Re: Rise and Fall times in IBIS

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

Prasad Rau wrote:
>
> 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
 
Received on Tue Mar 6 18:01:22 2001