Enable notifications for NUS and another service in BLE Central device avoiding NRF_ERROR_BUSY

RE: Enable notifications for NUS and another service in BLE Central device avoiding NRF_ERROR_BUSY

Bruno Santamaria — Thu, 31 May 2018 13:41:37 GMT

I think that might be it. I replaced ble_nus_c.c from the SDK with a modified copy which includes the message-buffer technique used in the other services. So far, it works, with no undesired effect.

I attach the code to this message, for anyone having the same problem. I think it would be reasonable to patch it as well in the SDK if no one reports any side-effect.

devzone.nordicsemi.com/.../ble_5F00_nus_5F00_c.c

RE: Enable notifications for two services in BLE Central device avoiding NRF_ERROR_BUSY

MartinBL — Thu, 31 May 2018 07:53:45 GMT

You might be on to something here. It seems like there is a chance of GATTC write collisions in the NUS client implementation.

I would recommend implementing your own version with a buffer.

RE: Enable notifications for two services in BLE Central device avoiding NRF_ERROR_BUSY

Bruno Santamaria — Wed, 30 May 2018 14:37:24 GMT

Thank you for your answer, Martin. You understood it correctly.

I've taken a look at the example you mention, although for SDK 12.3.0. I've seen that for the two services (HRS and BAS) it uses a buffer that waits for the BLE_GATTC_EVT_WRITE_RSP response from the SD. It is programmed independently inside the central-service c-files (ble_bas_c.c and ble_hrs_c.c). It looks great and it's independent of the configuration of main.c.

It seems that my problem lays on the NUS central service. Its structure differs from the one of HRS, BAS and the rest of standard service and doesn't implement the buffer to enable CCCDs. Do you know if there is any reason for this?

For example, the cccd_configure functions of NUS...

/**@brief Function for creating a message for writing to the CCCD.
 */
static uint32_t cccd_configure(uint16_t conn_handle, uint16_t cccd_handle, bool enable)
{
    uint8_t buf[BLE_CCCD_VALUE_LEN];

    buf[0] = enable ? BLE_GATT_HVX_NOTIFICATION : 0;
    buf[1] = 0;

    const ble_gattc_write_params_t write_params = {
        .write_op = BLE_GATT_OP_WRITE_REQ,
        .flags    = BLE_GATT_EXEC_WRITE_FLAG_PREPARED_WRITE,
        .handle   = cccd_handle,
        .offset   = 0,
        .len      = sizeof(buf),
        .p_value  = buf
    };

    return sd_ble_gattc_write(conn_handle, &write_params);
}

...and BAS...

/**@brief Function for creating a message for writing to the CCCD.
 */
static uint32_t cccd_configure(uint16_t conn_handle, uint16_t handle_cccd, bool notification_enable)
{
    NRF_LOG_DEBUG("Configuring CCCD. CCCD Handle = %d, Connection Handle = %d\r\n",
                                                            handle_cccd,conn_handle);

    tx_message_t * p_msg;
    uint16_t       cccd_val = notification_enable ? BLE_GATT_HVX_NOTIFICATION : 0;

    p_msg              = &m_tx_buffer[m_tx_insert_index++];
    m_tx_insert_index &= TX_BUFFER_MASK;

    p_msg->req.write_req.gattc_params.handle   = handle_cccd;
    p_msg->req.write_req.gattc_params.len      = WRITE_MESSAGE_LENGTH;
    p_msg->req.write_req.gattc_params.p_value  = p_msg->req.write_req.gattc_value;
    p_msg->req.write_req.gattc_params.offset   = 0;
    p_msg->req.write_req.gattc_params.write_op = BLE_GATT_OP_WRITE_REQ;
    p_msg->req.write_req.gattc_value[0]        = LSB_16(cccd_val);
    p_msg->req.write_req.gattc_value[1]        = MSB_16(cccd_val);
    p_msg->conn_handle                         = conn_handle;
    p_msg->type                                = WRITE_REQ;

    tx_buffer_process();
    return NRF_SUCCESS;
}

What do you recommend? I can't alter the SDK files in order to keep other projects working.

Should I create my own version of ble_nus_c.c using the structure and behavior of BAS and the other services? Would that suffice for this purpose of enabling two CCCDs?

RE: Enable notifications for two services in BLE Central device avoiding NRF_ERROR_BUSY

MartinBL — Wed, 30 May 2018 13:53:48 GMT

Hi,

So if I understand you correctly the problem is that you can't enable two CCCDs back-to-back because you get the busy error when you write to the second CCCD attribute?

I advise you to have a look at the BLE Heart Rate Collector Example. It shows how a central can enable CCCDs on two or more characteristics one after another. Shortly explained, the example sets up a buffer of "messages" to be written and then processes this buffer on every successive BLE_GATTC_EVT_WRITE_RSP event.