# Internal Design

## Key classes

- PowerInterface
  - Defines the `org.openbmc.control.Power` D-Bus interface.
  - The `state` property is set to power the chassis on or off. This contains
    the desired power state.
  - The `pgood` property contains the actual power state of the chassis.
- PowerControl
  - Created in `main()`. Handles the event loop.
  - Sub-class of PowerInterface that provides a concrete implementation of the
    `org.openbmc.control.Power` D-Bus interface.
  - Finds and loads the JSON configuration file.
  - Finds power sequencer device information.
  - Creates a sub-class of PowerSequencerDevice that matches power sequencer
    device information.
  - Powers the chassis on and off using the `power-chassis-control` named GPIO.
  - Monitors the chassis pgood status every 3 seconds using the
    `power-chassis-good` named GPIO.
  - Enforces a minimum power off time of 15 seconds from cold start and 25
    seconds from power off.
- DeviceFinder
  - Finds power sequencer device information on D-Bus published by
    EntityManager.
- Rail
  - A voltage rail that is enabled or monitored by the power sequencer device.
- PowerSequencerDevice
  - Abstract base class for a power sequencer device.
  - Defines virtual methods that must be implemented by all child classes.
- StandardDevice
  - Sub-class of PowerSequencerDevice that implements the standard pgood fault
    detection algorithm.
- PMBusDriverDevice
  - Sub-class of StandardDevice for power sequencer devices that are bound to a
    PMBus device driver.
- UCD90xDevice
  - Sub-class of PMBusDriverDevice for the UCD90X family of power sequencer
    devices.
- UCD90160Device
  - Sub-class of UCD90xDevice representing a UCD90160 power sequencer device.
- UCD90320Device
  - Sub-class of UCD90xDevice representing a UCD90320 power sequencer device.
- Services
  - Abstract base class that provides an interface to system services like error
    logging and the journal.
- BMCServices
  - Sub-class of Services with real implementation of methods.
- MockServices
  - Sub-class of Services with mock implementation of methods for automated
    testing.