Sending byte array over ble on nrf52840

[LuckyGinger]

Hello,

I am in the process trying to send multiple packets over ble but cannot seems to figure out this UART example.

I have read many posts that use `ble_nus_string_send` to send string data, however, I can't seem to find this function located anywhere.

I currently can only send one byte at a time.

I started on the ble_blinky example as I needed some buttons so I don't know exactly what I would be missing.

This is a little test hello world on button click for button 0 in main.c:

static void button_event_handler(uint8_t pin_no, uint8_t button_action)
{
    ret_code_t err_code;

    uint8_t data_value[] = {"HelloWorld!"};
    uint16_t len = sizeof(data_value);

    SEGGER_RTT_printf(0, "Length = %d\n", len);

    switch (pin_no)
    {
        case BUTTON0_BUTTON:
            SEGGER_RTT_WriteString(0, "Send button state change.\n");
            err_code = ble_btn_on_button0_change(m_conn_handle, &m_btn, data_value, len);
            if (err_code != NRF_SUCCESS &&
                err_code != BLE_ERROR_INVALID_CONN_HANDLE &&
                err_code != NRF_ERROR_INVALID_STATE &&
                err_code != BLE_ERROR_GATTS_SYS_ATTR_MISSING)
            {
                APP_ERROR_CHECK(err_code);
            }
            break;
        case BUTTON1_BUTTON:
            SEGGER_RTT_WriteString(0, "Send button state change.\n");
            err_code = ble_btn_on_button1_change(m_conn_handle, &m_btn, button_action);
            if (err_code != NRF_SUCCESS &&
                err_code != BLE_ERROR_INVALID_CONN_HANDLE &&
                err_code != NRF_ERROR_INVALID_STATE &&
                err_code != BLE_ERROR_GATTS_SYS_ATTR_MISSING)
            {
                APP_ERROR_CHECK(err_code);
            }
            break;
        default:
            APP_ERROR_HANDLER(pin_no);
            break;
    }
}

button_service.c:

uint32_t ble_btn_on_button0_change(uint16_t conn_handle, ble_btn_t * p_btn, uint8_t * button_state, uint16_t len)
{
    ble_gatts_hvx_params_t params;

    memset(&params, 0, sizeof(params));
    params.type   = BLE_GATT_HVX_NOTIFICATION;
    params.handle = p_btn->button0_char_handles.value_handle;
    params.p_data = button_state;
    params.p_len  = len;

    return sd_ble_gatts_hvx(conn_handle, &params);
}

I don't know if there is some external setting that I need to change to send more than one byte. I know this to be the case in some areas as it took me longer than I would like to admit to figure out that `app_button_enable` is required to enable buttons.

If I am totally off base please point me in the right direction.

[howard n2wx]

You're so close.  If you'd have searched for ble_nus_c_string_send() you'd have found it:  There's a ble_nus_c_string_send() in the ble_app_uart_c client example.   But note that you might be better off inspecting the ble_app_uart server example and dropping the NUS server into your button server project. The equivalent server project uses ble_nus_data_send() and can send about 247 bytes at a time.  By dropping into I mean settomg BLE_NUS_ENABLED to 1 in sdk_config.h, adding ble_nus.c service to the project along with other source files that you'll find in the ble_app_uart project that aren't in blinky along with the corresponding init calls..

[LuckyGinger]

I finally found this [1] and this [2] eventually leading me to change this:

    memset(&attr_char_value, 0, sizeof(attr_char_value));

    attr_char_value.p_uuid    = &ble_uuid;
    attr_char_value.p_attr_md = &attr_md;
    attr_char_value.init_len  = sizeof(uint8_t);
    attr_char_value.init_offs = 0;
    attr_char_value.max_len   = sizeof(uint8_t);
    attr_char_value.p_value   = NULL;

To this:

    memset(&attr_char_value, 0, sizeof(attr_char_value));

    attr_char_value.p_uuid    = &ble_uuid;
    attr_char_value.p_attr_md = &attr_md;
    attr_char_value.init_len  = 20;       // <-- changed len to 20
    attr_char_value.init_offs = 0;
    attr_char_value.max_len   = 20;       // <-- changed max len to 20
    attr_char_value.p_value   = NULL;

This seemed to have had the biggest impact on letting me send several bytes at a time. We will see what happens when I need to send several packets. I will definitly look into that BLE_NUS_ENABLED stuff though as 247 bytes at a time would be wildly helpful. My current understanding says the nrf52840 is only able to send 20 byte packets.

It has been a pretty steep learning curve for me. With your comment, I have become aware of the sdk_config.h file and its importance.

[1] https://devzone.nordicsemi.com/f/nordic-q-a/6395/error-using-the-sd_ble_gatts_hvx

[2] https://devzone.nordicsemi.com/f/nordic-q-a/4255/how-to-send-20-bytes-using-sd_ble_gatts_hvx

[howard n2wx]

What you're seeing is the classic BLE maximum packet size (MTU in BLE lingo). Larger MTUs are supported by your part when they're negotiated at connection time.

This thread https://devzone.nordicsemi.com/f/nordic-q-a/33541/negotiating-max-mtu-size addresses your part, sdk15, and negotiating the larger MTU..

Without question it's a very steep learning curve. I've been really happy with the nrf52 environment though because the docs are terrific and the sdk very well thought out so even though there's a lot to learn it's rapidly rewarded. Every day is like getting a shot of dopamine. Bet you'll have a similar experience.

[awneil]

howard n2wx said:

There's a ble_nus_c_string_send() in the ble_app_uart_c client example

IIRC, the name is misleading:

In 'C', a string is taken to mean an array of characters terminated by NUL.

The ble_nus_c_string_send() function it is not a string send - it just sends a buffer of entirely arbitrary bytes, and takes a second parameter to specify the length.