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

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

Vidar Berg — Tue, 07 Jan 2020 14:11:43 GMT

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.

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

Vincent25 — Fri, 03 Jan 2020 15:36:18 GMT

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

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

Vidar Berg — Fri, 03 Jan 2020 12:55:19 GMT

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