Re: using s2ibis2

Hello Syed,

Thank you for your responses.

>>> Syed Huq <shuq@cisco.com> 09/20/00 01:30AM >>>
>Betty,

>Not sure if you got any responses to your questions. My 2cents below:

>> 2) I encountered a DC convergence problem when I was using s2ibis2 to generate
an IBIS model for a 3-state Output pad (using HSPICE). So, I manually changed
the spice file generated by s2ibis2 for the pulldown curve to sweep from 6.6V to
-3.3V in intervals of -0.1V (instead of its default from -3.3V to 6.6V in
intervals of 0.1V). In the resultant .ibs file, I don't think I'm getting the
correct pulldown curve with the subtraction of the gnd clamp curve. Is this
because for s2ibis2 to generate the data correctly, the voltage values in the
output file have to be increasing? Any ideas on other workarounds?
>>
>> Many thanks,

>The pulldown curve from s2ibis2 does not subtract the gnd clamp. This is very
visible if you plot the data points. I am unable to comment further since I do
not know how your generated .ibs file looks like. Sometimes, I would reduce the
sweep range to get away from convergence issues. Then I could extrapolate the
end data points to the IBIS specified ranges(this is a valid approach).

I have attached my .ibs file. As a "trial", I have not included any package parasitics or c_comp. I changed the sweep range from -3.3V - 6.6V to -2.7 - 6.6V to clear the convergence issue in the files pdtout.spi and ddtout.spi ("out" being the name of my output pin). The file pdtout.spi specifies the typical pulldown curve spice file, and the file ddtout.spi specifies the pulldown curve with output disabled spice file. (The [Ground clamp] curve is generated by another file, gctout.spi.)

It seems to me when I look at the resultant data in pdtout.out, ddtout.out and in the .ibs file under keyword [Pulldown], the subtraction was done by s2ibis2. I have included excerpts from those files at the end of this message. If s2ibis2 does not do the "subtraction", then wouldn't the simulator be "double-counting" the clamp curves when it's using the IBIS model? (Since it is stated in the IBIS specification that it is assumed the simulator will sum the clamp curves with the appropriate pullup or pulldown curves when the buffer is driving high or low.)

What method do you use to extrapolate the end data points?

Many thanks,
Betty Luk

****** file excerpt: pdtout.out
        volt current vouts2i -2.7000e+00 1.664e-01 -2.6000e+00 1.598e-01 -2.5000e+00 1.531e-01 -2.4000e+00 1.464e-01 -2.3000e+00 1.396e-01 -2.2000e+00 1.327e-01 -2.1000e+00 1.258e-01 -2.0000e+00 1.188e-01 -1.9000e+00 1.118e-01 -1.8000e+00 1.047e-01 -1.7000e+00 9.760e-02 -1.6000e+00 9.048e-02 -1.5000e+00 8.334e-02 -1.4000e+00 7.622e-02 -1.3000e+00 6.914e-02 -1.2000e+00 6.210e-02 -1.1000e+00 5.516e-02 -1.0000e+00 4.834e-02 -9.0000e-01 4.169e-02 -8.0000e-01 3.529e-02 -7.0000e-01 2.921e-02 -6.0000e-01 2.359e-02 -5.0000e-01 1.861e-02 -4.0000e-01 1.451e-02 -3.0000e-01 1.083e-02 -2.0000e-01 7.221e-03 -1.0000e-01 3.611e-03 0. 1.091e-10 1.0000e-01 -3.535e-03 2.0000e-01 -6.921e-03 3.0000e-01 -1.016e-02 4.0000e-01 -1.324e-02 5.0000e-01 -1.617e-02 6.0000e-01 -1.895e-02 7.0000e-01 -2.158e-02 8.0000e-01 -2.405e-02 9.0000e-01 -2.636e-02 1.0000e+00 -2.851e-02 1.1000e+00 -3.050e-02 1.2000e+00 -3.233e-02 1.3000e+00 -3.400e-02 1.4000e+00 -3.550e-02 1.5000e+00 -3.683e-02 1.6000e+00 -3.797e-02 1.7000e+00 -3.890e-02 1.8000e+00 -3.961e-02 1.9000e+00 -4.011e-02 2.0000e+00 -4.046e-02 2.1000e+00 -4.071e-02 2.2000e+00 -4.090e-02 2.3000e+00 -4.104e-02 2.4000e+00 -4.115e-02 2.5000e+00 -4.125e-02 2.6000e+00 -4.132e-02 2.7000e+00 -4.139e-02 2.8000e+00 -4.145e-02 2.9000e+00 -4.150e-02 3.0000e+00 -4.154e-02 3.1000e+00 -4.158e-02 3.2000e+00 -4.162e-02 3.3000e+00 -4.166e-02

****** file excerpt: ddtout.out
        volt current vouts2i -2.7000e+00 1.215e-01 -2.6000e+00 1.154e-01 -2.5000e+00 1.093e-01 -2.4000e+00 1.031e-01 -2.3000e+00 9.695e-02 -2.2000e+00 9.075e-02 -2.1000e+00 8.454e-02 -2.0000e+00 7.832e-02 -1.9000e+00 7.211e-02 -1.8000e+00 6.592e-02 -1.7000e+00 5.976e-02 -1.6000e+00 5.365e-02 -1.5000e+00 4.761e-02 -1.4000e+00 4.167e-02 -1.3000e+00 3.585e-02 -1.2000e+00 3.019e-02 -1.1000e+00 2.474e-02 -1.0000e+00 1.953e-02 -9.0000e-01 1.467e-02 -8.0000e-01 1.021e-02 -7.0000e-01 6.294e-03 -6.0000e-01 3.133e-03 -5.0000e-01 9.967e-04 -4.0000e-01 1.728e-04 -3.0000e-01 1.994e-05 -2.0000e-01 1.351e-06 -1.0000e-01 4.619e-08 0. 1.098e-10 1.0000e-01 -1.066e-09 2.0000e-01 -1.158e-09 3.0000e-01 -1.210e-09 4.0000e-01 -1.254e-09 5.0000e-01 -1.293e-09 6.0000e-01 -1.331e-09 7.0000e-01 -1.367e-09 8.0000e-01 -1.401e-09 9.0000e-01 -1.436e-09 1.0000e+00 -1.470e-09 1.1000e+00 -1.504e-09 1.2000e+00 -1.537e-09 1.3000e+00 -1.570e-09 1.4000e+00 -1.603e-09 1.5000e+00 -1.636e-09 1.6000e+00 -1.670e-09 1.7000e+00 -1.702e-09 1.8000e+00 -1.735e-09 1.9000e+00 -1.769e-09 2.0000e+00 -1.802e-09 2.1000e+00 -1.836e-09 2.2000e+00 -1.871e-09 2.3000e+00 -1.905e-09 2.4000e+00 -1.941e-09 2.5000e+00 -1.977e-09 2.6000e+00 -2.015e-09 2.7000e+00 -2.054e-09 2.8000e+00 -2.080e-09 2.9000e+00 -2.086e-09 3.0000e+00 -2.093e-09 3.1000e+00 -2.096e-09 3.2000e+00 -2.102e-09 3.3000e+00 -2.107e-09