Bluetooth LE latency module

Use the Bluetooth® LE latency module to:

• Lower the Bluetooth LE connection latency either when Configuration channel is in use or when a firmware update is received by the Simple Management Protocol module (low latency ensures quick data exchange).

• Request setting the initial connection parameters for a new Bluetooth connection.

• Keep the connection latency low for the LLPM (Low Latency Packet Mode) connections to improve performance.

• Disconnect the Bluetooth Central if the connection has not been secured in the predefined amount of time after the connection occurred.

Module events

Source Module	Input Event	This Module	Output Event	Sink Module
Source modules for config_event	config_event	ble_latency
Bluetooth LE state module	ble_peer_conn_params_event
Bluetooth LE advertising module	ble_peer_event
Bluetooth LE state module
Simple Management Protocol module	ble_smp_transfer_event
Source modules for module_state_event	module_state_event
Power manager module	power_down_event
Bluetooth LE advertising module	wake_up_event
Buttons module
HID forward module
Motion module
Power manager module
Wheel module
			module_state_event	Sink modules for module_state_event

Note

See the Overview: Firmware architecture for more information about the event-based communication in the nRF Desktop application and about how to read this table.

Configuration

The module requires the basic Bluetooth configuration, as described in Bluetooth in nRF Desktop. The module is enabled for every nRF Desktop peripheral device.

You can use the option CONFIG_DESKTOP_BLE_SECURITY_FAIL_TIMEOUT_S to define the maximum allowed time for establishing the connection security. If the connection is not secured during this period of time, the peripheral device disconnects.

You can set the option CONFIG_DESKTOP_BLE_LOW_LATENCY_LOCK to keep the connection latency low for the LLPM connections. This speeds up sending the first HID report after not sending a report for some connection intervals. Enabling this option increases the power consumption - the connection latency is kept low unless the device is in the low power mode.

Implementation details

The Bluetooth® LE latency module uses delayed works (k_work_delayable) to control the connection latency and trigger the security timeout.

Note

The module does not request an increase in the connection latency until the connection is secured. Increasing the slave latency can significantly increase the amount of time required to establish the Bluetooth connection security level on some hosts.

The module listens for the following events related to data transfer initiated by the connected Bluetooth central:

• config_event - This event is received when the Configuration channel is in use.

• ble_smp_transfer_event - This event is received when the firmware update is received by Simple Management Protocol module.

When these events are received, the module sets the connection latency to low. When the Configuration channel is no longer in use and the firmware update is not received by Simple Management Protocol module (no mentioned events for LOW_LATENCY_CHECK_PERIOD_MS), the module sets the connection latency to CONFIG_BT_PERIPHERAL_PREF_LATENCY to reduce the power consumption.

Note

If the option CONFIG_DESKTOP_BLE_LOW_LATENCY_LOCK is enabled, the LLPM connection latency is not increased unless the device is in the low power mode.

When the device is in the low power mode and the events related to data transfer are not received, the connection latency is set to higher value to reduce the power consumption.

The ble_latency module receives Configuration channel events, but it is not configurable with the Configuration channel. The module does not register itself using the GEN_CONFIG_EVENT_HANDLERS macro.

Note

Zephyr’s Bluetooth API does not allow to use the LLPM connection intervals in the connection parameter update request. If the LLPM connection interval is in use:

• The nRF Desktop peripheral uses a 7.5-ms interval in the request.

• The nRF Desktop central ignores the requested connection interval, and only the connection latency is updated.

For more detailed information, see the Bluetooth LE connection parameters module documentation page.