Updating ATT MTU to 0 bytes (desired: 0) on connection 0x0.

[Quentin]

Hi,

I have two 52832 (both on SDK 14.2, S132 v5.1) talking to each other using a slightly modified UART service to support more than one connection.

I have this strange log message and I could not figure out from where it is coming from.

On the central :

<info> EVENTS: BLE_GAP_EVT_CONNECTED
<debug> ble_gatt: Peer on connection 0x0 requested an ATT MTU of 60 bytes.
<debug> ble_gatt: Updating ATT MTU to 0 bytes (desired: 0) on connection 0x0.
<error> ble_gatt: sd_ble_gatts_exchange_mtu_reply() returned unexpected value 0x7.
<info> EVENTS: BLE_GAP_EVT 0x55
<debug> ble_gatt: Peer on connection 0x0 requested a data length of 64 bytes.
<debug> ble_gatt: Updating data length to 0 bytes on connection 0x0.
<info> EVENTS: BLE_GAP_EVT_DATA_LENGTH_UPDATE_REQUEST
<debug> ble_gatt: Data length updated to 64 on connection 0x0.
<debug> ble_gatt: max_rx_octets: 64
<debug> ble_gatt: max_tx_octets: 64
<debug> ble_gatt: max_rx_time: 624
<debug> ble_gatt: max_tx_time: 624
<info> EVENTS: BLE_GAP_EVT_DATA_LENGTH_UPDATE
<info> EVENTS: Connection interval updated: 0x40, 0x40.

On the peripheral:

<debug> ble_gatt: Requesting to update ATT MTU to 60 bytes on connection 0x1.
<debug> ble_gatt: Requesting to update data length to 64 on connection 0x1.
<info> EVENTS: Connected
<debug> MODULE: Connected (handle=0x01)
<info> ADV: No Parameter Change
<info> ADV: Start Advertising - Success
<debug> ble_gatt: Data length updated to 64 on connection 0x1.
<debug> ble_gatt: max_rx_octets: 64
<debug> ble_gatt: max_tx_octets: 64
<debug> ble_gatt: max_rx_time: 624
<debug> ble_gatt: max_tx_time: 624
<debug> CONFIG: ATT MTU exchange completed. central 0x3C peripheral 0x3C
<info> EVENTS: BLE_GAP_EVT_DATA_LENGTH_UPDATE
<info> EVENTS: Connection interval updated: 0x40, 0x40.
<info> EVENTS: Disconnected

I tried with my android phone and the nrfConnect app, and everything seems to work :

<debug> ble_gatt: Requesting to update ATT MTU to 60 bytes on connection 0x2.
<debug> ble_gatt: Requesting to update data length to 64 on connection 0x2.
<info> EVENTS: Connected
<debug> MODULE: Connected (handle=0x02)
<info> ADV: No Parameter Change
<info> ADV: Start Advertising - Success
<debug> ble_gatt: ATT MTU updated to 60 bytes on connection 0x2 (response).
<debug> CONFIG: ATT MTU exchange completed. central 0x3C peripheral 0x3C
<info> EVENTS: BLE_GAP_EVT 0x3A
<info> EVENTS: BLE_GAP_EVT 0x12
<debug> ble_gatt: Data length updated to 27 on connection 0x2.
<debug> ble_gatt: max_rx_octets: 27
<debug> ble_gatt: max_tx_octets: 27
<debug> ble_gatt: max_rx_time: 328
<debug> ble_gatt: max_tx_time: 328
<debug> CONFIG: ATT MTU exchange completed. central 0x3C peripheral 0x3C
<info> EVENTS: BLE_GAP_EVT_DATA_LENGTH_UPDATE
<info> EVENTS: Connection interval updated: 0x1B, 0x1B.
<info> EVENTS: BLE_GAP_EVT 0x12
<info> EVENTS: BLE_GAP_EVT 0x12
<info> EVENTS: BLE_GAP_EVT 0x12
<info> EVENTS: BLE_GAP_EVT 0x50
<info> TRS: 0x02 : (0x11) CCCD Notification Enabled [op 0x01, auth req 0x00]
<debug> MODULE: RX : Message type 0x3D
<error> MSG_FROM_READER: RX : Message to tag (Conn ID = 0x01, Use legacy = 0x00)
<debug> MODULE: Send to tag (handle=0x02)
<info> TRS: 0x02 sd_ble_gatts_hvx(handle=0x10, len=6)
<info> EVENTS: BLE_GATTS_EVT_HVN_TX_COMPLETE

What surprise me the most is obviously the log message "Updating ATT MTU to 0 bytes (desired: 0) on connection 0x0.".
I have tried different things, but I am a bit lost. How can I debug this mess ?

Thanks for your help,

[Susheel Nuguru]

Hi Quentin,

Seems strange with that log. But it seems that ATT MTU size is set to 0 explicitly in the app. Can't you set a breakpoint at the place where you set this and see why your app is trying to set the ATT_MTU size to 0?

[Quentin]

I finally found what I was doing wrong. I had a first instance of nrf_ble_gatt_t m_gatt; declared in the context of my init function, but I also had NRF_BLE_GATT_DEF(m_gatt); somewhere else. Which mean that the init was declare with a different structure than the observer and the event handler was called with an uninitialized struct.

I think it is coming from the migration to 14.2. I have been following the migration instructions without thinking too much of what I was doing, and that's when bad things happen.

Anyway, thanks a lot for your help, and sorry for this stupid question.

[Susheel Nuguru]

there are no stupid questions in this forum. Only experiences that might help others. Thanks for coming back to give a reply. I am marking your answer as verified.