nRF52840 (BMD-340-A-R) + Raspberry Pi 3B+ max MTU size does not work

[Duy]

Dear Nordic Support,

I am using ble_nus service on nRF52840 (BMD-340-A-R) + Raspberry Pi 3B+ (https://github.com/IanHarvey/bluepy). The nRF52840 is the peripheral and the Pi 3B+ is the central. Using nRF5 SDK 15.3.

I set following parameters:

#define NRF_SDH_BLE_GAP_DATA_LENGTH      251

#define NRF_SDH_BLE_GATT_MAX_MTU_SIZE   247

The connection between the nRF52840 and the Raspberry Pi is OK, the MTU also changed. 

The issue is:

1/ The nRF52840 can only receive and send below 23 bytes.

2/ Same configuration for the nRF52840 but there is not problem when connecting with the Android.

Do you have any suggestion to fix this issue? Or to find the root cause of this? My temporary solution to make the nRF52840 send and receive more than 23 bytes with the Pi 3B+ is modifying the following line in ble_gatt.h:

#define BLE_GATT_ATT_MTU_DEFAULT          23 

change to

#define BLE_GATT_ATT_MTU_DEFAULT          247

I found the relevant case: https://devzone.nordicsemi.com/f/nordic-q-a/38563/max-mtu-for-characteristic-read-not-honoured-in-some-cases

Thank you and best regards,

       Duy Tran

[Simonr]

Hi Duy Tran

Let's see if I understand your problem correctly. Your nRF52840 (peripheral) is not able to receive/send more than 23 bytes when it is connected to a Raspberry Pi central, but there are no problems if the central is an Android phone for example, is that correct?

A few things you can check:

• What are the MTU settings of the Raspberry Pi? Is that set to transfer more than 23 bytes by default? I have not looked specifically at this example, but could it be that the Raspberry Pi also uses ble_gatt.h to decide the default MTU?
• Did you try the workaround from the relevant case you linked to? Please see the verified answer, and try that out.
• Where in the code is the NRF_SDH_BLE_GATT_MAX_MTU_SIZE set? Are you sure that that is the MTU size that is used by the application, and not the BLE_GATT_ATT_MTU_DEFAULT?

Best regards,

Simon

[Duy]

Hello Simonr,

Thank you for your answer, you understand the problem correctly.

I checks things that you told me:

1/ 

Simonr said:

What are the MTU settings of the Raspberry Pi? Is that set to transfer more than 23 bytes by default? I have not looked specifically at this example, but could it be that the Raspberry Pi also uses ble_gatt.h to decide the default MTU?

=> It is set to 247 already. There is a configuration param. 

2/ 

Simonr said:

Did you try the workaround from the relevant case you linked to? Please see the verified answer, and try that out.

=> I am still working on the relevant case. Not yet work. 

3/

Simonr said:

Where in the code is the NRF_SDH_BLE_GATT_MAX_MTU_SIZE set? Are you sure that that is the MTU size that is used by the application, and not the BLE_GATT_ATT_MTU_DEFAULT?

=> Below is the function I am using to check. I copied from an example. I also run the debug and it shows that the MTU is changed to 247 bytes.

/**@brief Function for handling events from the GATT library. */
void gatt_evt_handler(nrf_ble_gatt_t * p_gatt, nrf_ble_gatt_evt_t const * p_evt)
{
    if ((m_conn_handle == p_evt->conn_handle) && (p_evt->evt_id == NRF_BLE_GATT_EVT_ATT_MTU_UPDATED))
    {
        m_ble_nus_max_data_len = p_evt->params.att_mtu_effective - OPCODE_LENGTH - HANDLE_LENGTH;
        NRF_LOG_INFO("Data len is set to 0x%X(%d)", m_ble_nus_max_data_len, m_ble_nus_max_data_len);
    }

    m_wrench_send++;

    NRF_LOG_INFO("ATT MTU exchange completed. central 0x%x peripheral 0x%x",
                  p_gatt->att_mtu_desired_central,
                  p_gatt->att_mtu_desired_periph);
}

Best regards,

    Duy