[SI-LIST] Time_to_VM on a clock forwarded interface?
- From: "Todd DeRego" <tderego@xxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 22 Dec 2008 11:14:03 -0500
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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