Unable to send more than 4 bytes of data over BLE

Question

Hi,

I am trying to create a custom BLE service.

Following is the main code:

            err_code = ble_cus_flash_update(&m_cus, m_rx_buf, m_conn_handle);
            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);
            }

These are my BLE write functions:

    uint8_t flash_init_value [6] = {0};

    memset(&add_char_params, 0, sizeof(add_char_params));
    add_char_params.uuid              = FLASH_UUID;
    add_char_params.uuid_type         = p_cus->uuid_type;

    add_char_params.init_len          = 6;// (in bytes)
    add_char_params.max_len           = 6;
    add_char_params.p_init_value      = flash_init_value;

    add_char_params.char_props.read   = 1;
    add_char_params.char_props.notify = 1;

    add_char_params.read_access       = SEC_OPEN;
    add_char_params.cccd_write_access = SEC_OPEN;

    err_code = characteristic_add(p_cus->service_handle,
                                  &add_char_params,
                                  &p_cus->flash_handles);
    if (err_code != NRF_SUCCESS)
    {
        return err_code;
    }

    return NRF_SUCCESS;

uint32_t ble_cus_flash_update(ble_cus_t * p_cus, uint8_t * flash_data_byte, uint16_t conn_handle)
{
    ble_gatts_hvx_params_t params;
    uint16_t len = sizeof(flash_data_byte);

    memset(&params, 0, sizeof(params));
    params.type   = BLE_GATT_HVX_NOTIFICATION;
    params.handle = p_cus->flash_handles.value_handle;
    params.p_data = flash_data_byte;
    params.p_len  = &len;

    return sd_ble_gatts_hvx(conn_handle, &params);
}

My sdk_config file says MAX_MTU is defined 23

// <o> NRF_SDH_BLE_GAP_EVENT_LENGTH - GAP event length. 
// <i> The time set aside for this connection on every connection interval in 1.25 ms units.

#ifndef NRF_SDH_BLE_GAP_EVENT_LENGTH
#define NRF_SDH_BLE_GAP_EVENT_LENGTH 6
#endif

// <o> NRF_SDH_BLE_GATT_MAX_MTU_SIZE - Static maximum MTU size. 
#ifndef NRF_SDH_BLE_GATT_MAX_MTU_SIZE
#define NRF_SDH_BLE_GATT_MAX_MTU_SIZE 23
#endif

// <o> NRF_SDH_BLE_GATTS_ATTR_TAB_SIZE - Attribute Table size in bytes. The size must be a multiple of 4. 
#ifndef NRF_SDH_BLE_GATTS_ATTR_TAB_SIZE
#define NRF_SDH_BLE_GATTS_ATTR_TAB_SIZE 1408
#endif

I am using nrf Connect to debug BLE, which gives me output like this:

0x A1-A2-A3-A4-00-00

I am able to send 4 bytes successfully, but the rest of the values are not updating.
Any help would be really appreciated.

Regards,
Anurag

Einar Thorsrud · Accepted Answer

Hi Anurag, 
 The problem here is that the size you specify is always 4. See this part of your code: 
 uint32_t ble_cus_flash_update(ble_cus_t * p_cus, uint8_t * flash_data_byte, uint16_t conn_handle)
{
 ble_gatts_hvx_params_t params;
 uint16_t len = sizeof(flash_data_byte); 
 Here the len variable is set to the sizeof a pointer, which is always 4 byte. This is not what you want. There is no way to know the length of the input in this function, so you must pass the length as an additional parameter.

Unable to send more than 4 bytes of data over BLE

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