Subject: IBIS BIRD63.2 Comments/Questions

From: Bob Ross <[email-masked]>
Date: Wed, 09 Feb 2000 16:46:22 -0800

To Stephen Peters and IBIS Committee:

I am finally starting to look at BIRD62.2 and BIRD63.2 and have these
comments.  My first comments are on BIRD63.2.

Bob Ross
Mentor Graphics

1.  In general, I have problems with any slew rate definition.  We
have already mis-used it for the [Ramp] keyword where the only 
meaningful portion is dt, and we interpret it as the 20% to 80% value.

The BIRD63.2 definition is   

  (Tester_Vhigh - Tester_Vlow) / (deltaT for the above tester voltages)

Presumably, these values are unreduced 0% to 100%.

According to SSTL-2, for 2.5 V, EIA/JESD8-9, Sept. 1998, pg. 4,

  Slew = (VIHmin(ac) - VILmax(ac)) / (deltaT for the above (ac) voltages)

where the delta ac voltage could fall inside the delta tester voltage
according to the diagram.  This would produce a different unreduced values

                             _______ Tester_Vhigh
                            /
                           /<- VIHmin(ac)
                          /
                         /<- VILmax(ac)
       Tester_Vlow  ____/ 

Does JEDEC have an official definition of slew rate?

Where do you get the Tester_Vhigh and Tester_Vlow values if they
  are not the (ac) values?  Are they documented in data sheets?

If Tester_Vhigh and Tester_Vlow are given, then the numerator is
redundant - a source of error.

The 0% to 100% interpretation is inconsistent with the [Ramp]
intepretation and also with a potential connector model interpretation.

2.  According to the JEDEC document, the slew rate is a minimum value
(i.e., faster signals are allowed).  Should this be noted?

3. Should the Tester_Vhigh and Tester_Vlow values be deltas from
Tester_Vref to be consistent with the BIRD62.2 subparameters?

Tester_Vlow is listed as -1.0 V in the first example.

4. I do not like mixing differential and single-ended definitions
by value.  A new subparameter should to be defined if the Vth is to
be interpreted for differential operation.

I presume it is possible for a particular buffer in some technology
to be used both in differential and single-ended mode.  Sometimes
simulators can control whether single-ended or differential is used.

The differential example has the Tester_Vlow and Tester_Vhigh
voltages relative to the differential voltage - an inconsistency.
The slew rate voltage does not conform to the slew rate definition
in the differential example.  Is this intential? - is it always
a single ended definition?

5.  Cosmetic:  I would prefer the proposed Sub-Parameter names to be
the following:

  Vth_tester
  Vhigh_tester
  Vlow_tester
  Slew_setup_tester
  Slew_hold_tester

etc.

for the following consistency reason.  So far, all of the subparameters
of IBIS which are of the form:

  Xxxx_qqq = numerical value

or 

  Yyyy_zzz  word

follow the format of uppercase first letter and lower case remaining
letters (e.g., Si_location, R_pkg, Model_type, R_fixture, V_fixture_min,
S_overshoot_high, etc.)

Moving V as the first letter gives the major definition of the 
subparameter and preserves this convention.

We still remain case sensitive, so having a consistent convention
is important.  This was violated in BIRD63.2 where Tester_Slew_Setup
was defined, but Tester_Slew_setup was used in the examples.  So you
see the problem.

6. Under Usage rules, Tester_Skew_* is used instead of "slew".

---

From: Mike LaBonte <[email-masked]>
Date: Thu, 10 Feb 2000 00:42:48 -0500

BIRD 63.2 covers a way to document the input signal conditions under
which a receiver's data book setup and hold timings are specified. But I
do not see sufficient discussion concerning how an analysis tool is
expected to use this information.

For example, if a particular simulation shows an input signal that
differs radically from the conditions under which Tsetup and Thold are
specified, are we supposed to somehow compensate? If there were multiple
values of Tsetup and Thold each with a specification of input
conditions, then the analysis tool could adjust the Tsetup and Thold
values based on the input conditions seen.

If this BIRD somehow ties to BIRD 61.1, then this should be stated.
Maybe I will understand it once I read all of these new BIRDs.

Mike

---

From: "D. C. Sessions" <[email-masked]>
Date: Fri, 18 Feb 2000 14:55:01 -0700

> Maybe I will understand it once I read all of these new BIRDs.

At the very least, it provides a correlation point for closing the
timing analysis if the input waveforms DO match the criteria.  Which
was not explicit before.  (How often have we seen customers measure
input transitions at 0.8v and 2.0v regardless of reality?)

And, yes, BIRD 61.x is an attempt to define the response to nonideal
inputs.

-- 
D. C. Sessions
[email-masked]