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

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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