BLE connection timeout on startNotifications()

[banglejsdev]

I'm writing my first BLE app and would be grateful for any help anybody can provide.

Basically, I'd like my Bangle to receive notifications from a BLE peripheral device. Using the code below, my Bangle is able to successfully connect to the peripheral, get its service, and the service's characteristic. But after calling characteristic.startNotifications(), nothing happens for several seconds and my Bangle disconnects from the device without ever getting a 'characteristicvaluechanged' event. I've used the NRF Connect app and verified that when it subscribes to notifications from the device it does receive data as expected, though it can take up to 10 seconds before the device sends any data.

I've searched but couldn't find/understand enough info to understand what's going on at the BLE level, so if anybody can offer any suggestions about what to try next I would appreciate it. In particular, is there a notification timeout that may be causing my app to disconnect before it has received any data? If so, is there way to override such a timeout? Also, what is going on on the Bangle side after calling characteristic.startNotifications()? Does it send any data notifying the peripheral of its intent to subscribe (and disconnect if it doesn't receive an acknowledgement), or does it simply wait until it receives data from the peripheral?

Well, here's the code I'm using. Thanks for taking a look and offering any ideas you might have!

NRF.connect("68:5E:1C:2B:5F:00 public").then(function(ga) {
    gatt = ga;
    console.log("getting service...");
    gatt.device.on('gattserverdisconnected', function(reason) {
	console.log("disconnected: " + reason);
	E.showAlert(reason).then(function() {
            Bangle.showLauncher();
	});
    });
    return gatt.getPrimaryService("0000ffe0-0000-1000-8000-00805f9b34fb");
}).then(function(service) {
    console.log("getting characteristic...");
    return service.getCharacteristic("0000ffe1-0000-1000-8000-00805f9b34fb");
}).then(function(characteristic) {
    characteristic.on('characteristicvaluechanged', function(data) {
        console.log("notification data: ", data);
    });
    console.log("starting notifications...");
    return characteristic.startNotifications();
}).then(function() {
    console.log("waiting for notifications");
}).catch(function(e) {
    console.log("Error: " + e);
});

[gfwilliams]

Hi - the code you've got there looks good. Only thing I can think of is are you sure that "0000ffe1-0000-1000-8000-00805f9b34fb" is the characteristic with the notification? I guess if you called it on another one you might have a problem.

is there a notification timeout that may be causing my app to disconnect before it has received any data?

No, there shouldn't be. There's a timeout if the peripheral device doesn't respond to a certain number of polling intervals, but that's pretty standard as it should really always respond to keep a connection going.

When the device disconnects I see you have console.log("disconnected: " + reason); - what's the reason number that gets printed?

Also, what is going on on the Bangle side after calling characteristic.startNotifications()? Does it send any data notifying the peripheral of its intent to subscribe

Yes, that's exactly right. It tries to find the device's characteristic descriptor (CCCD) and then writes a value to it to enable notifications. IIRC if those fail startNotifications should fail - it shouldn't automatically trigger a disconnect. It's why it might be handy to see what the disconnect reason was.

Is your Bangle firmware up to date? I know we did have an issue when the CCCD was in a non-standard place (as in Adafruit Bluefruit modules) but I fixed that a year or more ago now, so any semi-recent firmware should be ok.

[banglejsdev]

Thank you very much for reviewing my code Gordon!

I've double-checked the device's UUIDs and am sure I am connecting to the correct device, service, and characteristic. The device only advertises one service with one characteristic, and as I mentioned, the NRF Connect app does receive data from it, which is why I thought there might be an issue on the Bangle side.

There's a timeout if the peripheral device doesn't respond to a certain number of polling intervals, but that's pretty standard as it should really always respond to keep a connection going.

So are you saying that after the Bangle connects to a peripheral, it periodically sends some kind of 'heartbeat'-like data to the device and if it doesn't receive a response, the Bangle will disconnect? Sorry but I forgot to write that the reason returned on disconnect is the number '8' (timeout). And sorry for my lack of understanding but does 'gattserverdisconnected' mean that the peripheral terminated the connection and not my Bangle?

