Send long packets for Android using low MTU sizes

[jinvers]

Hi,

In a ble_app_uart example I have problems to send long packets if the Android smartphone only accepts MTU sizes of 20 bytes. I'm using the nRF5_SDK_17.1.0, the S132 over the SES IDE and the nR52-DK board.

The application adjusts the MTU size to the value negotiated by the smartphone and it's set to 20 bytes. However, when I send longer packets the smartphone only receives a part. It looks like the device only send one packet and doesn't manages automatically the division of the full packet for sendidn it in different subpackets.

How could I setup the Softdevice so it could automatically manages this sending of a long packet in subpackets? Excuse me if there is other similar case about this, but I have not been able to find it.

Thank you in advance.

Regards,

Joel

[Sigurd]

Hi,

You can see how the ble_app_uart example handles this here:

void uart_event_handle(app_uart_evt_t * p_event)
{
    static uint8_t data_array[BLE_NUS_MAX_DATA_LEN];
    static uint8_t index = 0;
    uint32_t       err_code;

    switch (p_event->evt_type)
    {
        case APP_UART_DATA_READY:
            UNUSED_VARIABLE(app_uart_get(&data_array[index]));
            index++;

            if ((data_array[index - 1] == '\n') ||
                (data_array[index - 1] == '\r') ||
                (index >= m_ble_nus_max_data_len))
            {
                if (index > 1)
                {
                    NRF_LOG_DEBUG("Ready to send data over BLE NUS");
                    NRF_LOG_HEXDUMP_DEBUG(data_array, index);

                    do
                    {
                        uint16_t length = (uint16_t)index;
                        err_code = ble_nus_data_send(&m_nus, data_array, &length, m_conn_handle);
                        if ((err_code != NRF_ERROR_INVALID_STATE) &&
                            (err_code != NRF_ERROR_RESOURCES) &&
                            (err_code != NRF_ERROR_NOT_FOUND))
                        {
                            APP_ERROR_CHECK(err_code);
                        }
                    } while (err_code == NRF_ERROR_RESOURCES);
                }

                index = 0;
            }
            break;

if the phone does only support 20 bytes MTU, then 

m_ble_nus_max_data_len is 20, and the data you try to send will be split into chunks, where each chunk is max 20 bytes.

[jinvers]

Hi Sigurd,

Thank you for your reply. 

i also see that in the gatt_evt_handler() the management of the MTU size should also update the m_ble_nus_max_data_len value. But it doesn't look to work:

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

I understand this piece of code should work, isn't it?

Regards,

Joel

[Sigurd]

Hi,

Default value of m_ble_nus_max_data_len is 20. If there is an MTU update, the m_ble_nus_max_data_len will be updated in gatt_evt_handler() to reflect the new MTU value.

[jinvers]

Ok. Thank you.

Regards,

Joel

[jinvers]

Hi,

I have tested this using the Android nRF Connect app. I have the m_ble_nus_max_data_len  variable set to 20 in the nRF SDK. In nRFConnect Android app I do the next sequence:

- I connect to the Nordic UART Service of the Dk board program.

- I open the tab of this service. In the tab, I select Connect.

- I select the "Nordic UART Service" and the app presents the RX Characteristics values and the TX Characteristics values.

- I push the icon in the "Tx Characteristics". I understand that this will enable the reception of notifications.

- I push the icon in the "Rx Characteristics". It opens a window to write a command to my FW. I send it.

- The DK sends an answer that should be greater than 20 bytes. However, I can only receive 20 bytes and I the app doesn't receive more packets of 20 bytes.

How could enable the automatic sending of the rest of the packet from the DK to the nRFConnect app?

Regards,

Joel