RE: different rising and falling Ramp dV

The case I had in mind has the dV rising about 1/3 the dV falling. The dV
rising seems to be taken from the Rising V/T curve with 50 ohms to ground
and the dV falling seems to be taken from the Falling V/T curve with 50 ohms
to Vtt. There is no pullup in the power clamp for this buffer. I didn't
ask my supplier whether the pullup was always on but I don't think it is.
This buffer is used on a bus that has at least one pullup. This seems to be
a little different from what you discussed; perhaps an alternate method?
Perhaps I am missing something?

BTW, my simulator seems to be OK with this as my simulations match what we
measure in the lab.

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 2:58 PM
> To: ibis-users@vhdl.org
> Subject: RE: different rising and falling Ramp dV
>
>
> Sorry, I got dyslexic a bit... For the open-drain GTL buffer
> the resistor is not connected to GND, but to Vtt, and the
> "ON" endpoints of the Vt curve will definitely not go to GND,
> but the "OFF" endpoints should be at Vtt.
>
> Arpad
> ==============================================================
>
> -----Original Message-----
> From: Muranyi, Arpad [mailto:arpad.muranyi@intel.com]
> Sent: Friday, March 23, 2001 12:52 PM
> To: ibis-users@vhdl.org
> Subject: RE: different rising and falling Ramp dV
>
>
> Aubrey,
>
> In light of the original question from Mike and my answer to it,
> I don't expect anything different in this case. I assume that
> the weak pullup you are talking about is always on. So the
> two Vt curves you will make will be generated with a resistor
> to ground, and the rising and falling edge will characterize
> how the switched pulldown is turning on and off. The amplitude
> of the rising and falling edge will be the same, but may not
> start or end on the rail (GND) exactly due to the weak pullup.
>
> I am not sure if this answered your question, or whether I
> understood your question fully. Did this help?
>
> Arpad
> ===============================================================
>
> -----Original Message-----
> From: Aubrey_Sparkman@Dell.com [mailto:Aubrey_Sparkman@Dell.com]
> Sent: Friday, March 23, 2001 10:49 AM
> To: arpad.muranyi@intel.com; ibis-users@vhdl.org
> Subject: 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
> >
> >
>
>
>
>