Method and apparatus for clock selection and switching

Details Method and apparatus for clock selection and switching Method and apparatus for clock selection and switching

1998-06-25

2001-05-01

Beausoleil, Robert (Department: 2781)

Electrical computers and digital data processing systems: input/

Intrasystem connection

C710S108000, C710S104000

Reexamination Certificate

active

06226699

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to remote management of computer systems, and more particularly, to the isolation of a remote management device from its host system if the host system is not operable.
2. Description of the Related Art
Remote server management devices have been used to remotely access and administrate server computer systems. Remote server management devices allow a system administrator to dial in to a server or one server in a group of servers and monitor the status or administrate the server from a remote location. Remote server management devices are either embedded components in the server or expansion cards added to the server.
In some cases a server may lose power or become inoperable for some other reason. In such cases it is desirable to allow continued operation of the remote server management device so that the problems leading to the server fault can be observed. To allow such continued operation independent of the server, remote server management devices have been developed having battery backup power supplies.
Servers typically include peripheral component interconnect (PCI) buses to support peripheral cards. Some buses, including PCI buses, are fault intolerant and the bus lines must always be driven to known states to prevent improper operation. Also, floating bus lines can cause excess power usage due to switching. To provide uninterrupted operation of the remote server management device, the remote server management device must be isolated from the bus without deleteriously affecting the processing of its transactions.
One function of a remote server management device is to capture video display data related to the server so that it may be remotely viewed. Prior remote server management devices have stored the data in the system memory of the server. This situation is undesirable if the server loses power because the system memory is erased.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
SUMMARY OF THE INVENTION
In one aspect of the present invention, a circuit is provided for isolating a device from a bus. The circuit includes a local clock, a clock multiplexer, and control logic. The clock multiplexer has a first input terminal coupled to the local clock, a second input terminal coupled to the first clock line of the bus, a control input terminal, and an output terminal. The output terminal is adapted to provide an output clock signal to the device. The control logic is coupled to the control input of the clock multiplexer and adapted to detect a bus isolation event. The control logic is further adapted to generate a bus isolation signal in response to the bus isolation event and provide the bus isolation signal to the control input of the clock multiplexer.
In another aspect of the present invention, a method is provided for isolating a device from a bus. The method includes detecting a bus isolation event. An isolation signal is generated in response to the bus isolation event. One of a local clock signal and a first clock signal provided by the bus is selected based on the isolation signal. An output clock signal is generated based on the selected one of the local clock signal and the first clock signal. Generating the output clock signal includes changing the output clock signal at a predetermined rate until the output clock signal matches the selected one of the local clock signal and the first clock signal.


REFERENCES:
patent: 5313597 (1994-05-01), Gephardt
patent: 5784576 (1998-07-01), Guthrie et al.
Compaq Computer Corporation Technology Brief, “Remote Server Management with Integrated Remote Console”, Oct. 21, 1996.

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