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

From: Tom Dagostino <tom_at_.....>
Date: Mon Oct 22 2007 - 09:10:53 PDT
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




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Received on Mon Oct 22 09:13:36 2007

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