Introduction

Periodic advertising is a Bluetooth 5.0 feature based on extended advertisements. It allows non-connectable advertisements to be sent at fixed interval of time where advertising data can change between those intervals. One or more scanners can then listen for such advertisements. It is a form of multicast.

Some points to know about Periodic Advertisements:

1. Although extended advertiser does not need to be enabled when periodic advertising is enabled, the only way to get the pointer to the Periodic Advertisement train is through the SyncInfo field of the extended advertiser hence starting extended advertisement is mandatory.

2. At least one advertisement packet needs to be sent for periodic advertisements to be enabled.

3. It will use the same PHY as the auxiliary packet. Each periodic advertiser has same parameters as in connection. Additionally, the channel is also determined in the same way as in connection using the Channel Selection Algorithm #2.

Since it is based on extended advertisements, it uses data channels as opposed to advertisements channels. The overall process is shown in this figure.

Periodic advertisement modes

• Advertiser

  Advertising device is sending periodic advertisement train.

   

  

   

• Scanner

  User will obtain address and SID (advertising set ID) values by scanning. Device will synchronize with periodic advertisement train with user specifies address and SID. Synchronization will only happen when scanning is enabled. When device has received at least one AUX_SYNC_IND packet it is in synchronized mode. Device will then report received AUX_SYNC_IND packets. After device has synchronized, scanning can be stopped.

   

  

   

Note: Periodic advertising is a new feature; it's not yet implemented in Phones yet. Hence, periodic advertisement packets would not be visible in mobile platforms and in Bluetooth scanning apps.

Periodic advertisement mode is indicated with the ADV_EXT_IND packets, which point to AUX_ADV_IND packet, containing the actual information about the periodic advertisement mode, such as interval, hopping sequence, advertiser address, and so on. The advertiser will also send AUX_SYNC_IND packets at the identified interval containing the actual periodic advertisement data. The AUX_SYNC_IND along with AUX_CHAIN_IND make up a sequence of advertisements forming a periodic advertisement train. The advertiser will periodically send new AUX_ADV_IND packets, so that new scanners can synchronize to the data stream or existing scanners can resume a lost sync. The Bluetooth stack sends AUX_ADV_IND packet before every periodic advertisement so that new scanners will be able to synchronize quickly.

To add periodic advertising functionality to your application use the commands listed below. Refer the sample application attached to this document for implementation details. For details of each command please refer to the Bluetooth Software API Reference Manual.

Advertiser

max_advertisers in the Bluetooth configuration structure also configures the maximum number of periodic advertisers. Usually the structure is found in main.c file.

#else
  .sleep.flags = 0,
#endif // LFXO
  .bluetooth.max_connections = MAX_CONNECTIONS,        /* Maximum number of simultaneous connections */
  .bluetooth.max_advertisers = MAX_ADVERTISERS,        /* Maximum number of advertisement sets */
  .bluetooth.heap = bluetooth_stack_heap,              /* Bluetooth stack memory for connection management */

The MAX_ADVERTISERS can be overwritten in app.c using

void appMain(gecko_configuration_t *pconfig)
{
#if DISABLE_SLEEP > 0
  pconfig->sleep.flags = 0;
#endif
​
  /* Set maximum number of periodic advertisers */
  pconfig->bluetooth.max_advertisers = 1;

Feature Initialization

On the advertiser side, you can initiate using the periodic advertisement using this command. Please note this command should be called after gecko_init().

 gecko_init_periodic_advertising()

Enable/Disable Periodic advertising

This command can be used to start periodic advertisement on the given advertisement set. The stack will enable the advertisement set automatically if the set has not been enabled and the set can advertise using extended advertising PDUs.

// Enable periodic advertisement
gecko_cmd_le_gap_start_periodic_advertising(uint8 handle,
                                              uint16 interval_min,
                                              uint16 interval_max,
                                              uint32 flags)
​
// Disable
gecko_cmd_le_gap_stop_periodic_advertising(uint8 handle)

Set Periodic advertisement data

This command sets the pointers for the data to be sent in Periodic advertisement. Make sure to execute this command every time you want to change the periodic advertisement data. If periodic advertising is currently disabled for the specified advertising set, the data shall be kept by the Controller and used once periodic advertising is enabled for that set. The data shall be discarded when the advertising set is removed.

gecko_cmd_le_gap_bt5_set_adv_data(uint8 handle,
                                    uint8 scan_rsp,
                                    uint8 adv_data_len,
                                    const uint8 *adv_data_data)

The value of scan_rsp for Periodic advertisements is 8.

The maximum data length is 191 bytes for extended advertising.

