Receive regular Ble packets from multiple peripherals

[F_Vahid]

Dear Nordic support team,

We are currently working on a motion tracking project that involves sending quaternion matrices from 8 individual devices (custom boards with nRF52840) to a central device (nRF52840 dongle). The peripherals are programmed to send packets every 16.8ms (59.5 Hz refresh rate), and the central device receives these packets and prints them on the com port using the USB CDC ACM module. However, we have noticed that when only one device sends data, the data on the com port is regular, with delta_t~=15ms. But when multiple devices send data, the received data (two successive packets from one specific device) on the com port is irregular, and the timestamps vary from 1 to 80ms.

We do not believe that the issue is with the peripheral devices because they are programmed to send data regularly and independently from the central device (which does not send any data to the peripherals). We have also tested the USB module and found that it is rarely the cause of the issue, as it can write test data every 500us on the com port. However, we suspect that the problem is related to the central device's concurrent receive function, which may be affecting its performance.

We are currently using a code for the central device that can be found at https://github.com/NordicPlayground/nrf52-ble-app-uart-c-multilink. We have attached the main and configuration files for both the peripheral and central devices. We would greatly appreciate your help in resolving this issue, as we require the motion data to be received regularly. Is it possible to receive 8 packets every 15ms regularly?

Thank you for your support, help, and time.

Best regards.

[F_Vahid]

Dear susheel

Thank you for your kind and quick response. I understand that post-processing could cause delays in the central device's performance, and we have already removed the post-processing section (see below code). Currently, we are only writing the received packets to a buffer and sending them to the USB module in the ble_nus_c_evt_handler function. However, the irregular delays in receiving data from multiple devices still persist.

static void ble_nus_c_evt_handler(ble_nus_c_t * p_ble_nus_c, ble_nus_c_evt_t const * p_ble_nus_evt)
{
    ret_code_t err_code;
    uint16_t length;
    uint8_t dat_tab[36];
    char *pt;
    int i=0;
    int j=0;

    switch (p_ble_nus_evt->evt_type)
    {
        case BLE_NUS_C_EVT_DISCOVERY_COMPLETE:
            //NRF_LOG_INFO("Discovery complete.");
            err_code = ble_nus_c_handles_assign(p_ble_nus_c, p_ble_nus_evt->conn_handle, &p_ble_nus_evt->handles);
            APP_ERROR_CHECK(err_code);

            err_code = ble_nus_c_tx_notif_enable(p_ble_nus_c);
            APP_ERROR_CHECK(err_code);
            //NRF_LOG_INFO("Connected to device with Nordic UART Service.");
            break;

        
        
        case BLE_NUS_C_EVT_NUS_TX_EVT:
        NRF_LOG_HEXDUMP_DEBUG(p_ble_nus_evt->p_data, p_ble_nus_evt->data_len);
        memcpy(m_nus_data_array, p_ble_nus_evt->p_data, p_ble_nus_evt->data_len);
        length = p_ble_nus_evt->data_len;


