HID Service module

Use the HID Service module to handle the GATT Human Interface Device Service. The GATT Service is used to exchange HID data with the peer connected over Bluetooth®. For this reason, it is mandatory for every nRF Desktop peripheral.

Module events

Source Module	Input Event	This Module	Output Event	Sink Module
Bluetooth LE advertising module	ble_peer_event	hids
Bluetooth LE state module
Source modules for config_event	config_event
HID Service module	hid_notification_event
HID forward module	hid_report_event
HID state module
HID Service module
USB state module
Source modules for module_state_event	module_state_event
			config_event	Sink modules for config_event
			hid_notification_event	HID Service module
			hid_report_event	Sink modules for hid_report_event
			hid_report_sent_event	HID forward module
				HID state module
				Motion module
			hid_report_subscription_event	HID forward module
				HID state module
				Motion 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

Complete the following steps to configure the module:

1. Complete the basic Bluetooth configuration, as described in Bluetooth in nRF Desktop. Make sure that both CONFIG_DESKTOP_ROLE_HID_PERIPHERAL and CONFIG_DESKTOP_BT_PERIPHERAL options are enabled. The HID Service application module is enabled by the CONFIG_DESKTOP_HIDS_ENABLE option, which is implied by CONFIG_DESKTOP_BT_PERIPHERAL together with other GATT Services that are required for a HID device.

2. The CONFIG_DESKTOP_HIDS_ENABLE option selects the following Kconfig options:

   • The CONFIG_BT_HIDS option that automatically enables the GATT Human Interface Device (HID) Service.

   • The CONFIG_BT_CONN_CTX option that automatically enables the Bluetooth connection context, which is required by the GATT HID Service.

3. The nRF Desktop application modifies the defaults of Kconfig option values, defined by the GATT Human Interface Device (HID) Service, to tailor the default configuration to application needs. The configuration is tailored for either nRF Desktop mouse (CONFIG_DESKTOP_PERIPHERAL_TYPE_MOUSE) or nRF Desktop keyboard (CONFIG_DESKTOP_PERIPHERAL_TYPE_KEYBOARD). For more details, see the src/modules/Kconfig.hids file.

   Tip

   If the HID report configuration is identical to the default configuration of either nRF Desktop mouse or keyboard, you do not need to modify the GATT HID Service configuration. Otherwise, see GATT Human Interface Device (HID) Service documentation for configuration details.

The HID Service application module forwards the information about the enabled HID notifications to other application modules using hid_report_subscription_event. These notifications are enabled by the connected Bluetooth® Central. By default, the hids application module starts forwarding the subscriptions right after the Bluetooth connection is secured.

You can define additional delay for forwarding the notifications on connection (CONFIG_DESKTOP_HIDS_FIRST_REPORT_DELAY). Sending the first HID report to the connected Bluetooth peer is delayed by this period of time.

Note

The nRF Desktop centrals perform the GATT service discovery and reenable the HID notifications on every reconnection. A HID report that is received before the subscription is reenabled will be dropped before it reaches the application. The CONFIG_DESKTOP_HIDS_FIRST_REPORT_DELAY option is set to 500 ms for nRF Desktop keyboards (CONFIG_DESKTOP_PERIPHERAL_TYPE_KEYBOARD) to make sure that the input is not lost on reconnection with the nRF Desktop dongle.

Implementation details

The HID Service application module initializes and configures the GATT HID Service. The application module registers the HID report map and every HID report that was enabled in the application configuration. Detailed information about HID-related configuration in nRF Desktop is available in the HID state module documentation.

Sending HID input reports

After subscriptions are enabled in HID state module, the HID state module sends the HID input reports as hid_report_event. The HID Service application module sends the report over Bluetooth LE and submits the hid_report_sent_event to inform that the given HID input report was sent.

HID keyboard LED output report

The module can receive an HID output report setting state of the keyboard LEDs, e.g. state of the Caps Lock. The report is received from the Bluetooth connected host. The module forwards the report using hid_report_event, that is handled either by HID state module (for peripheral) or HID forward module (for central).

Right now, the only board that displays information received in the HID output report using hardware LEDs is the nrf52840dk_nrf52840 in keyboard build type configuration. The keyboard reference design (nrf52kbd_nrf52832) has only one LED that is used to display the Bluetooth LE peer state. Detailed information about the usage of LEDs to display information about Bluetooth LE peer state and system state to the user is available in the LED state module documentation. Detailed information about displaying state of the HID keyboard LEDs using hardware LEDs is available in HID state module documentation.

Bluetooth LE connections and disconnections

The module informs the GATT HID Service about the Bluetooth LE connections and disconnections using bt_hids_connected() and bt_hids_disconnected(), respectively.

Registered handlers

The GATT HID Service uses registered handlers to send the following information to the HID Service application module:

• Enabling or disabling a HID input report notification.

• Incoming HID output or feature reports.

• Switching between the boot mode and the report mode.

HID notifications

The hid_notification_event is used to synchronize the information about enabling or disabling the HID notifications for the HID input report. The event is submitted when the GATT HID Service calls a callback related to enabling or disabling the notifications and the event is received only by the hids application module.

Transport for configuration channel

The HID Service application module works as a transport for the Configuration channel and exchanges the Configuration channel HID reports over Bluetooth LE. The module communicates with the Configuration channel listeners using config_event.