RE: [SI-LIST] : HSPICE Start Up conditions

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From: Kai Keskinen ([email protected])
Date: Wed Mar 22 2000 - 11:15:27 PST

I have been given an encrypted driver model. It is a PECL driver. The app
notes say to use 4.5k pulldowns. The sim works now but it is extremely
sensitive to initial charge values on those caps. I'm told the physical chip
works with the recommended pull downs and coupling caps so is this a problem
with the model or something related to HSPICE? I suspect the model but I
can't really do much with it.

Kai Keskinen
Equipment and Network Interconnect
Nortel Subsystems and Performance Networks (NSPaN)
(613)-765-3506 (ESN 395)
[email protected] <mailto:[email protected]>

        -----Original Message-----
        From: Tom Dagostino [SMTP:[email protected]]
        Sent: Wednesday, March 22, 2000 1:50 PM
        To: si-list
        Subject: RE: [SI-LIST] : HSPICE Start Up conditions

        One thing I would look at is the output impedance of the driver.
With the 4.5K pull down there is little standing current keeping Zout high
(I'm assuming and ECL like driver). During the falling transition the
transistor is off and the 4.5K is the source of current.

        Tom Dagostino
        ICX Modeling Group
        [email protected]


                -----Original Message-----
                From: [email protected]
[mailto:[email protected]]On Behalf Of Kai Keskinen
                Sent: Wednesday, March 22, 2000 7:46 AM
                To: '[email protected]'
                Subject: [SI-LIST] : HSPICE Start Up conditions

                Hi SI Folk:

                I had a lot of problems getting a spice model of a gigabit
ethernet chip to run properly. The differential outputs are tied to ground
with 4.5kOhm resistors and then coupled to the long T-lines (W-element) with
~1nF caps. They are terminated with 100ohms across the differential pairs
just before the receiver. It turned out that if we did not set the initial
charging conditions on the coupling caps to a very narrow range of values,
the simulation never actually worked. One or both of the differential
outputs did not at all resemble what the chip should transmit even for very
long simulation times.

                My questions are: Why did the circuit not charge up the caps
and then work as would happen in reality? Is this a very common effect when
running HSPICE or other sims?


                Kai Keskinen
                Equipment and Network Interconnect
                Nortel Subsystems and Performance Networks (NSPaN)
                (613)-765-3506 (ESN 395)
                [email protected]
<mailto:[email protected]>

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