Subject: Ground/Power Noise modeling of sim switch outputs

From: Michael Gutzmann <[email-masked]>
Date: Mon, 23 Oct 95 11:30:27 +0100

Hello, IBISians !

(Sorry, but I've forgotten to name a subject for posting my
 mail. Therefore I sent this message once again. The content is
 the same as in my first mail.)

I want to use IBIS for modeling of Ground/Power Noise of simultaneously
switching drivers. 
Spec version 2.1 can handle static V/I curves and Vout(t) curves
for a given load. If I've got this information then I am able to 
determine the rising/falling waveform of the output current Iout(t)
of a switching driver.

What I need for example to calculate the voltage drop over the
ground inductance L_gnd_pin (see below) is the contribution of every 
(simultaneously) switching driver to the current I_gnd(t) flowing 
through their common ground pin inductance (if I assume for
simplification that the inductance of the ground path is caused only
by that ground pin).

                         o PWR
                         | 
                         |
                         C
                         C L_pwr_pin
                         C
                         |
                         |
                         V I_pwr(t)
       I_pwr_1(t)        |                      I_pwr_n(t)
                         |
     -----<--------------*-------------  -  -  ----->---
     |                   |                             |
     |                   V I_pwr_2(t)                  |
     |                   |                             |
----------          ----------                    ----------
|        | Iout_1(t)|        |  Iout_2(t)         |        | Iout_n(t)
|Driver_1|          |Driver_2|                    |Driver_n| 
|        |--->-|    |        |--->--|             |        |--->--|
|        |     |    |        |      |             |        |      |
----------    ---   ----------     ---            ----------     ---
     |        --- CL_1   |         --- CL_2            |    CL_n ---
     |         |         |          |                  |          |
     |         |         |          |                  |          |
     |        ---        |         ---                 |         ---
     |                   |                             |
     V I_gnd_1(t)        V I_gnd_2(t)                  V I_gnd_n(t)
     |                   |                             |
     |                   |                             |
     --------------------*--------------  -  -  --------
                         |
                         |
                         V I_gnd(t)
                         |
                         C
                         C L_gnd_pin
                         C
                         |
                         |
                        ---

It would be useful for Ground/Power Noise calculations to have the
currents of the internal ground/power path of every driver 
I_gnd_1(t)/I_pwr_1(t) ... I_gnd_n(t)/I_pwr_n(t) with respect to
the corresponding Vout(t) curves for specified loads.

Unfortunately it is not possible to describe these ground/power path
currents with current IBIS format in my opinion. I'm quite aware
of the fact that this request perhaps contradicts the basic 
concept of IBIS because these internal ground/power path currents are
not measurable from outside of the package.

But at my point of view I don't see any other way for an
efficient ground (power) noise analysis.

However, what do you think about the opportunities to model 
power/ground noise and the resulting switching noise at the
outputs of active/quiescent output buffers by using present
or future IBIS format ?

Any comments to that problem will be appreciated.

Thank you very much in advance

Michael Gutzmann

Cadlab
Group Analog System Engineering
Joint R&D Institute
University of Paderborn
Siemens Nixdorf Informationssysteme AG
Fuerstenallee 7
33102 Paderborn
Germany

phone: +49-5251-60-6164
fax  : +49-5251-60-6155
e-mail: [email-masked]

---

From: [email-masked] ( Bob Ross)
Date: Tue, 24 Oct 95 15:36 PDT

Michael:

I tend to agree with your assessment that knowing I_pwr_n(t) and I_gnd_n(t) 
obtained under the test load condition will give a more accurate 
Simutaneous Switching Output analysis.  There may be some feedback
interactions when switching several outputs which may cause the total
power and ground currents to be different than the sum of the components
found individually.  However, this approach should still be reasonably 
accurate under the test load conditions.

Presently, IBIS can be used to estimate the currents based on assuming
the Pullup table provides currents from the Power Bus, and the
Pulldown table provides currents from the Ground Bus.  The currents
that are provided by each BUS into the test loads during the
low-to-high and high-to-low transitions depend on the simulator
switching or blending algorithm with respect to the Pullup and Pulldown
tables.  So this approach can only be approximate.  The IBIS format does
not give any more details about internal dynanic impedances which would
effect the current distributions.

If providing I_pwr_n(t) and I_gnd_n(t) tables gives sufficient information,
the IBIS format could be easily extended using syntax similar to [Rising 
Waveform] and [Falling Waveform] to provide this data.  However, I am
wondering how this information can be accurately applied in real loading
situations (not just the test load case) to simulate actual power and ground
bounce noise.  Also, what are the best test loads?

Bob Ross,
Interconnectix, Inc.