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

From: Sudarshan H N <hn.sudarshan_at_.....>
Date: Thu Oct 18 2007 - 22:32:08 PDT
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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>

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Received on Thu Oct 18 22:32:38 2007

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