NRF52 Running S132V3.0 Soft Device as a Central can't maintain a connection with a Samsung Galaxy S7 peripheral

[Greg Nowak]

Hardware: I have an nRF52 device running the S132 running as a Central. I have 6 iOS/Android devices running as peripherals.

Background: I am attempting to have my central device maintain a connection with a variety of devices. My phone app is running the Uart Service following the Nordic Uart Service example. I've extended the UART Central Service on my hardware to accept multiple concurrent UART channels.

I am successfully able to connect and maintain a connections with: Samsung Galaxy S6, Note5 (devices running Android 6.0.1) iPhone 5S, iPhone6, and iPhone7 Nexus/Pixel running Android 7.1.1

I am, however, having trouble maintaining a connection with anything that is running Android 7.0.

Question: On the Galaxy S7 (Android 7.0) I'm seeing that a valid Bluetooth connection is established, my UART service discovery is successful (I get valid looking UART handles), and before I'm able to send anything to the phone the connection is dropped. This issue seems to affect the Android 7.0 phone I have for testing, but I'm not sure if it is an issue with Android 7.0 specifically or it is an issue with my Central code.

In the Multi-Connection Central example the Central continues to scan for peripherals after the first connection is made. During the Scanning window I'm seeing that no other Central bluetooth actions can take place. Is it possible that if my peripheral device has a 20 ms connection interval but my scan window is 200 ms that my peripherals would be disconnected during my scan window, since I could miss 10 connection intervals? Does the Soft Device have a way to guard against this?

For reference, this is how I have my scanning and connection intervals setup for my Central device.

SCAN_INTERVAL_SLOW             MSEC_TO_UNITS(2000, UNIT_0_625_MS) 
SCAN_WINDOW_SLOW               MSEC_TO_UNITS(200, UNIT_0_625_MS) 
SCAN_TIMEOUT_SLOW              MSEC_TO_UNITS(0, UNIT_0_625_MS)  

MIN_CONNECTION_INTERVAL   MSEC_TO_UNITS(7.5, UNIT_1_25_MS)  
MAX_CONNECTION_INTERVAL   MSEC_TO_UNITS(100, UNIT_1_25_MS) 
SLAVE_LATENCY             4  
SUPERVISION_TIMEOUT       MSEC_TO_UNITS(6000, UNIT_10_MS) 

[Jørgen Holmefjord]

Have you tried doing a sniffer trace to see if it gives any indications on why the connection is dropped?

[Greg Nowak]

I haven't tried sniffing the packets directly (I don't have a BLE Sniffer) but I'm getting the BLE_GAP_EVT_DISCONNECTED event from the on_ble_evt() callback.

I've been doing some research into the differences between Android 7.0 and other versions of Android. I found that my hardware also cannot connect to Android 5.1.1 devices either. My suspicion at this point has to do with the rotating MAC addresses on the peripheral. When the phone rotates its MAC address it seems as though I get a BLE Disconnect event. My Android 6.0.1 device seems to have a static MAC address throughout the entire connection process, where the S7 doesn't seem to have a fixed MAC address.

[Greg Nowak]

I should also note that i'm just doing a standard connection and device discovery, I am not using Nordic's "Peer Manager" or attempting to do any sort of encryption of the connection. At this point I figured it would be easier to get non-encrypted traffic working and add the encryption later.

[Greg Nowak]

Another update. The Disconnect Reason is BLE_HCI_CONNECTION_TIMEOUT.
I'm not sure why the Android 7.0 phones have this issue but no other phone I've tried has it.

[Jørgen Holmefjord]

Have you written your own Android application? I'm seeing similar issues with my Galaxy S7 with Android 7.0 with nRF Connect, but I can connect to the phone. If I try to read characteristics, the phone disconnects with reason BLE_HCI_CONNECTION_TIMEOUT. I have managed to get a connection sometimes though. I will look some more into this and try to do a sniffer trace to see if there is some unexpected going on.