nRF52840 random disconnection

[Dani]

Good morning,

We have a new equipment based on the nRF52840 and s140.

                Just now, a new problem has appeared and it is very strange, as it seems to be a ‘random’ error:

 

                We have already assembled tens of equipment, and most of them are perfectly working with any smartphone (using both your ‘nRF Connect’ application or our own application).

                But, we have a few smartphones that show a strange behavior: they are able to perfectly connect to some of our nRF52840-based equipment, and in a few ones, they are able to connect but they get disconnected after a few seconds. Same equipment is connecting to other smartphones without problems (without disconnecting).

                This is (step by step) what it is happening a few times:

• Equipment (nRF52840) is advertising.
• Smartphone stablish connection with equipment using pin number
• First time, the connection is perfectly working.
• If smartphone is disconnected and connected back again with same equipment (Already bonded), then connection is stablished but smartphone disconnects after 30”, approximately. Same behavior is happening with both nRF connect application and our own application. Looking at log in nRF application we see: ‘Connection terminated by peer (status 19)’or ‘GATT_CONN_TIMEOUT’

 

Why is it happening?

Which is the reason?

How can we solve this?

 

 Could you help us? (For your information, we have set MIN_CONN_INTERVAL to 7.5m and MAX_CONN_INTERVAL to 100ms)

Thank you

[Hung Bui]

Hi Dani, 

Please try to debug. Have you made sure you change the configuration of the characteristic to use BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITM  ?

I can see that CCCD has been written but after that there were no activity from the device. 
Please check why the device doesn't send the indication. 

[Dani]

Hi Hung,

The configuration of the characteristic using BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITM, must it apply to both CCCD and attribute values?

Dani

[Hung Bui]

Hi Dani, 
Yes , correct. This is so that you don't need to bond with MITM and the sniffer can be easier to decrypt the connection. 

[Dani]

Hi,

I have been doing some tests without bonding and with BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITM, and with SEC_PARAM_LESC=0. In CCCD in all services I have implemented this:

BLE_GAP_CONN_SEC_MODE_SET_OPEN(&cccd_md.read_perm);
BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITM(&cccd_md.write_perm);

Now, equipment is not disconnecting.

Please find attached a sniffer trace doing the following procedure:

- Equipment advertising

- Equipment connected and receiving data

- Equipment disconnects and starts advertising again

So, from that point, how to find the solution to the problem I have? I need to use bonding, and I'm using a 6 digit-pin.

DaniAndroid_Pairing_without_bonding_No_MITM_19_05_22.pcapng

[Hung Bui]

Hi Dani, 
It's possible for the sniffer to track Legacy pairing with Passkey. You just need to enter the passkey in the Sniffer menu and click the "Arrow" button (before input that into your phone):

But I found this method is not very reliable as the central and peripheral may change the LTK in further re-connections. 

It's also possible to track LESC pairing if you can find the LTK key. And I found this is a more reliable solution. 

To print the LTK out you can edit security_dispatcher.c in sec_info_request_process () as follows: 

    NRF_LOG_INFO("LTK: length =%d 0x",p_enc_info->ltk_len);
    for(int i=0;i<p_enc_info->ltk_len;i++) NRF_LOG_INFO("%x ",p_enc_info->ltk[i]);

You need to input the LTK into wireshark, and need to pay attention to the endianess. Basically if the log print out this (after you disconnect and reconnect to bonded device): 


You need to input this to the sniffer and click the arrow button: