[SI-LIST] Re: Series termination
- From: "Tom Biggs" <tbiggs@xxxxxxxxxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 12 Mar 2003 10:52:59 -0800
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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