Modelling bus switches and other series elements

Hello John:

   Happy new year to you also! I read your proposal, and I was wondering
why you felt the [Pulldown] keyword was needed, and why only the
[GND Clamp] keyword was required (and not both [GND Clamp and [PWR Clamp]
keywords). If you are trying to describe a "series pass transistor"
type of switch, then I think one waveform describing the switch when
"on, along with the [GND clamp] and [PWR clamp] keywords for when the switch
is off would suffice. Could you explain more why you chose the keywords
you did?

               Regards,
               Stephen Peters
               Intel Corp.

Happy New Year,

IBIS cannot correctly model series devices. This isn't
much of a problem for simple passive devices (R,L,C) or diodes.
However, it's a major handicap for bus-switches (e.g. 74CBT range
from Texas Inst).

I've seen seen this feature on the wish-list, but am not aware
of any proposals to handle it.

So here's my suggestion for extending IBIS to model series switches.
It requires 3 new keywords and one new modeltype.

1) Define a new keyword [Series Pin]
   This keyword is at the same level as, say, [Diff Pin]

   [Series Pin]
   |Pin1 Pin2 Buffer
       3 4 switchA |switchA is buffer from 3 to 4
       4 3 switchAneg |Buffer from 3 to 4 is non-symmetric
      12 13 switchA
      12 14 switchB |12 can be routed to 13 or 14
         

   The two pins of the series element are identified, along with
   the the associated series buffer. This buffer corresponds
   to measurements made at Pin1.

   If the buffer is not symmetric, a different buffer may be
   specified with the pin order swapped.

   In the case of a crossbar switch, all possible series
   connections must be listed. IBIS contains no information
   concerning how or when one of the connections is selected.

   The buffer name given here supersedes that given under the
   [Pin] keyword.

2) Define new model type SERIES-SWITCH
   Define new keywords [Series] and [Series OFF]

   Four curves are obligatory: [Pulldown]
                               [GND Clamp]
                               [Series]
                               [Series OFF]

   [GND Clamp] and [Series OFF] are measured for switch OFF.
   [Pulldown] and [Series] are measured for switch ON.

   [GND Clamp] + [Pulldown] are standard measurements, made at one pin.
   The other pin, identified under the [Series Pin] keyword,
   is assumed connected to ground.
 
   [Series] and [Series OFF] are through measurements ie between
   the two pins.

   The [Ramp] keyword is also required. The test load must
   include a voltage source.

   This gives enough information to model a switchable non-linear
   resistor.

   We could also define a model type SERIES which would not
   be switchable: only [Pulldown] and [Series] needed in this case.

   To allow for series frequency-dependent devices, additional
   R,L,C paths might be defined.

Looking forward to your feedback,
Regards,
John

-- 
John Fitzpatrick   <John.Fitzpatrick@ln.cit.alcatel.fr>    
Alcatel Telecom, 4 rue de Broglie, 22304 Lannion, France
Tel: +33(0)2.96.04.79.33  Fax: +33(0)2.96.04.85.09