Browse Prior Art Database

Harmonically-Designed Clock Trees

IP.com Disclosure Number: IPCOM000113885D
Original Publication Date: 1994-Oct-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 2 page(s) / 51K

Publishing Venue

IBM

Related People

Appel, WD: AUTHOR

Abstract

Disclosed is a method for assuring good signal quality on a clock tree of a high-speed processor card by maintaining certain geometric relationships among the branches of the tree, so that slope reversals are not introduced into the clock signals propagating along the branching structure. Such slope reversals can otherwise cause the triggering of a receiving device to become unreliable.

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Harmonically-Designed Clock Trees

      Disclosed is a method for assuring good signal quality on a
clock tree of a high-speed processor card by maintaining certain
geometric relationships among the branches of the tree, so that slope
reversals are not introduced into the clock signals propagating along
the branching structure.  Such slope reversals can otherwise cause
the triggering of a receiving device to become unreliable.

      The Figure shows multi-level tree structure of the type often
needed to conserve board wiring.  In this two-level example, a tree
driven by a driver DR divides into M branches at a node A, closest to
the driver, and into N branches at each node B, where M and N are
both greater than one.  An unwanted negative-phase re-reflection is
always generated in the A-to-B leg, since the characteristic
impedance of the node B is always less than that of the copper
transmission lines.  This re-reflection typically results in a
detrimental slope reversal at the receiving nodes C.

      However, if the clock tree is symmetrical about an axis
extending from the driver, and if certain integral ratios are
preserved in the lengths of the A-to-B and B-to-C branches, the phase
reversal can be cancelled, so that a high-quality, monotonically
rising, or falling, waveform can be produced as a direct consequence
of the "conservation of power."

      Specifically, the phase reversal is cancelled if this symmetry
is maintained, and if the condition...