[SI-LIST] Re: Series termination
- From: Ed Sayre III <esayre3@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 12 Mar 2003 15:07:21 -0500
Tom,
This really depends on the speed of the bus. Is it like the a
133/266MHz DDR bus?? Have you run an simulations of the environment to
identify the SI risk?? We have found that it really depends on the bus
topology as to whether terminations are needed. I would suggest you do an
accurate simulation (in SPICE for instance) of a cross section of the bits
of the bus with and without terminations. I would also suggest you go with
the smallest sized part, like and 0402.
I also agree that on chip terminations and Tx/Rx control are key to
raising the level of integration and signal fidelity on copper.
Good luck
-Dr Ed Sayre, jr
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NORTH EAST SYSTEMS ASSOCIATES, INC
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"High Performance Engineering & Design"
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Dr. Edward Sayre 3rd e-mail: esayre3@xxxxxxxx
NESA, Inc. http://www.nesa.com/
5 Lan Drive, Suite 200 Tel +1.978.392-8787 x 218
Westford, MA 01886 USA Fax +1.978.392-8686
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At 10:52 AM 3/12/2003 -0800, you wrote:
>Consider this situation:
>You have two BGAs with a 32 bit source synchronous high speed bus
>between them. The BGAs are only .5" apart. To add series termination to
>the 32 bit bus you have to separate the BGAs to make room for all of the
>resistors.
>
>You end up with the .5" net before you even get to the resistor! In this
>case wouldn't it make more sense just to leave the resistor out?
>
>This is why I really like the trend for ICs to have temperature and
>process controlled output impedences (referenced to an extenal resistor)
>so that there is no need for termination resistors.
>
> -tom
>
>-----Original Message-----
>From: Ingraham, Andrew [mailto:Andrew.Ingraham@xxxxxx]=20
>Sent: Wednesday, March 12, 2003 5:42 AM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Series termination
>
>
>If overshoot is acceptable, also check whether the overshoot would be
>clamped. If the overshoot doesn't clamp, it could ring back and cause
>double-clocking. Clamping on overshoots is a form of termination too
>because it dissipates some of the energy.
>
>If the electrical length is a whole multiple of a quarter wavelength of
>the clock frequency or a low harmonic, and it is poorly terminated, you
>could see a build-up of energy and overshoot over a few cycles that
>exceeds what you might otherwise expect. This is one case where the
>frequency (not just the risetime) matters.
>
>Series termination includes the internal driver impedance. Occasionally
>(if rarely), the driver impedance is reasonably close to the
>transmission line impedance with no need for an additional resistor.
>
>Regards,
>Andy
>
>
> > I agree with Scott in principle, it all depends on the rise time of=20
> > the oscillator and the length of the trace. However, there are some=20
> > cases where
> > there is an acceptable risk in not using a series term. (Checking
> > whether
> > the receiver can deal with the overshoot might not be a bad idea,
> > either.)
>...
>
>
>
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