Extend the MTU with SDK 15.2

[oalt]

Hi!

Is there an example to extend the Bluetooth MTU size to more than 20 bytes together with SDK 15.2?

I found only examples with older versions.

Thank you in advance.

Best regards

Oliver

[Edvin]

Hello Oliver,

There are examples in SDK15.2.0 that uses MTU larger than 20 Bytes, such as the ble_app_uart or ble_app_uart_c examples. Check out these examples to see how it is done.

Best regards,

Edvin

[oalt]

Hi!

When the MTU GAT request/response is sent, the MTU is set to 23 because the server rx mtu is 23. Where can I change this?

Here is the log:

 0> <debug> nrf_ble_gatt: Peer on connection 0x0 requested an ATT MTU of 256 bytes.
 0> <debug> nrf_ble_gatt: Updating ATT MTU to 43 bytes (desired: 43) on connection 0x0.
 0> <debug> nrf_ble_gatt: server rx mtu 23, desired 43
 0> <debug> nrf_ble_gatt: ATT MTU updated to 23 bytes on connection 0x0 (response).
 0> BLE data len is set to 0x14 (20)
 0> ATT MTU exchange completed. central 43 peripheral 43

[Edvin]

Hello,

Is this the log from the peripheral or the central? And either way, what is the other device (central/peripheral)? Is it the other example, ble_app_uart or ble_app_uart_c, or is it a phone? If it is a phone, and the phone is old, or another device that doesn't support MTU larger than 23, you can't increase the MTU further.

Best regards,

Edvin

[oalt]

My application is a peripherial. The log comes from the module mrf_ble_gatt.c.

I have extended the logging in the function on_exchange_mtu_rsp_evt() with one line of code:

NRF_LOG_DEBUG("server rx mtu %i, desired %i", server_rx_mtu, p_link->att_mtu_desired);

in line 211.

The peer seems to send a request of MTU=256. My test setting is 43 but the result is 23 because of server rx mtu in nrf_ble_gatt.c, function on_exchange_mtu_rsp_evt()

p_link->att_mtu_effective = MIN(server_rx_mtu, p_link->att_mtu_desired);

[Edvin]

Hello,

oalt said:

The peer seems to send a request of MTU=256.

By "peer" you mean central, right?

And still, what is your central device? A mobile phone or another nRF? Or something else?

What example do you test with? Did you test the ble_app_uart example? Did you get a higher MTU than 23 there? 

Can you describe where you set the setting to 43?

In the ble_app_uart example it is set in main.c -> gatt_init() -> nrf_ble_gatt_att_mtu_periph_set(&m_gatt, NRF_SDH_BLE_GATT_MAX_MTU_SIZE); where NRF_SDH_BLE_GATT_MAX_MTU_SIZE = 247

When I do the same logging as you, I get:

So the p_link->att_mtu_desired is the peripherals desired MTU, and the server_rx_mtu is the central (iPhone in my case).

Also know that data_len, which is the payload length = MTU - OPCODE_LENGTH - HANDLE_LENGTH, OPCODE_LENGTH = 1 and HANDLE_LENGTH = 2.

So it looks like it is your central that doesn't accept MTU over 23.

BR,

Edvin