Re: BIRD41.1 Comments

To Bob Ross and IBIS Forum:

Thanks to everyone (especially Bob) who contributed to the
41.1 debate in my absence.

Below are my views on the different comments received.

But first, a question:
   Will this Bird allow sufficient information be
   included in IBIS to build a good behavioural
   model for a FET switch?

I've tried to build a model using PSpice, but have not
fully succeeded (yet).

In my opinion, no vote should be taken on 41.1 until at
least one company (preferably a simulator company)
confirms that it can build a model, and then publishes some
simulation results on the reflector.

I'll submit an up-to-date version of Bird 41 later in the week
taking into account the comments received so far.

Regards,
John

My comments on comments:
 
> Keyword: [Series Pin Table]

I like Bob's proposal (except for the trailing slashes - could
they not be made optional?).

Although I agree with Arpad that we risk putting too much "logic"
into IBIS, might that not be a necessary risk? If buffers can
be programmed by external pins, then why not say so in the
IBIS model?

example:

[Buffer Modes]
|Fnl_Group Mode control_pins
1 On 1 2 | by default, is Off for other combinations
2 On 1# 2
3 On 1 2#
4 On 1# 2#

Hey, this looks like a truth table!
But its scope is very limited - only buffers whose mode
can be set by external pins.
This would be a more compact solution than Bob's, and
might be extendable to improve Bird 30.2.

 
> 1. Is non-symmetrical operation really needed? It will lead to
> problems in other areas.

No, but what problems?
It's important to define which side of the buffer is used to
make the measurements. I'll drop the reference to
non-symmetric devices.
  
  
> 1. Just [Rseries], [Lseries], and [Cseries] are needed. The
> proposed [R_c_series], [L_c_series] and [R_l_series] additions
> are unnecessary.
> All series elements should be allowed to be connected in
> series,

This is not sufficient. For example, there may be a parasitic
capacitance in parallel with Rseries.

(An alternative solution would be to adopt a Bird36-like notation).

 
> 2. The [Series Impedance] table can also be connected in series.

I disagree.
The only element that could be put in series with [Series Impedance]
is [Lseries]. With [Cseries], no DC measurements can be made.
With [Rseries], there would be a risk of double counting.
And what about a parallel parasitic capacitance?

 
> 3. The [Series Impedance] table could be defined with a voltage
> range of -Vcc to Vcc for closed switch operation or low
> impedance, it could
> also have a range from -2Vcc to 2Vcc for open operation.
> So its range needs to be defined.

I agree that the ranges need to be defined. -2Vcc to 2Vcc seems
OK to me for the OFF state. However -Vcc to Vcc might be
too much for the ON state. This is the same debate as for
clamping diodes.
Could we not limit the "measurements" to reasonable current values?
  
 
> 4. R_load is not needed.

I disagree.

R_load is needed to properly characterize a FET switch.
An interesting feature of these switches is that the
series impedance increases as the "input" signal
approaches the gate voltage level. I suggest that you
need a Rload of at least 10 X "on-resistance" to be
able to measure this.

I'm open to alternative suggestions. But this is the
only way I can see to reproduce the nice Vin. vs. Vout
curves given in the bus-switch databooks.

Perhaps we should explicitely define a series model
called FET_switch?

 
> 5. Also the Mode subparameter could be dropped. This could be
> handled another way for actual switches.

OK, I'll change the Bird to include the new suggestion.
 

 
> 1. What does C_comp mean in a series model?
> 3. Should the Series Pins be used for Differential terminations?

The definitions of all existing passive elements remain unchanged.
In the case of C_comp, it is a capacitance to ground, seen at the
pin through which the measurements are made.

Perhaps we should redine these passive (and parallel) elements,
(except for R_pkg, C_pkg, L_pkg),
to say that they are split 50-50 on each side of the series
element.
This would be better for differential terminations.

> 2. Should a Series Pin be used to connect to a supply or
> ground, as proposed above?

I don't understand the question.

 
> 4. Can several different series pin models be connected to the
> same pin?

Yes, if the series element is a switch.

  
> Arpad Muranyi raised the issue whether BIRD41.1 should capture
> the dynamic turn-on and turn-off characteristics of a switch.
> Bob suggested that there
> was sufficient complication already without this addition.

It should be possible to reuse the rising and falling waveforms.
But I agree there is no need for such an addition.
 

-- 
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