RE: [IBIS-Users] re: Warning message for IO_open_drain model

Lynne

 

The problem is not the load resistance but the voltage it is terminated to,
Vref = 0 in this case.  Lowering the Rref is not going to help, Rref should
be whatever resistance the datasheet says it should be.  There will be no
current flowing in this driver if both the load and the driver are tied to
ground.  From I=E/R, I will be 0 if E=0.  I've seen this mistake way too
many times and likely made it myself once or twice.

 

I'm not sure what you mean by (Rref of 50 ohms for the transmission line
load, in parallel with the pullup load.)   Could you expand on it? What do
you mean in parallel with the pullup load?

 

Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@teraspeed.com
www.teraspeed.com

Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827 

From: owner-ibis-users@server.eda.org
[mailto:owner-ibis-users@server.eda.org] On Behalf Of Lynne D. Green
Sent: Thursday, October 25, 2007 10:02 AM
To: 'Ibis Model'; 'Server EDA'
Subject: RE: [IBIS-Users] re: Warning message for IO_open_drain model

 

Hello, Timmy,

 

The warning says if you hook the circuit up (in simulation), it will only
switch a few millivolts, not enough for the output to cross Vmeas.  You
might try setting Rref and Vref to the normal load for this buffer.  (Rref
of 50 ohms for the transmission line load, in parallel with the pullup
load.)

 

One could also look at the V-t curves, to see where their endpoint voltages
are.

 

If you would like your model reviewed, the IBIS Model Review Committee
welcomes models that pass the IBIS parser.

 

Best regards,

Lynne

 

 

"IBIS training when you need it, where you need it."

 

Dr. Lynne Green
Green Streak Programs
http://www.greenstreakprograms.com
425-788-0412
lgreen22@mindspring.com

 

  _____  

From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf
Of Ibis Model
Sent: Wednesday, October 24, 2007 11:04 PM
To: Server EDA
Subject: [IBIS-Users] re: Warning message for IO_open_drain model

Hi everyone,

 

I get a warning message "TYP VI curves cannot drive through Vmeas given load
Rref=1e+006 Ohms to Vref=0V" for IO_open_drain model. 

 

What does this message mean ? 

 

Regards,

Timmy

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