RE: different rising and falling Ramp dV

Mike,

The reason dV can be different for rising and falling edges is because
the voltage swing does not have to be the same (in contrary of your
statement).

A falling edge ramp requires that the R_load resistor is connected to
the supply rail. This means that the falling edge characterizes the
pulldown turning on. A rising edge ramps requires that the R_load
resistor is connected to GND. This means that the rising edge ramp
characterizes how the pullup turns on. Since the pullup and pulldown
IV curves do not have to be the same (strength), you can get different
swings for these two edges, hence the dV numbers will also be different.

Now, if you consider an open drain type buffer, where the rising and
falling edges are characterized with R_load connected to the same rail,
your statement is correct, the swing will be the same, therefore dV
should also be the same. However, notice that in this case one edge
describes how the device turns on, the other edge describes how the
same device turns off.

Arpad
=======================================================================

-----Original Message-----
From: Mike LaBonte [mailto:mike@labonte.com]
Sent: Thursday, March 22, 2001 3:19 PM
To: ibis-users@vhdl.org
Subject: different rising and falling Ramp dV

Many IBIS models have different values for dV in the dV/dt_r and dV/dt_f.
The clipping below from IBIS spec shows an example. But the usage rules
imply that dV must be 60% of the voltage swing. If the voltage swing is
measured between the 2 steady-state voltages, then how could you have
different values for rise and fall dV? If there is variation in how
simulators handle the Ramp dV values, then this may matter.

Mike LaBonte

|===========================================================================
==
| 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