RE: How Can I limit Gatt Data bytes that I write? (Not With Zero-Fill)

Sigurd — Mon, 08 May 2017 08:54:50 GMT

You set this in the Attribute metadata(ble_gatts_attr_md_t), which is a part of the GATT Attribute(ble_gatts_attr_t). This is then used when you add a characteristic declaration.

Code snippet:

ble_gatts_attr_md_t attr_md;

attr_md.vlen    = 1;

You can see it used in e.g. the Nordic UART Service when adding the RX characteristic:

static uint32_t rx_char_add(ble_nus_t * p_nus, const ble_nus_init_t * p_nus_init)
{
    ble_gatts_char_md_t char_md;
    ble_gatts_attr_t    attr_char_value;
    ble_uuid_t          ble_uuid;
    ble_gatts_attr_md_t attr_md;

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

    char_md.char_props.write         = 1;
    char_md.char_props.write_wo_resp = 1;
    char_md.p_char_user_desc         = NULL;
    char_md.p_char_pf                = NULL;
    char_md.p_user_desc_md           = NULL;
    char_md.p_cccd_md                = NULL;
    char_md.p_sccd_md                = NULL;

    ble_uuid.type = p_nus->uuid_type;
    ble_uuid.uuid = BLE_UUID_NUS_RX_CHARACTERISTIC;

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

    BLE_GAP_CONN_SEC_MODE_SET_OPEN(&attr_md.read_perm);
    BLE_GAP_CONN_SEC_MODE_SET_OPEN(&attr_md.write_perm);

    attr_md.vloc    = BLE_GATTS_VLOC_STACK;
    attr_md.rd_auth = 0;
    attr_md.wr_auth = 0;
    attr_md.vlen    = 1;

    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  = 1;
    attr_char_value.init_offs = 0;
    attr_char_value.max_len   = BLE_NUS_MAX_RX_CHAR_LEN;

    return sd_ble_gatts_characteristic_add(p_nus->service_handle,
                                           &char_md,
                                           &attr_char_value,
                                           &p_nus->rx_handles);
}

RE: How Can I limit Gatt Data bytes that I write? (Not With Zero-Fill)

Daniel T. Lee — Mon, 08 May 2017 06:16:56 GMT

Thanks for the reply!

Sorry for my misunderstanding, but I don't get it.

The variable length flag that you mean is ble_gattc_write_params_t write_params?
Parameter with options such as write_op, flags, handle, offset, len, p_value when I send data with Central(the one who send Data, writes on Other side's characteristic data),??

RE: How Can I limit Gatt Data bytes that I write? (Not With Zero-Fill)

RK — Sun, 07 May 2017 04:29:49 GMT

make the characteristic variable length and each time you change it, write the actual length. It looks to me like you're using a characterisic without the variable length flag set.