CAF: Bluetooth LE advertising module

Configuration
Implementation details
- Reaction on Bluetooth peer operation
- Avoiding connection requests from unbonded centrals when bonded

When enabled for an application, the Bluetooth® LE advertising module is responsible for controlling the Bluetooth LE advertising for Bluetooth LE Peripheral device.

This module can only work together with the CAF Bluetooth LE state module. The Bluetooth LE state module is a core Bluetooth module in the Common Application Framework (CAF).

Configuration 

The following Kconfig options are available for this module:

Read about some of these options in the following sections.

Enabling the module 

To enable the Bluetooth® LE advertising module, complete the following steps:

Enable and configure the CAF Bluetooth LE state module.
Define the Bluetooth device name using the CONFIG_BT_DEVICE_NAME Kconfig option.
Define the Bluetooth device appearance using the CONFIG_BT_DEVICE_APPEARANCE Kconfig option.
Set the CONFIG_CAF_BLE_ADV Kconfig option.
Create a configuration file that defines Bluetooth LE advertising data.

In the configuration file, define the following arrays of bt_data:

ad_unbonded - Defines undirected advertising data when unbonded.
ad_bonded- Defines undirected advertising data when bonded.
sd- Defines scan response data.

For example, the file contents should look like follows:

#include <zephyr.h>
#include <bluetooth/bluetooth.h>

static const struct bt_data ad_unbonded[] = {
      BT_DATA_BYTES(BT_DATA_FLAGS, (BT_LE_AD_GENERAL | BT_LE_AD_NO_BREDR)),
      BT_DATA_BYTES(BT_DATA_UUID16_ALL,
                    0x0f, 0x18,          /* Battery Service */
      ),
};

static const struct bt_data ad_bonded[] = {
      BT_DATA_BYTES(BT_DATA_FLAGS, BT_LE_AD_NO_BREDR),
      BT_DATA_BYTES(BT_DATA_UUID16_ALL,
                    0x0f, 0x18,   /* Battery Service */
      ),
};

static const struct bt_data sd[] = {};

Specify the path to the configuration file with the CONFIG_CAF_BLE_ADV_DEF_PATH Kconfig option.

Note

The configuration file should be included only by the configured module. Do not include the configuration file in other source files.

By default, the module uses indirect advertising. Set the CONFIG_CAF_BLE_ADV_DIRECT_ADV option to use directed advertising. The directed advertising can be used to call the selected peer device to connect as quickly as possible.

Note

The module will not advertise directly towards a Central that uses Resolvable Private Address (RPA). The Bluetooth LE Peripheral does not read the Central Address Resolution GATT characteristic of the Bluetooth LE Central, so the Peripheral does not know if the remote device supports the address resolution of directed advertisements.

Changing advertising interval 

Set the CONFIG_CAF_BLE_ADV_FAST_ADV Kconfig option to make the Peripheral initially advertise with a shorter interval. This lets you speed up finding the Peripheral by Bluetooth Centrals.

If the device uses indirect advertising, it will switch to slower advertising after the period of time defined in CONFIG_CAF_BLE_ADV_FAST_ADV_TIMEOUT (in seconds).
If the device uses directed advertising, the Bluetooth® LE advertising module will receive ble_peer_event with ble_peer_event.state set to PEER_STATE_CONN_FAILED if the Central does not connect during the predefined period of fast directed advertising. The ble_peer_event is submitted by CAF: Bluetooth LE state module. After the event is received, the device will switch to the low duty cycle directed advertising.

Switching to slower advertising is done to reduce the energy consumption.

Using Swift Pair 

You can use the CONFIG_CAF_BLE_ADV_SWIFT_PAIR option to enable the Swift Pair feature. The feature simplifies pairing the Bluetooth Peripheral with Windows 10 hosts.

Note

Make sure to add the Swift Pair data to advertising packets for unbonded device in the configuration file if you enable CONFIG_CAF_BLE_ADV_SWIFT_PAIR option. The Swift Pair data must be added as the last member of ad_unbonded array.

Power-down 

When the system goes to the Power-down state, the advertising stops.

If the Swift Pair feature is enabled with CONFIG_CAF_BLE_ADV_SWIFT_PAIR, the device advertises without the Swift Pair data for additional CONFIG_CAF_BLE_ADV_SWIFT_PAIR_GRACE_PERIOD seconds to ensure that the user does not try to connect to the device that is no longer available.

Implementation details 

The Bluetooth® LE advertising module is used only by Bluetooth Peripheral devices.

The Bluetooth® LE advertising module uses Zephyr’s Settings to store the information if the peer for the given local identity uses the Resolvable Private Address (RPA).

Reaction on Bluetooth peer operation 

If the application supports Bluetooth LE bond management (CONFIG_CAF_BLE_BOND_SUPPORTED), the Bluetooth LE bond module defined for the application is used to control the Bluetooth bonds. The Bluetooth LE bond module broadcasts information related to bond control using ble_peer_operation_event.

The Bluetooth® LE advertising module reacts on ble_peer_operation_event related to the Bluetooth peer change or erase advertising. The module performs one of the following operations:

If there is a peer connected over Bluetooth, the Bluetooth® LE advertising module triggers disconnection and submits a ble_peer_event with ble_peer_event.state set to PEER_STATE_DISCONNECTING to let other application modules prepare for the planned disconnection.
Otherwise, the Bluetooth advertising with the newly selected Bluetooth local identity is started.

Avoiding connection requests from unbonded centrals when bonded 

If CONFIG_BT_FILTER_ACCEPT_LIST is enabled and Bluetooth local identity that is in use already has a bond, the device will mark incoming scan response data requests and connection requests as acceptable. This is done to prevent Bluetooth Centrals other than the bonded one from connecting with the device.