Subject: Re: [IBIS-Users] Query on the IBIS file verification
From: Ingraham, Andrew (a.ingraham@ieee.org)
Date: Mon Jan 05 2004 - 08:06:31 PST
Message1. I have generated an IBIS model for a 3-state Output buffer. When I disable the output buffer IBIS model and give a sweep voltage from -ref_voltage to 2*(ref_voltage) on the output pin (just as is done to generate the IV tables for IBIS model), I get very small currents in order of uA even on beyond rail voltage values. ...
If you are going a volt beyond the rails and seeing no significant current, it may be an HSPICE "feature". Some of the B-element nodes are listed as "optional." Do not omit the optional nodes!
If you omit them, HSPICE behaves exactly like you described (the clamps turn on and off with the pull-up/down devices).
2. In one of the I/O buffer IBIS model transient analysis, the ouput waveform does not match the model's V-T table. I am using the same setup (same Rload with same termination and same input signal used to generate the IBIS model) for the analysis. I am not able to figure out the mismatch.
Make sure to add something to represent the IC package (R_pkg or R_pin). The HSPICE IBIS implementation doesn't include any IBIS package parasitics.
When I run ibischk3 on the IBIS model I am getting the following warnings which I am not able to eleminate :
a. The [Rising Waveform] with [R_fixture]=50 Ohms and [V_fixture_max]=1.6V has MAX column DC endpoints of 0.76V and 1.59V,but an equivalent load applied to the model's I-V tables yields different voltages(0.76V and 1.57V), a difference of 0.21% and 2.22%,respectively.
b. The [Falling Waveform] with [R_fixture]=50 Ohms and [V_fixture_max]=1.6V has MAX column DC endpoints of 0.76V and 1.59V,but an equivalent load applied to the model's I-V tables yields different voltages(0.76V and 1.57V), a difference of 0.21% and 2.22%,respectively.
These differences are not huge, you're almost there. Check that the SPICE simulation actually uses 50.0 ohms to 1.60V, the same exact supply voltage, and that the package resistance is correct and in the simulation.
Regards,
Andy
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