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]

Hi Uri,

Did you try the sample I sent in my last reply?

I am out of office now, but I will check the traces when I return next week. Meanwhile, please give the sample I sent a try.

[uri_ee]

Hi Hieu,

Yes I tried the sample and I have managed to get the full length message from the peripheral device. At this time I am trying to update my code to match the sample you provided. Now I am having a new problem. When I try any other value than 23 I get this error:

NordicSemiException ble_gattc_exchange_mtu_req: Failed to ble_gattc_exchange_mtu_req. Error code: NRF_ERROR_INVALID_PARAM

[Hieu]

Hi Uri,

Have you noted the comments I wrote in the code? Duplicating it here for reference:

        # If the peer (peripheral device) initiates the MTU exchange, this value will be automatically used
        # as this device (Python/interface device) preferred MTU.
        # By default, the Heart Rate Collector example also uses this value to configure the
        # SoftDevice/interface device. See further notes in open()
        self.adapter.default_mtu = 111

        self.mtu_exchanged = False
        self.nus_tx_uuid = None
        self.peer_mtu = 0

    def open(self):
        self.adapter.driver.open()
        if config.__conn_ic_id__.upper() == "NRF51":
            raise AssertionError ("NOT SUPPORTED: config.__conn_ic_id__.upper() == ""NRF51""")
        elif config.__conn_ic_id__.upper() == "NRF52":
            gatt_cfg = BLEConfigConnGatt()

            # In this configuration, we are configuring the MAXIMUM value that the SoftDevice/interface device can
            # ever support. It can only be reconfigured after driver.ble_disable().
            # See: https://infocenter.nordicsemi.com/topic/com.nordic.infocenter.s140.api.v7.2.0/structble__gatt__conn__cfg__t.html#a3fac0ed97a3a75cf21e9ed8432b95193
            gatt_cfg.att_mtu = self.adapter.default_mtu
            gatt_cfg.tag = CFG_TAG
            self.adapter.driver.ble_cfg_set(BLEConfig.conn_gatt, gatt_cfg)

            self.adapter.driver.ble_enable()

Another source of that error is, as we explained before, if the negotiation already happened. Which brings us to the sniffer traces.

Are you certain that the device in trace Three and Four is the same as in trace Five?

In trace Three and Four, we can see that the peripheral device initiate the MTU negotiation:

However, that didn't happen in trace Five.