(BLE): How can I send more than 20bytes from a server with notify?

[Vincent25]

Project context:
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Server BLE  for home trainer diy

Target  (Bluetooth server) : Bluefruit nRF52 The feather
Library : BluefruitBLE
Client : nRF Connect (Android)

- no descriptor added

- work fine for an other characteristic (CyclingSpeedAndCadence Measurement)

Problem description:
-----------------------
I work with the characteristic (CyclingPowerMeasurement (0x2A63, UUID GATT), which size is 32 bytes.

I check the characteristic with nRF Connect:
 - When Read : Good size (32 bytes) , and the good values.
 - When Notify: Only 20 bytes, and not the good values.

Some suggestions:
---------------------
I think there is a limitation of 20 bytes..
but i don't know how to modify or check it


thanks!!!!

[Vidar Berg]

Hi

The default payload length is 20 bytes, but it can be extended after a connection is established provided both devices have support for long MTU (a majority of new phones do). In our SDK examples, we use the  GATT Module to handle the negotiation to increase the packet length, but I'm not sure if the BluefruitBLE library does the same?

Message sequence chart for reference: GATTS ATT_MTU Exchange

[Vincent25]

Hi,

Thank you very much for your answer Vidar.

I changed in the file ble_gatt.h the default value:

//#define BLE_GATT_ATT_MTU_DEFAULT          23 changed as   #define BLE_GATT_ATT_MTU_DEFAULT          40

and I could see that I have the good MTU value of 40 before and after the connexion with the client.

But i didn't change  the result on nRFConnect.

After a write , see the characteristic on nRFConnect

And after a notify:

the field is shown as a binary and the size is 20 bytes...

Thank you very much.

Vincent

[Vidar Berg]

Hi Vincent,

The max. length of the notification packet may be limited by the MTU size, but since you're able to read 32-byte I wonder if it might just be the way the send notify function is implemented in your FW or by in the library you use. The packet length is specified by the application, not by the MTU size.