        // Send readable data through CDC ACM
        char cdc_data[BLE_NUS_MAX_DATA_LEN];
        if(m_nus_data_array[0]<=20)
        {
          sprintf(cdc_data, "%d,%d,%d,%d,%d\r\n", m_nus_data_array[0],m_nus_data_array[1],m_nus_data_array[2],m_nus_data_array[3],m_nus_data_array[4]);
           ret_code_t ret = app_usbd_cdc_acm_write(&m_app_cdc_acm,cdc_data,strlen(cdc_data));
        }
        else if(m_nus_data_array[0]==51){
        //if(length<15)
        //{
        //sprintf(cdc_data, "%d,%d,%d\r\n%d,%d,%d\r\n%d,%d,%d\r\n%d,%d,%d\r\n%d,%d,%d\r\n",m_nus_data_array[0],m_nus_data_array[1]-100,m_nus_data_array[6]-125,
        //m_nus_data_array[0],m_nus_data_array[2]-100,m_nus_data_array[7]-125,m_nus_data_array[0],m_nus_data_array[3]-100,m_nus_data_array[8]-125,
        //m_nus_data_array[0],m_nus_data_array[4]-100,m_nus_data_array[9]-125,m_nus_data_array[0],m_nus_data_array[5]-100,m_nus_data_array[10]-125);
        //}
        //else
        //{
        //float step_rate = ((float) m_nus_data_array[24])/10;
        //long int step_count = (m_nus_data_array[13])|(m_nus_data_array[14]<<8)|(m_nus_data_array[15]<<16);
        //long int distance = (m_nus_data_array[16])|(m_nus_data_array[17]<<8)|(m_nus_data_array[18]<<16);
        //float speed = (float) m_nus_data_array[19]/10;
        //float calories = ((float) ((m_nus_data_array[21])|(m_nus_data_array[22]<<8)|(m_nus_data_array[23]<<16)))/10;
        //float sdnn = m_nus_data_array[25]*10+((float) m_nus_data_array[26]/10);
        //float rmssd = m_nus_data_array[27]*10+((float) m_nus_data_array[28]/10);
        //float sdsd = m_nus_data_array[29]*10+((float) m_nus_data_array[30]/10);
        //float sd1 = m_nus_data_array[31]*10+((float) m_nus_data_array[32]/10);
        //float sd2 = m_nus_data_array[33]*10+((float) m_nus_data_array[34]/10);
        //float bsi = m_nus_data_array[35]+((float) m_nus_data_array[36]/100);
        
        //sprintf(cdc_data, "%d,%d,%d\r\n%d,%d,%d\r\n%d,%d,%d\r\n%d,%d,%d\r\n%d,%d,%d,%d,%d,%d,%d,%f,%d,%f,%f,%f,%f,%f,%f,%f,%f\r\n",
        //m_nus_data_array[0],m_nus_data_array[1]-100,m_nus_data_array[6]-125,
        //m_nus_data_array[0],m_nus_data_array[2]-100,m_nus_data_array[7]-125,
        //m_nus_data_array[0],m_nus_data_array[3]-100,m_nus_data_array[8]-125,
        //m_nus_data_array[0],m_nus_data_array[4]-100,m_nus_data_array[9]-125,
        //m_nus_data_array[0],m_nus_data_array[5]-100,m_nus_data_array[10]-125,m_nus_data_array[11],m_nus_data_array[12],
        //step_count,distance,speed,m_nus_data_array[20],calories,step_rate,sdnn,rmssd,sdsd,sd1,sd2,bsi);
        //}
        }


        //ret_code_t ret = app_usbd_cdc_acm_write(&m_app_cdc_acm,cdc_data,strlen(cdc_data));
         //ret = app_usbd_cdc_acm_write(&m_app_cdc_acm,cdc_data,strlen(cdc_data));
         //ret = app_usbd_cdc_acm_write(&m_app_cdc_acm,cdc_data,strlen(cdc_data));
         //ret = app_usbd_cdc_acm_write(&m_app_cdc_acm,cdc_data,strlen(cdc_data));
         //ret = app_usbd_cdc_acm_write(&m_app_cdc_acm,cdc_data,strlen(cdc_data));
        
        //if(ret != NRF_SUCCESS)
        //{
        //    //NRF_LOG_INFO("CDC ACM unavailable, data received: %s", m_nus_data_array);
        //}
            break;


        case BLE_NUS_C_EVT_DISCONNECTED:
            //NRF_LOG_INFO("Disconnected.");
            scan_start();
            break;
    }
}

Could there be any problems in the BLE configuration on either the central or peripheral side that could be causing this issue? Is the ble_nus_c_evt_handler function the best place for writing data to the USB port?

Any help you can provide in resolving this issue would be greatly appreciated. Thank you again for your support and assistance.