Cannot alter MTU after connecting a peripheral device using the python library "pc_ble_driver_py"

[uri_ee]

Hello, 

I'm using the nRF device to communicate with a peripheral device I get some characteristic notifications but only 20 bytes for a packet.

When using the 'nRF connect' GUI I manage to update the MTU from 23 to any larger value and get the full notification.

But I cannot alter MTU after connecting a peripheral device using the python library "pc_ble_driver_py".

Iv'e tried many ways like setting the default_mtu and the att_mtu on adapter initialization:

driver = BLEDriver(
    serial_port=port,
    auto_flash=auto_flash,
    baud_rate=baud_rate,
    retransmission_interval=retransmission_interval,
    response_timeout=response_timeout,
    log_severity_level=driver_log_level,
)
driver.ble_gattc_exchange_mtu_req
adapter = BLEAdapter(driver)
adapter.default_mtu = 247
adapter.driver.open()
gatt_cfg = BLEConfigConnGatt()
gatt_cfg.att_mtu = adapter.default_mtu
adapter.driver.ble_cfg_set(BLEConfig.conn_gatt, gatt_cfg)
adapter.driver.ble_enable()


And to use the adapter.att_mtu_exchange function to but for no avail.
when i execute tyhe command : 
self.ble_adapter.att_mtu_exchange(self.conn_handler, 23)
 

it's running fine but when i try to increece the mtu to a different level like:
self.ble_adapter.att_mtu_exchange(self.conn_handler, 247)
 (or any number greater then 23)

it allways raise the folowing exception:

NordicSemiException: MTU exchange request failed. Common causes are: missing att_mtu setting in ble_cfg_set, different config tags used in ble_cfg_set and connect.

Is there any way to overcome this hurdle?

Thanks,
Uri

[Hieu]

I am a bit surprised. Do you not even have a MTU higher than 23 at all?

As you can see in the second screenshot in my last reply, when I set the default_mtu field to 121, the negotiation happened automatically, and the two devices successfully agreed on 121 as the MTU.

That experiment was based on the heart_rate_collector.py example. When I tried the code unmodified, I observed the negotiation to ended with MTU of 247 instead.

How are you setting up your project? Can you try to use the HRS sample unmodified and see if the MTU is updated automatically? 

[Hieu]

Hieu said:

I believe the idea is that the first MTU exchange should have already set MTU to the highest value both the Client and the Server can support, so there is no need to set it to any other value.

While it could be inconsequential, this seems a bit too inflexible and basically render this API meaningless after the first MTU exchange. That makes me doubt my own understanding. I am out of office until the end of the week, but I will double check in the Bluetooth spec and with my team next week.

Following up on this point, it is required by the Bluetooth Core Specification, Vol 3, Part G, Section 4.3.1 Exchange MTU:

4.3.1 Exchange MTU
This sub-procedure is used by the client to set the ATT_MTU to the maximum
possible value that can be supported by both devices when the client supports
a value greater than the default ATT_MTU for the Attribute Protocol. This subprocedure
shall only be initiated once during a connection.

[uri_ee]

Hieu said:

How are you setting up your project? Can you try to use the HRS sample unmodified and see if the MTU is updated automatically? 

Hello Hieu.

I run the HRS example code but cannot find any reference to the MTU setting in logs.

Can you please instruct me.

[Hieu]

Hi Uri,

The MTU log in my screenshot is from the HRS device. With a device running the HRS example, without any modification necessary, you should see it log by default via UART (which is connected to a Virtual COM port in our development kits).

If you have difficulty seeing that or if you are using a different hardware, please give me some details and I will see if there is an alternative.

Please also note that it is the Easter holiday at my location, and my responses would be delayed due to a staffing situation, and my being out of office from Wednesday this week. I am sorry for the inconvenience.

[uri_ee]

Hello Hieu.

Hieu said:

If you have difficulty seeing that or if you are using a different hardware, please give me some details and I will see if there is an alternative.

Yes, i would need That alternative.