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

From: Sudarshan H N <hn.sudarshan_at_.....>
Date: Tue Oct 23 2007 - 10:05:54 PDT
Hi All,

Thanks Tom ,Todd Scott and Arpad for your valuable answers. Arpad, with your
clear explanation i am able to solve all the problems i had in my modeling.

One general question i have to the forum is , in the IBIS website is all the
mails are stored in some central location. Because i was accessing IBIS
forum mails from my other email id 3 months back. Now i am accessing from
other email id.

I think sometime back there was a question on analog pads IBIS model. Do i
need to put a NC for these pads in pin mapping section ?

Regards
Sudarshan

On 10/23/07, Todd Westerhoff <twesterh@sisoft.com> wrote:
>
>  Oops,
>
>
>
> You're right: -VDDQ to 2*VDDQ.
>
>
>
> 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 *Muranyi,
> Arpad
> *Sent:* Monday, October 22, 2007 2:01 PM
> *To:* ibis@eda.org; ibis-users@eda.org
> *Subject:* RE: [IBIS-Users] RE: [IBIS] I-V and V-t curve mismatch
>
>
>
> 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 2.5Vsupply?
>
> 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 Tue Oct 23 10:06:59 2007

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