From: D. C. Sessions (email@example.com)
Date: Fri Nov 05 1999 - 09:55:15 PST
> Be careful if you intend to use an inverter as a clock buffer. The alvc
> stuff is pretty fast, but not nearly fast enough for your needs. That
> particular part is about 4nS spec into a heavy load, and a minimum Tpd spec
> of 1nS. At 240MHz Tpd alone is 50% of your cycle interval. That
> translates to linear amplifier. I don't know who did your ASIC's or the
> system design, but 240MHz at SSTTL is going to be very hard to do.
> One of the HSTL, or GTL variants is a much better choice, or alternatively
> distribution of a lower frequency clock with PLL's inside each chip
> multiply up to the necessary value. If you have already committed the
> ASIC's this could be a problem.
> Go look at Lattice Semiconductor for the 3nS GAL, GAL16LV8.
Hrrrmm.. How well specified are these at 2.5v?
Remember, this is going into an SSTL-2 application.
> At 07:17 PM 11/4/99 -0800, you wrote:
> > Thank you very much for the reply!
> > The 240 Mhz part is to buffer the clocks from clock synthesizer
> >to multiple ASICs as a clock source. The ASICs have 3.3v IO but
> >are 5V tolerate. I saw and IDT part# IDT74ALVC16244A that may
> >be promising. Have you had any experience with that part?
> > Do you know of any GaAs vendor that may provide faster
> >parts for my needs? I wonder what people use for buffering
> >the clocks for AMD's 200 Mhz system clock or DDR clocks?
> > Can you give me a pointer for the GAL part#/vendor info?
> > Thanks in advance!
> > >
> > > From your post, I guess that you want 3.3V SSTTL levels. 240MHz is really
> > > beyond any practical upper limit for SSTTL. There isn't much out there that
> > > can reliably get below a Tpd of 1.5nS, which would typically be suitable to
> > > 150-166MHz at the high-end. If you want to reach beyond 200MHz, it is time
> > > to use low-swing logic.
> > >
> > > The application sounds like a phase-splitter. If it is, instead of an
> > > inverter try a precision delay line, either buried in etch, or off the
> > shelf.
> > >
> > > The 120MHz mux / FF can be implemented in either a 5nS or 3nS GAL. Which
> > > grade is appropriate depends on the specifics of your Tsu and Th
> > > requirements.
> > >
> > > Regards,
> > >
> > > Steve.
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