DevAcademy Bluetooth LE connection problems

[Lars M]

Hi,

I am going through the DevAcademy Bluetooth LE lessons, using nRF5 DK. But I run into problems directly in the exercise of the first lesson. Everything goes fine to step 4.2, but connecting doesn’t work. Connection is established, but almost immediately disconnection happens. LED2 is lit for a short moment. The print outs are like this:

No matter how many times I try, disconnection always happens this way. I have also tested to connect via the Bluetooth low energy desktop app using the Nordic BLE dongle. Then the connection works fine. I have tested with both nRF Connect SDK 2.3.0 and 2.0.0, and the scenario is the same for both versions.

In the desktop app it is possible to read the button state and activate notifications (and receive updates when the button is pressed). But I am not able to control LED3. I have put a breakpoint in write_led() in lbs.c (C:\...\v2.3.0\nrf\subsys\bluetooth\services\lbs.c), but the breakpoint isn't hit when I write to the LED characteristic. Though, a breakpoint in read_button() will be hit when I click "Nordic LED and Button Service", so I should be in the correct file.

 

Best regards,

Lars

[Lars M]

Now I have tested to connect via another phone, a Samsung Galaxy A13 5G. Everything seems to work fine: connection, reading button status, button notifications, controlling the LED and disconnecting. I have attached the Wireshark file where all these features are tested. Obviously, something differs from my first phone (Samsung Galaxy S21 5G). There is an opcode, LL_CONNECTION_UPDATE_IND, that is present in the communication where things don’t work. That is where the 7.5 ms connection interval is set. Can that be the problem?

After CONNECT_IND is sent from a phone, the next few opcodes differ in order between the two phones. Why is that? Does It matter?

I also notice that the RGB_LED is lit red on my nRF52840 dongle (with the sniffer firmware) as long as the connection is established. Is that correct?

/Lars

Capture dk and second phone.pcapng

[Einarh]

Lars M said:

There is an opcode, LL_CONNECTION_UPDATE_IND, that is present in the communication where things don’t work. That is where the 7.5 ms connection interval is set. Can that be the problem?

Yes this seems to be the main difference I can see. You could try to set a minimum preferred connection interval in your peripheral's prj.conf file:

CONFIG_BT_PERIPHERAL_PREF_MIN_INT=[number]

You could for instance set it to 39, which seems to be the connection interval with the other phone.

Lars M said:

After CONNECT_IND is sent from a phone, the next few opcodes differ in order between the two phones. Why is that? Does It matter?

It looks like the phones are doing the same things just in a slightly different order, I don't think it should matter.

Lars M said:

I also notice that the RGB_LED is lit red on my nRF52840 dongle (with the sniffer firmware) as long as the connection is established. Is that correct?

I'm not sure what the led is supposed to indicate and I'm not sure where it's documented, but indicating an established connection would make sense yes.

-Einar

[Lars M]

Interesting suggestion. Now I have tested to set CONFIG_BT_PERIPHERAL_PREF_MIN_INT=39 in prj.conf, but it is the same behaviour. I see in autoconf.h that the setting has taken effect. Without this config in prj.conf, it is CONFIG_BT_PERIPHERAL_PREF_MIN_INT=24 in autoconf.h (should be 30 ms, not 7.5 ms). So even with the default 24 it should be much bigger than 7.5 ms. Is it my phone that doesn't care about it, or is there something wrong with the config?

Seems strange to use a red colour when something is correct. But maybe the LED is used that way.

/Lars

Capture dk and first phone, CONFIG_BT_PERIPHERAL_PREF_MIN_INT 39.pcapng

[Einarh]

It's probably your phone that ignores it yes. The peripheral can state its preferences but in the end it's up to the central unit to set the connection parameters. You could play around with other parameters and see if they make a difference, maybe you're able to communicate with your phone if you request to use PHY 1M for example.

[Lars M]

I have made my own BLE-program (called MMS), which is a light version of NUS. Here the connection works fine. It is also possible to send characters to and receive notifications from my dk, and also disconnect. The only difference in the connection process seems to be the lack of LL_CONNECTION_UPDATE_IND in my project. Instead there is an empty PDU (packet 116 compared to 213). Maybe there is some info from the device that makes the phone want to use 7.5 ms in the first case, but not in my project. I have tried to compare the autoconf.h-files, but haven’t yet found something that could be interesting to adjust. Do you see something in the connection process that may cause different behaviours of the phone?

PHY 2M seems to work, and there is 50 ms connection interval.

/Lars

Capture MMS first phone, connect UART disconnect.pcapng