RE: [SI-LIST] : Clock tree

Andrew Ingraham (Andrew.Ingraham@digital.com)
Tue, 18 Aug 1998 10:49:07 -0400

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> The following can help avoid a shelf or non-monotonicity:
>
> 1. If the load capacitances on the stubs are different, the stub with
less loading can be made longer
> to compensate. In other words, the stubs should have the same
electrical length.
>
> 2. If the stubs have an impedance of Zo, the main line should be Zo/2
for a "T" junction.
>
> 3. The impedance of the driver should be matched to the line with a
series resistor.

You can get non-monotonicity even with a single load on the clock line,
if the receiver's package model inductance is large enough.
Mis-matching the driver to the line might help push the glitch beyond
the threshold region, but at the cost of increased overshoot and
ringing.

In such cases, the simulated waveform at the die pads generally lacks
the misbehavior seen at the external pins, so you might reason it away.

Regards,
Andy

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