Method and apparatus for entering a low-power mode and controlling an external bus of a data processing system during low-power mode

A bus protocol for controlling a bus having at least one bit, the bus protocol controlling the bus in a manner which allows a data processor, which is coupled to the bus, to selectively enter a low-power mode of operation, the data processor having a bus grant status which determines the data processor ability to communicate with the bus, the method comprising:

• broadcasting a request on the bus, the request being broadcast by the data processor to indicate external to the data processor that the data processor intends to enter into a low-power mode of operation;
• placing the data processor into a mode wherein the data processor waits for a permission signal from the bus which allows the data processor to enter the low-power mode of operation;
• allowing the data processor to enter the low-power mode of operation if either the permission signal is received from the bus or a predetermined amount of time passes and a watchdog permission signal is received from external to the data processor;
• placing the data processor into a state wherein the at least one bit of the bus is driven to an electrical value, the electrical value being determined by the bus grant state of the data processor.

A system comprising:

• an execution unit for executing computer instructions, the execution unit having a normal mode of operation and a low-power mode of operation, the low-power mode of operation is entered by broadcasting a low-power request external to the execution unit and waiting for the execution unit to receive permission to enter the low-power mode of operation from external to the execution unit;
• a bus coupled to the means for executing computer instructions, the bus having at least one conductor for communicating information within the system, the at least one conductor being used to transmit the low-power request;
• and
• a bus controller coupled to the means for executing computer instructions through the bus, the bus controller granting the means for executing computer instructions selective access to the bus, the bus controller receiving the low-power request from the bus and sending the permission to enter the low-power mode of operation to the means for executing computer instructions, a time between the receipt of the low-power request and the sending of the permission being variable depending upon an amount of low-power mode preparation that is performed by the bus controller in order to prepare the system for the means for executing entering the low-power mode of operation.

A method for allowing a processor to enter a low-power mode of operation comprising:

• sending data external to the processor using a bus coupled to the processor, the data being used to indicate that the low-power mode of operation is requested;
• placing the processor into a mode wherein the processor waits for receipt of an externally-generated bus cycle termination control signal which signifies that the data has been processed, the time between the sending of the data and the receipt of the externally-generated bus cycle termination control signal being variable;
• and
• entering the low-power mode of operation in response to the externally-generated bus cycle termination control signal.

A method for placing a system into a low-power mode of operation, the system having a processor and a bus controller, the method comprising:

• informing the bus controller that the processor desires to enter a low-power mode of operation;
• generating a permission control signal in the bus controller and transmitting the permission control signal to the processor, the permission control signal indicating that the processor may enter the low-power mode of operation;
• placing the processor into a low power mode of operation;
• informing the bus controller that the processor has entered the low power mode of operation;
• and
• placing at least one peripheral into a low-power mode of operation in response to the processor being placed into the low-power mode of operation.