CODEWITHSUNDEEP
google-chromearduinonotificationsbluetooth-lowenergygatt
Quentin Bernet
Quentin Bernet

Reputation: 43

BLE Browser API startNotifications() returns "GATT Error: Not supported."

I've recently implemented a BLE notifications system on an ESP32 board. My system has a specific service uuid and a specific characteristic uuid. I've set the notify property to the characteristic:

    characteristic = service->createCharacteristic(
    UUID,
    BLECharacteristic::PROPERTY_READ |
    BLECharacteristic::PROPERTY_WRITE |
    BLECharacteristic::PROPERTY_NOTIFY
);

When I log the characteristic's properties into Chrome dev tools, the "notify" property appears to be "true":

Notify:               true

But, when I use the startNotifications() on my characteristic I get this error message:

Uncaught (in promise) DOMException: GATT Error: Not supported.

This is the code I use :

try {
    this.batteryLevelCharacteristic.startNotifications()
    .then(characteristic => {
        characteristic.addEventListener('characteristicvaluechanged',
                                handleCharacteristicValueChanged);
        console.log('Notifications have been started.');
    });
  } catch(e) {
    console.log(e);
  }

I don't really understand the error... any idea ? :(

Thanks in advance !

NB : I use chrome Version 87.0.4280.88 on macOS Catalina 10.15.7

Upvotes: 4

Views: 359

Answers (1)

agrath
agrath

Reputation: 966

Does your characteristic (defined on the ESP32) that you are trying to subscribe/notify to have the descriptor 0x2902 (CCC Descriptor) defined?

https://github.com/oesmith/gatt-xml/blob/master/org.bluetooth.descriptor.gatt.client_characteristic_configuration.xml

This is a requirement of the web-bluetooth implementation in Chrome per https://groups.google.com/a/chromium.org/g/web-bluetooth/c/TlHnA3hTksw

The other issue you could be running into is a problem with concurrency when writing to characteristics and introducing a delay could help. The comment from programmerRaj using an alert could be adding this delay, or the issue could also be due to interactivity (in-browser often you need to interact with the document such as a click event to grant permission).

I also ran into a similar problem; at least the same error. My BLE pererphial is defined/running in nodejs on a Raspberry pi 4. For me, the characteristic does have the descriptor and I've already tried a delay. After a lot of trial and error, I figured out the root cause of this.

The 0x2902 characteristic was duplicated, added by the framework (bleno) and also declared on the peripheral. I found this by looking at the device "on the air" using a BT app on my phone, and discovered the duplicate characteristic descriptor. Interestingly, the BT app could still subscribe to notifications but Chrome/web-bt was more fussy and threw this error when the descriptor was double defined.

Hope this helps someone else in the future!

Upvotes: 0

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