Oh yes--I did see the other post about the CCCD being in a non-standard place but due to my poor understanding of the BLE protocol didn't realize its potential relevance to my issue. Regardless, my Bangle 1 and Bangle 2 are both running the latest firmware, 2v27, and the behavior is the same on both of my Bangles.

Out of curiosity, instead of calling startNotifications(), I tried calling readValue() and my Bangle received (always the same) six bytes of data from the device and then immediately (got) disconnected. But for my use case my Bangle can't be continuously re-connecting to the peripheral to read data--I'm expecting the peripheral to notify me when it detects an event has occurred.

I will contact the developer of the peripheral too and see if they can offer me any insight into what's going wrong.

Thanks again as always for all your time, help, and dedication!

[gfwilliams]

are you saying that after the Bangle connects to a peripheral, it periodically sends some kind of 'heartbeat'-like data to the device

Yes, it's how Bluetooth LE works. The connecting device constantly polls the peripheral device at the 'connection interval', and sends any data that needs sending or sees if there's anything to receive. It's part of the connection, so if the peripheral device stops responding for a certain number of those intervals then it's treated as a disconnect, and I think that might be what you're seeing.

does 'gattserverdisconnected' mean that the peripheral terminated the connection and not my Bangle?

It's called for any disconnection, but the reason code tells you why. In this case the Bangle called the connection dead because the connected device didn't respond within a certain amount of time

I tried calling readValue() and my Bangle received (always the same) six bytes of data from the device and then immediately (got) disconnected

Is it possible the Bangle gets disconnected after a certain time period anyway, regardless of what you do?

I can think of some possibilities:

• The peripheral has specific connection interval requirements and the Bangle doesn't meet those so it disconnects. It's possible that the high connection interval the Bangle has to the PC interferes, so you could try NRF.setConnectionInterval(50) to slow it down (at the expense of slower code upload) and leave some time for the peripheral
• The peripheral requires pairing, and if the Bangle doesn't respond it bails out of the connection. Did you have to pair on you phone when you used NRF Connect?

Espruino can do pairing if that's what it is. You could try https://www.espruino.com/Reference#l_BluetoothDevice_passkeyRequest:

// before
NRF.setSecurity({mitm:1, keyboard:1}); // I think...
// when connecting...
device.on('passkeyRequest', function() { console.log("Passkey requested"); });
// then you can use device.sendPasskey(...)

But if you have contact with the developer that'd be great - they may have some ideas for why it disconnects

[banglejsdev]

Thanks so much for your explanation and suggestions! I will experiment with the various connection settings and see if there's one that keeps the connection alive. The peripheral works as expected with the NRF Connect app without pairing. Hopefully the developer of the peripheral will also offer some tips. I'll post another comment with my findings and hopefully the solution, soon.

[enaon]

I will experiment with the various connection settings

NRF.connect("68:5E:1C:2B:5F:00 public",{minInterval:7.5, maxInterval:15}).then(function(ga) {

maybe also try connection intervals like so

[banglejsdev]

@enaon thank you SO much for taking the time to read this post and suggesting those settings! Those were the first values I just tried and now my Bangle doesn't disconnect and receives all expected notifications from the peripheral!

And many thanks to @gfwilliams for your help and the whole Espruino team for making such wonderful and versatile devices!

[enaon]

and now my Bangle doesn't disconnect and receives all expected notifications from the peripheral!

nice, this is the case with some unicycles too ( I think they use the hc05 BT module}, if the intervals are not fixed and small, they disconnect. You may want to play with them a bit, find the largest your device works ok with.

[gfwilliams]

That's great - glad it sorted it for you! I just checked and by default Espruino uses 20ms as the lowest connection interval (200ms max). Maybe I should change that to 7.5? As I understand it there's a range, and both devices give a range they're happy with and agree on something in that range.

However whether or not the connection breaks if an interval can't be arranged is up to the peripheral - it doesn't have to disconnect.

[enaon]

I just checked and by default Espruino uses 20ms as the lowest connection interval

nice info, this explains it, I had problems keeping the connection alive with some peripherals from day one, so I always use minInterval:7.5, I think it was a hint by fanoush.

Maybe I should change that to 7.5?

It is no problem since you provide an easy way to set them, but if it breaks nothing, maybe it is a nice idea.