nrf52840-DK Central nRF Connect SDK 1.8 with BT_SECURITY_L4 connection dropped

Question

Hi, 
 I am using the nrf52840-DK with the nRF Connect SDK version 1.8. 
 I have modified the central_multilink sample to connect to a BLE peripheral which requires a secure connection. 
 I request BT_SECURITY_L4 via a call to bt_conn_set_security ( conn , BT_SECURITY_L4) in the 'connected' callback handler. 
 The trace I have captured in Wireshark suggests that after I have established a connection using a passkey, there is some additional pairing traffic, suggesting that some negotiation (for the secure connection) or similar is occurring, but is not completing... 
 
 The initial connection looks similar to a successful attempt (e.g. from an Android phone), but shows limited 'keys', as follows: 
 
 A connection from an android phone shows additional 'keys' (and securely connects). I am not clear whether this is the problem, or how I can gain control of this via the nRF Connect SDK

Kenneth · Answer

When establish connection from the nRF52840, can you lower the connection interval? I can see that nRF52840 establish connection with 200ms interval, while Android establish connection with 30ms. Reason for asking: 
 Android does 82 of these packets in 7.2seconds->succeed. nRF52840 does 66 packets in 14.2seconds->Silabs terminate. 
 Edit: 
 Our softdevice controller team have tried to replicate the issue, but they are not able to based on the provided info. Are you able to provide some more details on how to reproduce it? Their initial thought is that there is some problem when calling bt_conn_set_security(conn, BT_SECURITY_L4) function from the connected callback, maybe try to look into this. 
 Edit2: 
 Since you are doing LESC with passkey (BLUETOOTH CORE SPECIFICATION Version 5.3 | Vol 3, Part H page 1582). It supposed to be 20*4 packets, 4 packets per passkey bit. Protocol should abort if confirm value for subsequent bit does not match. Maybe the passkey is wrong somehow? https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/zephyr/reference/bluetooth/connection_mgmt.html?highlight=bt_conn_auth_cb#c.bt_conn_auth_cb 
 For instance is the passkey zero-padded? https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/zephyr/reference/bluetooth/connection_mgmt.html?highlight=bt_conn_auth_cb#c.bt_conn_auth_cb.passkey_entry

Paul Clark · Answer

The passkey is correct - it is a 6 digit code with no zeros anyway. If I deliberately set a wrong passkey, this is detected/handled. 
 The connection interval appears to be the issue ! I lowered the interval from 
 
 0x00A0 to 0xx0050 and the connection succeeded. 
 
 Many thanks - and well diagnosed!

nrf52840-DK Central nRF Connect SDK 1.8 with BT_SECURITY_L4 connection dropped

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