From: [email protected]
Date: Thu Mar 29 2001 - 12:36:39 PST
Andy,
Wouldn't your .param statement give him a stream that started low (0101...)?
Wouldn't you need
.param v1=3.3v v2=0v td=1ns tr=100ps tf=100ps pw=1.9ns per=4ns
to give a stream that starts high (1010...)? If it matters...
But I agree with your comment of "discourage setting "td" to 0, because
there can be times when SPICE does something funny". It does happen, just
when you don't want it to.
Aubrey Sparkman
Signal Integrity
[email protected]
(512) 723-3592
> -----Original Message-----
> From: Ingraham, Andrew [mailto:[email protected]]
> Sent: Thursday, March 29, 2001 1:41 PM
> To: 'Hassan Ali'
> Cc: si-list
> Subject: RE: [SI-LIST] : Bit stream definition in HSPICE
>
>
> You wrote:
>
> > In HSPICE, which of the following is the correct 500Mbps, tr=100ps,
> > tf=100ps, 101010101... bit stream definition?
> >
> > (1)
> >
> > Vin in gnd pulse(v1 v2 td tr tf pw per)
> > .param v1=0v v2=3.3v td=0ns tr=100ps tf=100ps pw=1.8ns per=4ns
> >
> > (2)
> >
> > Vin in gnd pulse(v1 v2 td tr tf pw per)
> > .param v1=0v v2=3.3v td=0ns tr=100ps tf=100ps pw=1.8ns per=3.8ns
>
> I think neither is what you want. Try:
>
> .param v1=0v v2=3.3v td=1ns tr=100ps tf=100ps pw=1.9ns per=4ns
>
> "per" *IS* the period or repetition rate. No adjustment
> necessary for rise
> and fall times.
>
> However, "pw" is the time spent *at* 3.3 V. You would likely
> want to adjust
> "pw" to account for the rise and fall times. If the rise and
> fall times are
> equal, and you want the pulsewidth to be 2 ns between 50%
> points, subtract
> one risetime from 2 ns to get 1.9 ns. You could also write:
> pw='(1/500MEG)-(tr+tf)/2'
>
> I discourage setting "td" to 0, because there can be times
> when SPICE does
> something funny there and I want to see it if it happens.
> You should see
> nothing change for the first 1 ns (in this case). If
> anything does change,
> investigate why.
>
> (Actually, in this case I might set "td" to 0.95 ns or
> '1ns-tr/2', so that
> the waveform crosses 50% at exactly 1.0 ns, just to make my
> measurements
> easier.)
>
> Andy
>
>
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