Subject: Re: [IBIS-Users] Vinh for Differential Part
From: Vinu Arumugham (vinu@cisco.com)
Date: Fri Dec 06 2002 - 15:19:26 PST
Tom is correct that datasheets that spec. Vdiff=0 timing reference also specify a
transition rate. If the simulation shows slow slew rates or poor transitions that
violate the datasheet requirement, then the datasheet receiver input timings are
no longer valid. The receiver manufacturer has to provide timing derating
information.
So, it is not clear to me how setting Vdiff to non-zero value or using Vinh not
equal to Vinl will help in this situation.
Thanks,
Vinu
Fred Balistreri wrote:
> Why not use Vmeas or Vinh, Vinl which would be more accurate. I think
> Tom is right on this one.
>
> Best Regards,
>
> Tom Dagostino wrote:
> >
> > The problem with setting Vdiff to 0 is slow slew rates or poor transitions
> > will not give accurate timing. All of the datasheets that I checked had a
> > limitation on the maximum transition time (between 1 and 5 nsec) for the
> > Vdiff = 0 timing.
> >
> > Tom Dagostino
> > Device Modeling Division
> > Teraspeed Consulting Group LLC
> > 2926 SE Yamhill St.
> > Portland, OR 97214
> > (503) 430-1065
> > tom@teraspeed.com
> > http://www.teraspeed.com
> >
> > -----Original Message-----
> > From: vinu@cisco.com [mailto:vinu@cisco.com]
> > Sent: Friday, December 06, 2002 12:40 PM
> > To: tom@teraspeed.com
> > Cc: 'Timothy Coyle'; ibis-users@eda.org
> > Subject: Re: [IBIS-Users] Vinh for Differential Part
> >
> > The device might switch when the differential inputs are 100mV apart.
> > However,
> > if the datasheet timing reference point is the zero crossing, Vdiff should
> > be
> > set to 0 in the IBIS model and the simulator should use this Vdiff value to
> > report timing. Setting a different Vdiff value like 100mV in the IBIS model
> > will result in a timing inaccuracy due to double counting. Likewise for
> > Vinh/Vinl.
> >
> > Thanks,
> > Vinu
> >
> > Tom Dagostino wrote:
> >
> > > The switching point for a differential input happens when the difference
> > in
> > > the input voltage reaches the Vdiff value. In this case the common mode
> > > voltage is 1.2 Volts and the inputs need to be at least 100 mV different
> > to
> > > cause the input to recognize state. Thus 1.15 and 1.25 volts would be the
> > > appropriate Vinl/Vinh values.
> > >
> > > Tom Dagostino
> > > Device Modeling Division
> > > Teraspeed Consulting Group LLC
> > > 2926 SE Yamhill St.
> > > Portland, OR 97214
> > > (503) 430-1065
> > > tom@teraspeed.com
> > > http://www.teraspeed.com
> > >
> > > -----Original Message-----
> > > From: owner-ibis-users@server.eda.org
> > > [mailto:owner-ibis-users@server.eda.org]On Behalf Of Vinu Arumugham
> > > Sent: Friday, December 06, 2002 9:30 AM
> > > To: Timothy Coyle
> > > Cc: ibis-users@server.eda.org
> > > Subject: Re: [IBIS-Users] Vinh for Differential Part
> > >
> > > Since these threshold voltages are used for timing measurements and since
> > > datasheets specify timing at the zero crossing or Vddq/2 level, would it
> > not
> > > be better to set Vdiff=0, Vinl=Vinh=1.2V?
> > >
> > > Thanks,
> > > Vinu
> > >
> > > Timothy Coyle wrote:
> > >
> > > > Hello,
> > > > I have a differential device in an IBIS file. I define a Vdiff of 100mV
> > > > for my differential signal thresholds. However, I also have to define a
> > > > Vinh and Vinl
> > > > for the single-ended positive and negative differential Input model
> > types
> > > > in IBIS. The IBIS spec (3.2) states that:
> > > >
> > > > If a pin is a differential input pin, the differential input
> > > > | threshold (vdiff) overrides and supersedes the need for Vinh
> > > > | and Vinl.
> > > >
> > > > However, if I do not define a Vinh and Vinl for my differential inputs,
> > > > then the IBIS parser gives me a warning. It also seems that some
> > > > simulators use this
> > > > Vinh and Vinl for the single-ended differential signals. If this is an
> > > LVDS
> > > > input, switching from 1 to 1.4 V, then I could put thresholds of 1.1 and
> > > > 1.3?
> > > >
> > > > I'm curious as to what other people do and why the parser gives me this
> > > > warning?
> > > >
> > > > Regards,
> > > > Tim
> > > >
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> --
> Fred Balistreri
> fred@apsimtech.com
>
> http://www.apsimtech.com
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