Subject: Rise and Fall times in IBIS

From: "Prasad Rau" <[email-masked]>
Date: Tue, 06 Mar 2001 16:16:15 -0800

Hi,

I have a question regarding the dv/dt_r and dv/dt_f  values that IBIS
reports.
It is unclear to me whether IBIS models an unloaded buffer in
determining these
values or it simulates using  Cref capacitance value to determine
values for dv/dt_r and dv/dt_f. It was not obvious to me from the IBIS
documentation.

Appreciate any feedback I can get in this regard.

Thanks,

Prasad Rau

---

From: Mike LaBonte <[email-masked]>
Date: Tue, 06 Mar 2001 21:01:13 -0500

Prasad,

The IBIS spec mentions R_load, as shown below:

|=============================================================================
|     Keyword:  [Ramp]
|    Required:  Yes, except for inputs, terminators, Series and Series_switch
|               model types.
| Description:  Defines the rise and fall times of a buffer.  The ramp rate
|               does not include packaging but does include the effects of the
|               C_comp parameter.
|  Sub-Params:  dV/dt_r, dV/dt_f, R_load
| Usage Rules:  The rise and fall time is defined as the time it takes the
|               output to go from 20% to 80% of its final value.  The ramp
|               rate is defined as:
|
|               dV          20% to 80% voltage swing
|               -- = ----------------------------------------
|               dt   Time it takes to swing the above voltage
|
|               The ramp rate must be specified as an explicit fraction and
|               must not be reduced.  The [Ramp] values can use "NA" for the
|               min and max values only.  The R_load subparameter is optional
|               if the default 50 ohm load is used.  The R_load subparameter
|               is required if a non-standard load is used.
|-----------------------------------------------------------------------------
[Ramp]
| variable      typ             min             max
dV/dt_r         2.20/1.06n      1.92/1.28n      2.49/650p
dV/dt_f         2.46/1.21n      2.21/1.54n      2.70/770p
R_load = 300ohms

Admittedly it fails to say much about it, but Ramp is determined with a
purely resistive load. Bear in mind that simulators may not use Ramp data
where Rising and Falling Waveforms exist. These are where dynamic behavior
and capacitive loads are really considered.

Mike LaBonte