RE: [IBIS-Users] RE: [IBIS] I-V and V-t curve mismatch

From: Muranyi, Arpad <Arpad_Muranyi_at_.....>
Date: Mon Oct 22 2007 - 11:01:22 PDT
While both Tom and Todd are suggesting the right thing,
I think both need some correction, or clarification...
 
Todd wrote "-2*VDD to 2*VDD" in which the first -2*VDD
should have read -VDD.
 
Tom's comment about using typical VDD is correct for the
range calculation but you need to be careful an not
apply that suggestion to the VDD of the power you
apply to the device under test and the VDD-relative
calculations (in case you don't measure it VDD relative
directly).
 
So to sum it up, my numbers in my first reply were
calculated as follows:
 
For the pulldown the sweep range is always:
-VDD_typ to 2*VDD_typ  =  -3.3 to 6.6,
regardless of which case you are generating,
i.e. whether the supply voltage to the chip 
is typ=3.3, min=3.0, or max=3.6 volts (assuming
that the chip's pulldown is connected to GND
or 0 volts).
 
For the pullup, which is swept VDD relative,
the numbers for the sweep ranges in GND relative
orientation will work out this way:
 
-VDD_typ to 2*VDD_typ with respect to VDD_typ = 6.6 to -3.3
-VDD_typ to 2*VDD_typ with respect to VDD_min = 6.3 to -3.6
-VDD_typ to 2*VDD_typ with respect to VDD_max = 6.9 to -3.0
 
Note that this numbers are only this confusing because
you are looking at the sweep ranges in a GND relative
way, while you are really doing the same rule as above
for the pulldown with respect to VDD.
 
Of course the direction of the sweep is irrelevant, as
long as the device doesn't act up.  Some times devices
can do funny things depending on which direction you
do the sweep.  Also, to help the simulator to find an
operating point I suggest that you start the seep at
the same rail voltage the device is driving to (i.e.
VDD for logic high or GND for logic low) and then do
the sweep you need for the IBIS model.  There is a
better chance to get the simulator converge that way...
 
I hope this helps,
 
Arpad
===========================================================
________________________________

From: owner-ibis-users@server.eda.org
[mailto:owner-ibis-users@server.eda.org] On Behalf Of Todd Westerhoff
Sent: Monday, October 22, 2007 10:09 AM
To: ibis@server.eda.org; ibis-users@server.eda.org
Cc: tom@teraspeed.com
Subject: [IBIS-Users] RE: [IBIS] I-V and V-t curve mismatch



I agree with Tom.

 

The choice of -2*VDD to 2*VDD is somewhat arbitrary ... it's simply
meant to ensure that you have valid V/I data for any voltage that the
part will be exposed to.  Thus, you don't need to adjust the voltage
ranges based on the MIN and MAX cases.  For most technologies, if you
have overshoot that's equal to the supply voltage, you have bigger
problems than model accuracy !

 

Todd.

 


Todd Westerhoff
VP, Software Products
SiSoft
6 Clock Tower Place, Suite 250
Maynard, MA 01754
(978) 461-0449 x24
twesterh@sisoft.com
www.sisoft.com

________________________________

From: owner-ibis@eda.org [mailto:owner-ibis@eda.org] On Behalf Of Tom
Dagostino
Sent: Monday, October 22, 2007 12:11 PM
To: 'Sudarshan H N'; 'Mirmak, Michael'
Cc: ibis@eda.org; ibis-users@eda.org
Subject: RE: [IBIS] I-V and V-t curve mismatch

 

Model all three corners from -Vdd to 2*Vdd where Vdd is the typical
voltage.

 

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@server.eda.org [mailto:owner-ibis@server.eda.org] On
Behalf Of Sudarshan H N
Sent: Monday, October 22, 2007 7:43 AM
To: Mirmak, Michael
Cc: ibis@server.eda.org; ibis-users@server.eda.org
Subject: Re: [IBIS] I-V and V-t curve mismatch

 

Hi Michael & All,

Can you people answer to my question regarding the voltage ranges in MIN
and MAX case as explained below. As Arpad mentioned in one of the
earlier mails the range for MIN and MAX case is

typ:  -3.3 to 6.6

min:  -3.6 to 6.3

max:  -3.0 to 6.9

How do we select this range for MIN and MAX case. Is there any document
which explains the selection criteria for these corners ? If there is no
logic , then how can we decide if the supply is 1.8 V supply or 2.5V
supply?

Let me know your answers.

Regards
Sudarshan

 

On 10/19/07, Sudarshan H N < hn.sudarshan@gmail.com> wrote:

Hi Michael, 

Thanks for your detailed reponse. I made a small mistake in putting the
voltage ranges as you mentioned. As i was writing a new tool , at this
point of time i didnt give much  attention to printing correct voltage
range , and i was only worried about the correct extraction of V-t and
I-V tables.

I have considered the formula you have mentioned for calculating
Vcc-relative Pullup and Powerclamp tables. I changed the voltage range
to (3.3, 3.0 and 3.6 which is the actual case) and now i am able to get
rid of all these errors.

But still i have one query regarding the values for I-V curves when it
is in the Min or MAX corner. I asked about this in my previous mail.
Lynne and Arpad gave some answers  but that didnt solve my problem.

The problem with MIN and MAX corner is , what is the voltage range we
need to consider for MIN and MAX case . For example if we consider 3.3 V
supply, the typical range would be -3.3 to 6.6 and this is the voltage
range in the IBIS file for all 3 corners. As per the cookbook we need to
consider voltage range -Vcc to +2Vcc and hence for the MIN case the
range will be -3.0 to +6.0. So in that case what current values we need
to put for the voltage range -3.3 to -3.0 and +6.0 to +6.6 whose values
will be missing in MIN corner simulations.

