# US 8,543,005 B2 · Intentionally skewed optical clock signal distribution

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- Number: US 8,543,005 B2
- Document ID: US8543005B2
- Status: Granted
- Kind: B2
- Application number: 12/990,494
- Filed: 2008-04-30
- Published: 2011-03-03
- Granted: 2013-09-24
- Earliest claimed: 2008-04-30
- Assignees: Hewlett-Packard Development Company, L.P.
- Applicants: Nathan L. Binkert; Norman P. Jouppi; Robert S. Schreiber; Jung Ho Ahn
- Inventors: Nathan L. Binkert; Norman P. Jouppi; Robert S. Schreiber; Jung Ho Ahn
- Web page: https://patentgrep.com/patent/US8543005B2

## CPC

- H04B 10/1121: {One-way transmission}
- G06F 1/105: {in which the distribution is at least partially optical}
- H04B: TRANSMISSION
- G06F: ELECTRIC DIGITAL DATA PROCESSING (computer systems based on specific computational models G06N)

## Abstract

Embodiments of the present invention relate to systems and methods for distributing an intentionally skewed optical-clock signal to nodes of a source synchronous computer system. In one system embodiment, a source synchronous system comprises a waveguide, an optical-system clock optically coupled to the waveguide, and a number of nodes optically coupled to the waveguide. The optical-system clock generates and injects a master optical-clock signal into the waveguide. The master optical-clock signal acquiring a skew as it passes between nodes. Each node extracts a portion of the master optical-clock signal and processes optical signals using the portion of the master optical-clock signal having a different skew for the respective extracting node.

## Claims

### Claim 1

A source synchronous system comprising:
a waveguide;
an optical-system clock optically coupled to the waveguide, the optical-system clock to generate and inject a master optical-clock signal into the waveguide, wherein a length of the waveguide is a whole integer multiple of a period of a clock cycle of the master optical-clock signal; and
a number of nodes optically coupled to the waveguide, wherein the master optical-clock signal acquires a skew as it passes between the nodes and each node extracts a portion of the master optical-clock signal and processes optical signals using the extracted portion of the master optical-clock signal having a respective skew for the respective extracting node.

### Claim 2 (depends on 1)

The system of claim 1 wherein the waveguide further comprises a loop configuration having a length that is substantially equal to:
L
=
m
⁢
c
n
⁢
T
CLK
where L is the length of the waveguide, m is a whole number, c is the speed of light in free space, n is the refractive index of the waveguide, and TCLK is the period or time needed to complete one clock cycle of the optical-clock signal.

### Claim 3 (depends on 1)

The system of claim 1 wherein each node processes the optical signals using the extracted portion of the master optical-clock signal having the respective skew for the respective extracting node further comprises the respective extracting node generating a local electronic clock synchronous with the skewed extracted portion of the master optical-clock signal, and the node reading data received on optical data signals and transmitting data onto optical data systems using the locally generated clock.

### Claim 4 (depends on 1)

The system of claim 1 further comprising the optical signals are sent between nodes with the same skew as the master optical-clock signal.

### Claim 5 (depends on 1)

The system of claim 1 further comprising a retimer disposed between two ends of the waveguide for retiming the optical signals, wherein the retimer determines the phase difference between a clock cycle of the optical-clock signal returning to the retimer and a clock cycle of an electronic-clock signal generated by an electronic-system clock of the retimer, and uses the phase difference to determine the time delay needed to re-time the optical signals so that the optical signals pass each node having the same skew as the master optical-clock signal.

### Claim 6 (depends on 5)

The system of claim 5 wherein the retimer further comprises:
an optical-to-electrical converter configured to convert the optical-clock signal into electronic-clock signal and optical signals generated by the nodes into electronic signals;
an asynchronous device configured to delay the electronic-clock signal and the electronic signals; and
an electrical-to-optical converter electrically coupled to the asynchronous device and configured to convert the electronic-clock signal into an optical-clock signal and the electronic signals into optical signals.

### Claim 7 (depends on 6)

The system of claim 6 wherein the asynchronous device further comprises one of:
a delay-lock loop for delaying the electronic signals;
memory configured to time delay the electronic signals by storing and retrieving the electronic signals in a first-in, first-out manner; and
an electronically tunable crystal composed of a material having a refractive index that can be varied in accordance with an appropriate amount of an applied voltage that delays the optical signals.

### Claim 8 (depends on 1)

The system of claim 1 further comprising a second waveguide having substantially the same path as the waveguide and optically coupled to each node so that each node can transmit optical signals to other nodes that are optically coupled to the second waveguide using each nodes extracted portion of the master optical clock.

### Claim 9

A method for processing optical signals using a skewed optical-clock signal in a source synchronous computer system, the method comprising:
generating a master optical-clock signal;
injecting the master optical-clock signal into a waveguide;
extracting a portion of the master optical-clock signal at each node optically coupled to the waveguide, the portion of the master optical-clock signal having a different skew at each node;
retiming the master optical-clock signal; and
processing optical signals at an extracting node using the portion of the master optical-clock signal having the respective skew for the extracting node.

### Claim 10 (depends on 9)

The method of claim 9 wherein processing optical signals using the portion of the master optical-clock signal having the different skew for the respective extracting node further comprises
reading the data in a received optical signal based on the extracted portion of the master optical-clock signal having the respective skew for the respective extracting node; and
transmitting data onto an optical signal in the waveguide by the respective extracting node based on the extracted portion of the master optical-clock signal having the respective skew for the respective extracting node wherein the transmitted optical signal has the equivalent time delay represented by the respective skew for the respective node in the extracted portion of the master optical-clock signal.

### Claim 11 (depends on 9)

The method of claim 9 wherein processing optical signals at an extracting node using the portion of the master optical-clock signal having the respective skew for the extracting node further comprises:
the respective extracting node generating a local electronic clock synchronous with the skewed extracted portion of the master optical-clock signal,
the node reading data received on optical data signals using the locally generated clock, and
the respective extracting node transmitting data onto optical data signals using the locally generated electronic clock.

### Claim 12 (depends on 9)

The method of claim 9, wherein retiming the master clock signal includes retiming the optical signals at a single point along the waveguide so that each node receives the re-timed optical signals with the same skew as the master optical-clock signal.

### Claim 13 (depends on 12)

The method of claim 12 wherein retiming the optical signals further comprises:
extracting the optical signals from the waveguide;
converting the optical signals into an electronic signals using an optical-to-electrical converter;
time delaying the electronic signal; and
converting the time delayed electronic signal into re-timed optical signals that are injected into the waveguide using an electrical-to-optical converter.

### Claim 14 (depends on 13)

The method of claim 13 wherein time delaying the electronic signal further comprises determining an amount of time Tdelay by which to delay the optical signals in accordance with the following:
Tdelay=n·TCLK−Treturn
where TCLK is the period of one complete clock cycle, Treturn is the time for one clock cycle to complete a trip around the waveguide and return to the point at which the clock cycle was injected, and n is an integer satisfying the condition:
0≦(n·TCLK−T1 CLK cycle)≦Treturn