traces on central side:

initiate connection with CA A4 F4 EA DE 1B
sd_ble_gap_connect

[APPL]: BLE_GAP_EVT_CONNECTED
[APPL]: device_manager_event_handler
[APPL]: >> DM_EVT_CONNECTION (conn_handle 0)
[APPL]:[CA A4 F4 EA DE 1B]: Connection Established
Starting discovery
[APPL]: << DM_EVT_CONNECTION
db_discover_evt_handler 0
[GATTC]: gatt block service discovered at peer
Discovery complete
here central is waiting

traces on device side :
starting advertising
[APPL]: device_manager_event_handler
[APPL]: >> DM_EVT_DISCONNECTION (conn_handle 3)
[APPL]: << DM_EVT_DISCONNECTION
[APPL]: BLE_GAP_EVT_CONNECTED (conn_handle 3)
[APPL]: device_manager_event_handler
[APPL]: >> DM_EVT_CONNECTION (conn_handle 3)
[APPL]:[FD 82 F6 90 01 74]: Connection Established
[APPL]: << DM_EVT_CONNECTION
[APPL]: receiving BLE_GAP_EVT_SEC_PARAMS_REQUEST (i do nothing for this event)
here device is waiting