Minimum Requirements for Scanning Filters

[RPDben]

Hi,

I am currently trying to implement a beacon scanner alongside peripheral mode BLE on an nRF52832. As I am only scanning for beacons, I was hoping I would not need to include any code other than "nrf_ble_scan.c" and ignore the rest of the central libraries.

My code is working well, expect the filters aren't firing, even though there is definitely a match. I tested this by setting my beacon UUID to that of the NUS service and ran the "ble_nus_c" example before running my custom code. The "ble_nus_c" worked well with my beacon and correctly gave me an NRF_BLE_SCAN_EVT_FILTER_MATCH event. However, in my custom code, this event never fires. I am initialising the scanning module in exactly the same way as the example. Code Below:

static ble_uuid_t const m_beacon_uuid = { .uuid = 0x0001, .type = BLE_UUID_TYPE_VENDOR_BEGIN };

NRF_BLE_SCAN_DEF( m_scan );

static void scan_evt_handler( scan_evt_t const* p_scan_evt )
{
 ret_code_t err_code;

switch( p_scan_evt->scan_evt_id )
 {
 case NRF_BLE_SCAN_EVT_SCAN_TIMEOUT:
 {
 NRF_LOG_INFO( "Scan timed out." );
 scan_start( );
 }
 break;

case NRF_BLE_SCAN_EVT_FILTER_MATCH:
 {
 NRF_LOG_INFO( "Filter Match" );
 } break;

case NRF_BLE_SCAN_EVT_NOT_FOUND:
 {
 NRF_LOG_INFO( "%02X, %02X, %02X, %02X, %02X, %02X",
 p_scan_evt->params.p_not_found->peer_addr.addr[0],
 p_scan_evt->params.p_not_found->peer_addr.addr[1],
 p_scan_evt->params.p_not_found->peer_addr.addr[2],
 p_scan_evt->params.p_not_found->peer_addr.addr[3],
 p_scan_evt->params.p_not_found->peer_addr.addr[4],
 p_scan_evt->params.p_not_found->peer_addr.addr[5] );
 }break;

default:
 break;
 }
}

static void scan_init( void )
{
 ret_code_t err_code;
 nrf_ble_scan_init_t init_scan;

memset( &init_scan, 0, sizeof( init_scan ) );

init_scan.connect_if_match = false;

err_code = nrf_ble_scan_init( &m_scan, &init_scan, scan_evt_handler );
 APP_ERROR_CHECK( err_code );

err_code = nrf_ble_scan_filter_set( &m_scan, SCAN_UUID_FILTER, &m_beacon_uuid );
 APP_ERROR_CHECK( err_code );

err_code = nrf_ble_scan_filters_enable( &m_scan, NRF_BLE_SCAN_UUID_FILTER, false );
 APP_ERROR_CHECK( err_code );
}

static void scan_start( void )
{
 ret_code_t ret;

ret = nrf_ble_scan_start( &m_scan );
 APP_ERROR_CHECK( ret );
}

Does the "nrf_ble_scan" module also require "db_discovery" or similar to be initialised in order to allow these filters to work?

Thanks!

EDIT: To clarify, the scanning module correctly starts scanning, and using the NRF_LOG_INFO() in the BLE_SCAN_EVT_NOT_FOUND event I can see my central is scanning and finding devices, including the one that should cause a filter match, but the filter is never returning as true

[Amanda Hsieh]

Hi Ben, 

The NRF_BLE_SCAN_EVT_FILTER_MATCH event is given in the nrf_ble_scan.c. You should debug the code to find out the reason why it didn't get.   

-Amanda H.

[RPDben]

Hi Amanda,

What do you mean by this? I have put a breakpoint on this part but it is never hit. Where should I be debugging the code? I am struggling as there are many devices discovered by the central so it is hard to pinpoint where I should put a breakpoint

Thanks,

[Amanda Hsieh]

Hi Ben, 

These filters are intended to decide what device to connect to. You can see how they are used in nrf_ble_scan.c, in the nrf_ble_scan_on_adv_report() function.

The softdevice doesn't filter based on these filter settings. It is up to the application (nrf_ble_scan.c). Depending on whether all_filter_mode is true or false, you need to match all the set filters, or only one in order to trigger the NRF_BLE_SCAN_EVT_FILTER_MATCH event or not. 

It will also call nrf_ble_scan_connect_with_target() which will connect to the target in case of NRF_BLE_SCAN_EVT_FILTER_MATCH if .connect_if_match is true.

So the assumption that the softdevice will filter out the advertisements and only pass the filter matched advertisements to BLE_GAP_EVT_ADV_REPORT event is not true. All advertisements will be forwarded to BLE_GAP_EVT_ADV_REPORT, but only the filter matches will be passed onto the NRF_BLE_SCAN_EVT_FILTER_MATCH event.

Is your code able to get inside the nrf_ble_scan_on_adv_report()? Does it match all the set filters? 

-Amanda H.