The next figure illustrates the second topology, which makes use of a battery charge controller and fuel gauge component connected directly to the platform's core silicon over a lightweight peripheral bus such as IÂC. In this configuration, Windows queries and is notified about power subsystem changes through communications over the IÂC bus. Instead of using a device driver for the battery or charging subsystem, the ACPI control method environment is extended with support for a Simple Peripheral (SPB) Operation Region. The SPB operation region allows the ACPI control method code to communicate to the battery charge controller and fuel gauge components connected to the core silicon over IÂC.
All PCs are expected to expose the batteries and charging subsystem through the ACPI control method interface. The battery miniport interface should not be used for platform-specific battery charging subsystems. There are ACPI specification-defined control methods that allow Windows to poll for battery information and status. Similarly, there is an event-driven model to allow the hardware platform to notify Windows of battery and power source changes, such as a transition from AC to battery power.
The Windows power manager periodically requests status information from the battery including the charge capacity remaining and the current rate of drain. This request originates in the power manager, a higher-level user interface component, or application. The power manager turns the request into an I/O Request Packet (IRP) to the battery device(s). When the battery is exposed through the ACPI control method interface, the control-method battery driver (cmbatt.sys) executes the appropriate ACPI control methods. In the case of status information, the _BST (battery status) method is executed.
The _BST method requires the ACPI firmware to obtain current information from the power subsystem and then packages that information in a buffer with format specified by the ACPI specification. The specific code required to access the battery status either from the embedded controller or the battery charger connected through IÂC is contained within the ACPI firmware and part of the code comprising the _BST method. The net result of the _BST method is the buffer of information required which is returned to the control-method battery driver. The control method battery driver finally converts the buffer to the format required by the battery driver and Windows power manager.
When the battery status changes, including remaining capacity or charging status, the ACPI firmware issues a Notify(0x80) on the control method battery device. The Windows control method battery driver then evaluates the _BST method and returns the updated information to the power manager.
When the battery static data changes, including last full charge capacity, design capacity and cycle count, the ACPI firmware issues a Notify(0x81) on the control method battery device. The Windows control method battery driver then evaluates the _BIX method and returns the updated information to the power manager.
Platforms that have their battery and power subsystem connected to the typical embedded controller use the ACPI Embedded Controller operation region to facilitate communications between the ACPI control method environment and the platform hardware.
When the embedded controller detects a change in battery state, including a change in charging state or remaining capacity as specified by _BTP, the embedded controller generates an SCI and sets the SCI_EVT bit in the embedded controller status command (EC_SC) register. The Windows ACPI driver will communicate with the embedded controller and issue a query command (QR_EC) to request specific information about the notification to be issued. The embedded controller then sets a byte value corresponding to the _QXX method to be executed. For example, the embedded controller and ACPI firmware can define value 0x33 to be an update to the battery status information. When the embedded controller sets the value 0x33 as the notification, the ACPI driver will execute the _QXX method. The contents of the _QXX method will typically be a Notify(0x80) on the control method battery device in the namespace.
Platforms may also connect their battery and power subsystem connected to the core chipset through a low-power serial bus such as IÂC. In these designs, the ACPI GenericSerialBus operation region is used to communicate between the ACPI control methods and battery subsystem hardware. Connecting the battery subsystem hardware to a GPIO interrupt allows the ACPI control methods to be executed when a battery status changes.
Describing the GPIO controller and mapping of GPIO lines to ACPI events allows the control methods for battery status and notification to be executed when a GPIO interrupt from an IÂC device is raised. Describing the GenericSerialBus operation region allows the ACPI code for battery status to communicate over the IÂC bus and read registers and information from the battery fuel gauge and charging subsystem.
The battery status can be executed by ACPI control methods by sending and receiving commands over the IÂC bus to which the battery subsystem hardware is connected. The control method code backing the status and battery static information methods reads and writes data from the GenericSerialBus operation regions described in the ACPI namespace. The control method code reads the data from the fuel gauge device or static information about the battery capacity and cycle counts over the IÂC bus through the GenericSerialBus operation region.
An interrupt can be generated by the battery subsystem hardware when the state changes and the interrupt is physically connected to a GPIO line on the core silicon. The GPIO line can be mapped to a specific control method execution using the _AEI object under the GPIO controller described in ACPI. When the GPIO interrupt occurs, the Windows ACPI subsystem runs the method associated with the specific GPIO line which can in turn do a Notify() on the control method battery device, causing Windows to re-evaluate the status and static information methods to update the battery status.
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