# RE: [SI-LIST] : Bit stream definition in HSPICE

From: Ingraham, Andrew ([email protected])
Date: Thu Mar 29 2001 - 11:40:49 PST

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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