RE: different rising and falling Ramp dV

Arpad,
Would you comment on what you expect for something like AGTL+ which is
almost an open drain, but has a weak pullup? The weak pullup alone can't
pull a 50 ohm resistor up very far.

Aubrey Sparkman
Signal Integrity
Aubrey_Sparkman@Dell.com
(512) 723-3592

> -----Original Message-----
> From: Muranyi, Arpad [mailto:arpad.muranyi@intel.com]
> Sent: Friday, March 23, 2001 10:28 AM
> To: ibis-users@vhdl.org
> Subject: 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
>
>