Periodic advertisement has the highest priority in the current implementation (BLE SDK 2.11.1.0). For collisions, Periodic advertisements is always overriding other advertisements. If you see collisions, try increasing the interval of advertisements.

Scanner

max_periodic_sync in the Bluetooth config is used to configure the maximum number of synchronizations the Bluetooth stack needs to support. Usually the config is mentioned in main.c however, MAX_PERIODIC_SYNC can be overwritten in app.c using

void appMain(gecko_configuration_t *pconfig)
{
#if DISABLE_SLEEP > 0
  pconfig->sleep.flags = 0;
#endif
​
  /* Set maximum number of periodic advertisers */
  pconfig->bluetooth.max_periodic_sync = 1;

Feature Initialization

To enable Periodic Advertisement Synchronization in the Bluetooth stack, the following initialization function must be called after the generic gecko_init function

void gecko_bgapi_class_sync_init()

Enable extended scan response

This command can be used to enable or disable the extended scan response event.

gecko_cmd_le_gap_set_discovery_extended_scan_response(uint8 enable)

Set it 0 to disable and 1 to enable.

Establish synchronization

This command can be used to establish a synchronization with periodic advertisement from the specified advertiser and begin receiving periodic advertisement packets. More details here.

gecko_cmd_sync_open(uint8 adv_sid,
                      uint16 skip, uint16 timeout,
                      bd_addr address,
                      uint8 address_type)

The adv_sid parameter specifies the value that must match the Advertising SID subfield in the ADI (Advertise Data Info) field of the received advertisement for it to be used to synchronize. It can be obtained from the event gecko_evt_le_gap_extended_scan_response_id

evt->data.evt_le_gap_extended_scan_response.adv_sid

The skip parameter specifies the maximum number of consecutive periodic advertisement events that the receiver may skip after successfully receiving a periodic advertisement packet. The timeout parameter specifies the maximum permitted time between successful receives. If this time is exceeded, synchronization is lost.

The advertiser address_type, and advertiser address parameters specify the periodic advertising device to listen to. It can be obtained from the event gecko_evt_le_gap_extended_scan_response_id

evt->data.evt_le_gap_extended_scan_response.address,
evt->data.evt_le_gap_extended_scan_response.address_type

Successfully synchronization leads to the generation of this event:

gecko_evt_sync_opened_id

Whenever a periodic advertisement packet is received, this event is generated:

gecko_evt_sync_data_id

Close synchronization

This command can be used to close a synchronization with periodic advertisement or cancel an ongoing synchronization establishment procedure.

gecko_cmd_sync_close(uint8 sync)

where sync is the Periodic advertisement synchronization handle.

When the synchronization is closed, lost or when an ongoing sync process is cancelled this event is generated:

gecko_evt_sync_closed_id

Test setup

We will use 2 devices (WSTK radio boards) to demonstrate this feature. This feature works with any Silicon Labs radio chip which supports Bluetooth 5. This functionality outlined in this KBA was implemented using

Advertiser: BGM13S22

Create an SoC - Empty example for your radio board and overwrite the contents of app.c file with the contents of advertiser_app.c file attached to this KBA. This program generates a sequence of random numbers every second as the data for periodic advertisement. This data is also visible on the console.

Scanner: BGM13P22

Create an SoC - Empty example for your radio board and overwrite the contents of app.c file with the contents of scanner_app.c file attached to this KBA. This program looks for the synchronizeSync service in the scanned advertisement data.

Set DEBUG_LEVEL to 1 in app.h to enable printing on the console on both devices. Observe the data broadcasting on the advertiser and being reflected on the scanner.

Observations

Use the energy profiler in Simplicity studio to evaluate the current consumption. The scanner goes into energy saving mode and wakes up every 200ms to receive sync packets from the advertiser. The advertiser sleeps when not advertising as shown in the figure below.

Code assembly instructions:

As mentioned above, we will use 2 devices (WSTK radio boards) to demonstrate this feature.

Advertiser: Create an SoC - Empty example for your radio board and overwrite the contents of app.c file with the contents of advertiser_app.c file attached to this KBA. In the .isc file click Custom BLE GATT Profile and click import icon on the right to import the attached gatt.xml file. This program generates a sequence of random numbers every second as the data for periodic advertisement. This data is also visible on the console.

Scanner: Create an SoC - Empty example for your radio board and overwrite the contents of app.c file with the contents of scanner_app.c file attached to this KBA. This program looks for the Health Thermometer service in the scanned advertisement data.

Set DEBUG_LEVEL to 1 in app.h to enable printing on the console on both devices. Observe the data broadcasting on the advertiser and being reflected on the scanner.