[SI-LIST] Re: Time_to_VM on a clock forwarded interface?

Todd,

It might help, conceptually, if you were to think of the reference load
calibration process as "calibrating out the implicit 'clock-to-out' time
of the IBIS model", which is undefined and can even vary, for the same
IBIS model, from simulator to simulator.

-db


> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Beal, Weston
> Sent: Monday, December 22, 2008 9:53 AM
> To: Todd DeRego; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Time_to_VM on a clock forwarded interface?
> 
> Todd,
> 
> You are correct that you should calculate the timing based on flight
> time compensated numbers, or in other words, use the time-to-vmeas.
This
> is the only way that you can correlate your simulation result times
with
> the timing numbers supplied by the IC vendors. I wrote a short
> application note about this some years ago. You can access it at
> http://supportnet.mentor.com/reference/appnotes/library/3906.pdf
> The application note talks about doing these calculations in ICX, but
> the principle is the same for any simulator. If you have further
> questions after reading the note referenced, feel free to contact me
> directly.
> 
> Regards,
> Weston
> 
> 
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Todd DeRego
> Sent: Monday, December 22, 2008 8:14 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Time_to_VM on a clock forwarded interface?
> 
> I'm trying to figure out the correct way to simulate the flight time
of
> a clock on a clock forwarded interface (SDRAM memory, in this case).
> 
> We have used XTK previously to extract flight times and are using
> HyperLynx now but my question is really about the concept and not the
> specific tool.
> 
> The data flight time out of the tool makes sense:
> 
> Tco (related to clock) -->
>              <-- Time_to_VM (based on datasheet AC test load)
>              Simulation Flight Time to SDRAM (based on etch & receiver
> capacitance) -->
> 
> But doing the clock the same way doesn't seem right to me since there
> isn't really isn't a Tco for the clock since the clock represents
"time
> zero" for the rest of the timing.
> 
> If I just use the simulated flight time out of the tool with no
> Time_to_VM adjustment, the flight time ends up much longer than the
data
> flight times and breaks the interface.
> 
> I was thinking the simulated flight time isn't really based on the
same
> "time zero" that I'm applying the Data Tco from. I really want to
apply
> the Data Tco to the time the clock passes through Vmeas (1.25V in this
> case) not when the tool starts it simulation.
> 
> Because of this I redid the Time_to_VM simulation for the clock output
> using 0pF load (versus 15pF load in the IBIS model). The resulting 0pF
> Time_to_VM is less than the 15pF Time_to_VM (which was making the
final
> result flight time for the clock negative, which would make sense with
a
> positive value Tco but there isn't a Tco for the clock) and makes the
> clock flight time make more sense compared to the data flight times.
> 
> In past simulations, we have applied the test load Time_to_VM to
clocks
> in clock forwarded interfaces. But when we couldn't make timing work
on
> this interface and started to look deeper, we are wondering if we
should
> have been doing that for any clock forwarded interface.
> 
> Anybody have any thoughts on this?
> 
> Thanks!
> Todd
> 
> Todd DeRego
> Principal Hardware Engineer - Signal Integrity Cedar Point
> Communications Derry, NH tderego@xxxxxxxxxxxxxxxxx
> 
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