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

From: Lynne D. Green <lgreen22_at_.....>
Date: Mon Oct 29 2007 - 14:11:24 PDT
Hi, Tom,
 
It all depends on where the pullup resistor is located.  I assumed that the
pullup was on-die with the buffer (esp with the high value, since some I/Os
are designed that way). You are right that there are other cases I should
have considered, particularly the case of the pullup being at the load.
 
I think we both qualify as good engineers with an occasional unstated
assumption.
 
- Lynne


  _____  

From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf
Of Tom Dagostino
Sent: Friday, October 26, 2007 10:23 AM
To: 'Lynne D. Green'; 'Ibis Model'; 'Server EDA'
Subject: RE: [IBIS-Users] re: Warning message for IO_open_drain model



Lynne

 

I assumed (bad engineer, bad engineer) that the 1M resistor came from the
datasheet and not a made up number.  I too think it is a very high value but
I had no basis to question it from the information in the question.  I've
seen a lot of open sink drivers in all the models we have done and have
found many way too weak to drive a 50 Ohm pullup to any reasonable Vmeas.
Open sink driver applications seem to be split between three different
categories; 1. very low power status bits, 2.  LED drivers and other
applications where speed is not an issue and only require a mA or so and  3.
High speed wired OR applications.  A 1M pullup would fall into the first
category but it still seems to be on the high side.  But if your application
is battery powered it may make sense.

 

I'm not sure you have your transmission line theory correct.  Let's take a
single 1V, 1psec edge propagating down a 6 inch transmission line terminated
into 50 Ohms to ground.  Let's assume the output impedance of the driver is
50 Ohms.  Let's also assume the Zo of the transmission line is 50 Ohms and
the speed of propagation is 170 psec per inch.  When the waveform is
launched into the transmission line the only impedance seen by the driver is
the Zo of the line.  The voltage at the output of the driver is now 0.5V
The signal has not been able to propagate down to the length of the line
yet.  As time goes by (6*170psec) the signal eventually reaches the end of
the line and is totally absorbed by the termination, there is no reflection.
So the driver sees a 50 Ohm load from the time it launches the edge down the
line until the power is removed.  The output voltage of the driver will
remain at 0.5V

 

If we were to replace the 50 Ohm termination with a short the driver again
would see a 50 Ohm transmission line when it launches the edge.  Again the
output voltage of the driver will be 0.5V.  After 6*170psec the edge will
get to the short and a large reflection will occur.  This reflection will
take 6*170psec to get back to the driver.  Up until the reflection returns
to the driver the impedance seen by the driver is 50 Ohms.  So for
12*170psec the driver sees 50 Ohms. After that time the reflection returns
to the driver and the driver now "knows" there is a short at end of the
line.  The output voltage from the driver is now 0V.  And you can see the
logic with a open at the end of the line.  The drive sees 50 Ohms for
12*170psec and then sees an open until the power is removed.  After the
reflection returns from the open the output voltage is 1V.

 

Regards

 

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: Friday, October 26, 2007 9:46 AM
To: tom@teraspeed.com; 'Ibis Model'; 'Server EDA'
Subject: RE: [IBIS-Users] re: Warning message for IO_open_drain model

 

Hi, Tom,

 

Thanks for the catch.  I must have confused Rref with a resistor-loaded
table.

 

Another thing what caught my eye was the value of 1e6.  Wouldn't this be
effectively be an "open", as it is much larger than any realistic Z0?  (I
seem to remember values like that preventing a Vmeas crossing.)

 

For Rref: When the buffer is driving, one sees a Thevenin equivalent of the
pullup (to Vcc) in parallel with the 50-ohm transmission line (to Vcc or
GND) ["parallel" when both go to Vcc].  Many people take the rule of thumb
to use Z0, which works as long at ZUP>>Z0.

 

Best regards,

Lynne

 

 

 

  _____  

From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf
Of Tom Dagostino
Sent: Thursday, October 25, 2007 10:43 AM
To: 'Lynne D. Green'; 'Ibis Model'; 'Server EDA'
Subject: 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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Received on Mon Oct 29 14:12:47 2007

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