[SI-LIST] Re: Series termination
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 12 Mar 2003 08:41:33 -0500
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
> the
> oscillator and the length of the trace. However, there are some 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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