Redundant clock source

• US 20040046613A1
• Filed: 09/11/2002
• Published: 03/11/2004
• Est. Priority Date: 09/11/2002
• Status: Active Grant

• Current Assignee: Hewlett Packard Enterprise Development LP (Hewlett-Packard Enterprise Company)
• Original Assignee: Hewlett-Packard Development Company, L.P. (HP Inc.)
• Inventors: Wissell, Daniel
• Granted Patent: US 7,397,314 B2
• Time in Patent Office: Days
• US Class Current: 331/46
• CPC Class Codes: G06F 11/1604 where the fault affects the clock signals of a processing unit and the redundancy is at or within the level of clock signal generation hardware
• CPC Class Codes: G06F 11/20 using active fault-masking, e.g. by switching out faulty elements or by switching in spare elements
• CPC Class Codes: H03H 7/0115 comprising only inductors and capacitors H03H7/075, H03H7/09, H03H7/12, H03H7/13 take precedence
• CPC Class Codes: H03H 7/1775 Parallel LC in shunt or branch path H03H7/1791 takes precedence
• CPC Class Codes: H03H 7/48 Networks for connecting several sources or loads, working on the same frequency or frequency band, to a common load or source phase shifters providing two or more output signals H03H7/21

Abstract

A redundant clock source provides a stable clock source for digital system. The clock source uses two oscillators to generate a clock signal. If one of the oscillators fails, the clock signal is generated from the other oscillator until the failed oscillator is replaced. Special filtering of the waveforms produced by the oscillators makes the clock source is resistant to jitter from the oscillators and transients that occur when an oscillator fails. This allows the clock source to not only use a redundant oscillator in an attempt to eliminate a single point of failure, but to also provide a stable clock signal even if one oscillator fails.

Claims

1. A clock system comprising:

first and second oscillators having the same frequency;

means for coupling a first waveform derived from said first oscillator and a second waveform derived from said second oscillator to produce a third waveform;

a resonant filter connected to receive said third waveform and generate clock edges; and

an output buffer connected to receive said clock edges and generate a master clock signal.

2. The clock system of claim 1 in which said coupling means comprises:

a first filter receiving the output of said first oscillator to produce said first waveform;

a second filter receiving the output of said second oscillator to produce said second waveform, said second waveform being different from said first waveform; and

an analog summer combing said first and second waveforms to produce said third waveform.

3. The clock system of claim 2 wherein said first filter is an allpass filter and said second filter is a lowpass filter.

4. The clock system of claim 2 wherein said analog summer comprises:

a first transformer having primary and secondary windings, said primary winding of said first transformer being connected to receive the output of said first filter;

a second transformer having primary and secondary windings, said primary winding of said second transformer being connected to receive the output of said second filter; and

a junction connecting said secondary winding to provide said third waveform.

5. The clock system of claim 1 in which said coupling means comprises:

a multiplexer receiving said first and second waveforms as inputs;

means for choosing an output of said multiplexer from said first and second waveforms; and

a buffer connected to receive a waveform chosen by said means for choosing and to produce said third waveform.

6. The clock system of claim 5, said choosing means comprises:

an oscillation detector receiving input from said first oscillator and having an output controlling said multiplexer, whereby said multiplexer selects the input from said first oscillator if said detector does not detect a failure in said first oscillator, and selects the input from said second oscillator if said detector detects a failure in said first oscillator.

7. A method of producing a clock signal in a redundant clock system compring the steps of:

deriving first and second waveforms from a first and second oscillators, said oscillators having a same frequency;

coupling said first and second waveforms to produce a third waveform; and

filtering said third waveform with a resonant filter to produce clock edges; and

passing said clock edges to an output buffer, said output buffer generating a master clock signal.

8. The method of claim 7 further comprising:

receiving the output of said first oscillator and producing said first waveform;

receiving the output of said second oscillator and producing said second waveform, said second waveform being different from said first waveform; and

combining in an analog summer said first and second waveforms to produce said third waveform.

9. The method of claim 8 further comprising:

filtering the output of said first oscillator with an allpass filter and filtering the output of said second oscillator with a lowpass filter.

10. The method of claim 8 further comprising:

passing said first waveform through a first transformer;

passing said second waveform through a second transformer; and

connecting said first and second transformers at a junction such that said first and second waveforms are summed to produce said third waveform.

11. The method of claim 7 further comprising:

receiving said first and second waveforms as inputs of a multiplexer;

choosing an output of said multiplexer from said first and second waveforms; and

passing a waveform chosen by said multiplexer though a buffer to produce said third waveform.

12. The method of claim 7 further comprising:

monitoring said first waveform with an oscillation detection logic to control said multiplexer, whereby said multiplexer selects the input from said first oscillator if said oscillation detection logic does not detect a failure in said first oscillator, and selects the input form said second oscillator if said oscillation detection logic detects a failure in said first oscillator.