/100pF loads gives overly pessimistic values."

If data book numbers are copied into IBIS models without verifying 
simulation vs. time-domain measurements, then the data book capacitance
numbers could lead to faulty simulations.  Faulty simulations, even if they
do provide conservative results, are a nuisance.  We don't like finding out
that we have spent labor fixing problems that didn't really exist.

We rank models based on our confidence in them & models created from data
book info have the lowest ranking.  The same driver with 2 different
package models can have very different waveforms.  If an IBIS model uses
data book info, the model should have a disclaimer to say so.

-- 
Bob Canright		Convex Computer Corp.	Richardson, Texas
214-497-4474 (desk)	[email-masked]	214-497-4500 (FAX)
disclaimer:
It's not my typing, the Sun keyboard repeeeatsss.