nRF52840 Dongle only receives a notification packet with a payload 20 bytes or less

[GameCodingNinja]

I'm using the Nordic Python libraries to receive the BLE data. I can send the data from our board with the nRF52840 chip on it. If I send the data to my Android phone with the nRF Toolbox app, I can send a packet with a payload of 60 bytes. Sending the same thing to the Nordic dongle, it only allows for a payload of 20 bytes or less.

The Python examples suggest an MTU of 250. Is there anything I can do to increase the payload size to send more bytes?

[Kenneth]

Hi,

First you should check that the characteristic in question support more than 20bytes by checking the length field when created. If that is okey, then I suggest to check out the DLE and ATT MTU exchange procedures to handle more than 20bytes payloads:

https://infocenter.nordicsemi.com/topic/com.nordic.infocenter.s140.api.v7.3.0/group___b_l_e___g_a_p___d_a_t_a___l_e_n_g_t_h___u_p_d_a_t_e___p_r_o_c_e_d_u_r_e___m_s_c.html 

https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.s140.api.v7.3.0%2Fgroup___b_l_e___g_a_t_t_c___m_t_u___e_x_c_h_a_n_g_e.html 

Best regards,
Kenneth

[GameCodingNinja]

I'm using NCS 1.7.0 and I'm sending the data out through ncs\v1.7.0\nrf\include\bluetooth\services\nus.h  bt_nus_send(). I haven't setup any custom services or characteristics and just trying to send the data through the built in Nordic libraries. My phone with the Nordic app gets at least 60 bytes but Nordic's dongle only receives 20.

I apologize that I'm new to BLE and still feeling my way around it. I had to program the dongle with the nRF Connect for Desktop "Bluetooth Low Energy" app. Perhaps this is hard coded for the app that is installed. The app that this tool installs, is it a project/sample I can build and install myself?

[Kenneth]

Can you check if you have set this:
https://github.com/NordicSemiconductor/pc-ble-driver-py/blob/ab41fb33bbd4e05bd05754955c3b44ff7038614f/pc_ble_driver_py/examples/heart_rate_collector.py#L83 

Best regards,
Kenneth

[GameCodingNinja]

I used that example to write my Python app so I already have that line in my code. I even tried to send the value to the BLEConfigConnGatt constructor. Below is some code from my Python app. This still made no difference.

self.adapter.default_mtu = 250
self.adapter.driver.open()
gatt_cfg = BLEConfigConnGatt(att_mtu=self.adapter.default_mtu)
gatt_cfg.att_mtu = self.adapter.default_mtu
gatt_cfg.tag = 1
self.adapter.driver.ble_cfg_set(BLEConfig.conn_gatt, gatt_cfg)
self.adapter.driver.ble_enable()

[Kenneth]

You don't by any chance have the possibility to make an on-air sniffer log, so I may see if there occurs MTU exchange?
https://www.nordicsemi.com/Products/Development-tools/nRF-Sniffer-for-Bluetooth-LE 
https://infocenter.nordicsemi.com/topic/ug_sniffer_ble/UG/sniffer_ble/intro.html 

[GameCodingNinja]

I'll have to give that a try. I do have a nRF52840 DK board. Looks pretty involved. Might try when I get a chance. Might be worth trying to setup a service and characteristic to see if that works better.