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Chrome OS EC Charge Manager

cros-ec boards with USB-C / USB-PD support have several independent charge-related tasks, controlling several charge ports. These tasks use different protocols (PD, BC1.2, type-C) to determine the voltage and input current limit of an attached charger. Charge manager is kept up to date with these current limits, and using its selection logic, chooses the active charge port (there can be only one) and its current limit.

charge_manager_update() is the basic function tasks call to inform charge manager of a power supply change.

Port / limit selection is handled by a deferred task charge_manager_refresh(). This task is queued every time a change occurs that may change port or current limit selection.

Port Selection Logic

  1. Prefer override port over all others.
  2. Prefer higher priority supplier (ex. PD over USB-C over BC1.2) over lower-priority.
  3. Prefer a charger capable of supplying more power if supplier priority is tied between ports.
  4. Prefer the current charge port (don’t switch) in case the above are all tied.

Extra Selection Notes

Charge Override

Override is set at the port level -- charge manager will ignore its normal port selection logic and choose the override port as its active charge port. Also, override supports a “don’t charge” parameter, where no port will be selected.

Override is automatically removed when a dedicated charger is plugged, or when the charger is removed from the override port.

Delayed Override

If a dual-role device is chosen as the charge supplier via override, it may be required to perform a power swap so that the port can switch from a source to a sink role. The power swap takes time, and is not guaranteed to complete -- this is known as a delayed override. Once the swap takes effect and the dual-role device begins supplying power, the requested port will become the override port. If the device never begins supplying power, the requested port will not become the override port.

If the device doesn’t supply power within a timeout period (based upon worst-case PD protocol timings), we abandon our delayed override.

Charge Ceiling

Charge manager selects the active port based upon the reported charge limit of the attached supply. Sometimes, we may want to limit the amount of current that we draw from a supply (ex. if we’re in the processes of negotiating to a higher current limit) without influencing charge manager’s port selection. charge_manager_set_ceil is used to set an artificial limit in this case. The “charge ceiling” will be applied when assigning a current limit, but the charge supplier limit will be used for port selection.

New Power Request

pd_set_new_power_request() is called with a port parameter when a port was previously active and becomes non-active, or when a port was previously non-active and becomes active. The PD task may take actions based upon port selection, for example, switching to a higher-power PDO.

Rejected Port

The callback function board_set_active_charge_port() returns a value that indicates whether the port change was accepted or not. A port may be rejected in certain cases, in which case the “next best” port will be selected as active by charge manager.

Safe Mode

To accommodate boot with low, missing or software-disconnected batteries, it's often necessary to relax port selection and ILIM restrictions to prevent device brown-out. On power-up, charge manager starts in safe mode, with the following behavior changes:

  1. All chargers are considered dedicated (and thus are valid charge source candidates) for the purpose of port selection.
  2. Charge ceilings are ignored. Most significantly, ILIM won't drop on PD voltage transition.
  3. CHARGE_PORT_NONE will not be selected. Power-on default charge port will remain selected rather than CHARGE_PORT_NONE.

After leaving safe mode, charge_manager reverts to its normal behavior and immediately selects charge port and current using standard rules.



Unit tests:

Basic functionality:

charge_manager_update_charge() - Tell charge manager about a new charge source.

charge_manager_update_dualrole() - Tell charge manager about a change to the dualrole capability of the port partner.

charge_manager_set_ceil() - Tell charge manager to set an artificial charge limit on a port, despite its capabilities.

charge_manager_force_ceil() - A specialized version of charge_manager_set_ceil() that causes the charge limit to go into effect immediately. It’s used only in rare cases when we don’t have time to wait for the normal charge_manager_refresh() to pick up the update.

charge_manager_set_override() - Tell charge manager to override its normal port selection logic and charge from a given port, or don’t charge from any ports.

charge_manager_leave_safe_mode() - Leave safe mode (typically called when battery is confirmed to supply sufficient power) and use standard port / ILIM selection rules.

State checking:

charge_manager_get_override() - Returns current override status, see charge_manager_set_override().

charge_manager_get_active_charge_port() - Returns current active charge port.

charge_manager_get_power_limit_uw() - Returns the power limit selected by charge manager

charge_manager_get_charger_current() - Returns the ILIM selected by charge manager


charge_manager_save_log() - Writes a lot entry containing charge_manager state data.

Source port control:

charge_manager_source_port() - Inform charge_manager that a port has become a PD source, or is no longer a PD source, for controlling source current limit.

charge_manager_get_source_pdo() - Returns PDO index of the source cap to send, given saved information about how many ports are currently sources.


board_set_active_charge_port() - Called when active charge port changes.

board_set_charge_limit() - Called when active charge limit changes.

pd_set_new_power_request() - Called when a port becomes active or becomes non-active.