I hope i have explained the problem correctly and let me know your
answers.

Thanks & Regards
Sudarshan

 

On 10/18/07, Mirmak, Michael <michael.mirmak@intel.com > wrote:

Sudarshan,

 

Thanks for the message.  The major issue appears to be problems in
generating the Vcc-relative information, but some general supply issues
are cropping up too.  Assuming this is just a regular I/O buffer without
internal terminations...

 

1) I would expect the Pullup I-V tables to pass through the zero V, zero
I axis intercept.  Yet they do not, with the max and min data shifted by
~0.3 V above and below 0 V, respectively.  This suggests that the math
used to generate the tables may be incorrect.  Remember that, if the
data at Vout is collected ground-relative, you can make the power clamp
and pullup table data in Vcc-relative by the formula (Vtable_corner =
Vcc_corner - Vout_corner), where xxx_corner refers to a value collected
for typ, min or max.  For example, if Vcc_min is 4.5 V, data collected
ground-relative for 0.0 at the output would be entered in the table for
the 4.5 V row.

 

The supply voltages and the Pullup I-V axis intercepts (when plotted
ground-relative) don't agree, which implies a problem with the supply
listed for the [Voltage Range] keyword not matching either the real
circuit or the values used to generate the Vcc-relative tables.

 

2) The voltage for the buffer is 5 V +/- 0.5, but the V-t fixture is 3.3
V +/- 0.3 V.  This complicates the math (it gets very hard for me to
calculate the intercepts if they don't match, at least before my first
cup of coffee), but it also suggests that the buffer V-t and I-V data
sets were collected using different settings or otherwise using
different conditions.  I would recommend checking the actual fixtures
used for the V-t extraction; matching the fixture voltages to the
voltage supply would make checking easier.

 

Calculating the right I-V intercepts vs. V-t levels will be easier once
these issues are addressed.

 

- Michael Mirmak

  Intel Corp.

  Chair, EIA IBIS Open Forum

 

________________________________

From: owner-ibis@server.eda.org [mailto:owner-ibis@server.eda.org] On
Behalf Of Sudarshan H N
Sent: Thursday, October 18, 2007 2:16 AM
To: ibis@server.eda.org; ibis-users@server.eda.org
Subject: [IBIS] I-V and V-t curve mismatch

Hello Experts,

I am finding problem with one of my IBIS model in I-V and V-t curve
mismatch. Please find the attached model for the refernence.

ERROR - Model dummy: The [Rising Waveform]
      with [R_fixture]=80 Ohms and [V_fixture]=0V
      has TYP column DC endpoints of  0.01V and  2.66v, but
      an equivalent load applied to the model's I-V tables yields
      different voltages (-0.00V and  3.90V),
      a difference of  0.25% and 31.85%, respectively.

I wrote a new tool to generate IBIS models and i am not able to make out
where it is going wrong.
I actually tried to manually calculate the currents at the points , that
it has reported in the above error.

As it is a rising waveform having a R_fixture to ground  the current at
2.66v (steady state) would be 2.66v/50 = 33.25mA.

I looked at the voltage corresponding to 33.25mA in pullup table and i
found it is coming around 0.66v. There is a mismatch in the way i am
caluculating also. But i observed a different voltage of 0.66v compared
to what ibischk4 has reported i.e, 3.9v. 

Let me know is the way i am cross checking is correct or not ? If not
let me  know the exact procedure.

Also let me know what might have been gone wrong in the curves. From the
shape of the curves i am not seeing any problem with the way i have
generated. 

Please let me know your answers as soon as possible.

Note : Please dont consider any other values apart from the curves like
Ramp , C_comp etc as i have dummy values for these place holders.

Thanks & Regards
Sudarshan



-- 
This message has been scanned for viruses and 
dangerous content by <http://www.mailscanner.info/> MailScanner, and is 
believed to be clean. 

 



-- 
This message has been scanned for viruses and 
dangerous content by MailScanner <http://www.mailscanner.info/> , and is

believed to be clean. 


-- 
This message has been scanned for viruses and 
dangerous content by <http://www.mailscanner.info/> MailScanner, and is 
believed to be clean. 
-- 
This message has been scanned for viruses and 
dangerous content by MailScanner <http://www.mailscanner.info/> , and is

believed to be clean. 

-- 
This message has been scanned for viruses and
dangerous content by MailScanner, and is
believed to be clean.


--------------------------------------------------------------------
|For help or to subscribe/unsubscribe, e-mail majordomo@eda-stds.org
|with the appropriate command message(s) in the body:
|
|  help
|  subscribe   ibis       <optional e-mail address, if different>
|  subscribe   ibis-users <optional e-mail address, if different>
|  unsubscribe ibis       <optional e-mail address, if different>
|  unsubscribe ibis-users <optional e-mail address, if different>
|
|or e-mail a request to ibis-request@eda-stds.org.
|
|IBIS reflector archives exist under:
|
|  http://www.eda-stds.org/pub/ibis/email_archive/ Recent
|  http://www.eda-stds.org/pub/ibis/users_archive/ Recent
|  http://www.eda-stds.org/pub/ibis/email/         E-mail since 1993
Received on Mon Oct 22 11:02:23 2007

This archive was generated by hypermail 2.1.8 : Mon Oct 22 2007 - 11:02:53 PDT