Browse Prior Art Database

Tuning Path Delays for Synchronous Clocking

IP.com Disclosure Number: IPCOM000115214D
Original Publication Date: 1995-Mar-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 2 page(s) / 53K

Publishing Venue

IBM

Related People

Johnson, DWJ: AUTHOR

Abstract

Disclosed is a method for minimizing overall system clock skew by utilizing intentional board skew to offset chip clocking differences.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 58% of the total text.

Tuning Path Delays for Synchronous Clocking

      Disclosed is a method for minimizing overall system clock skew
by utilizing intentional board skew to offset chip clocking
differences.

      For minimum cycle time, the clocks must arrive at each chip
synchronously and latch to pin delays must be the same.  Any
deviation from this introduces clock skew which increases minimum
cycle time, so has a direct impact on the maximum speed at which a
product can run.

      Logic delays, clock and logic path delays are studied.  The
clock paths on the card are given a built in amount of skew in order
to optimize cycle time.

      For instance, take a case where two modules exchange data
bidirectionally.  Suppose (for simplicity) that logic delays into and
out of the latches are the same, that is, that once the clock hits
the latch, they have the same setup and launch delays.  Now suppose
one clock tree delay of 1ns, the other has a clock tree delay of 6ns
(this is the time the clock takes from arriving at the pin of the
module to the latch).  (This is close to an actual case resulting
from different logic families and obtainable clock tree structures).
If the clock leaves the clock driver at the same time and travels the
same path, the clock hits both chips synchronously.  Because of the
difference in clock tree delays, the latches are now skewed by the
amount of difference in path length.  This difference can be
cancelled by shortening the clock path on the...