Connection Interval

[Filya]

Hi everyone,

I'm new to working with Bluetooth/

My problem is that the connection interval time sent by the central device does not converge with the time I see using an oscilloscope or a configured timer for 1 ms.
The principle of operation of my program is that the peripheral device sleeps for a certain time and wakes up at a specified interval. The central device at this time is trying to connect to it. Using the timer, I count the time from when the device woke up to the time of the BLE_GAP_EVT_CONNECTED event and to the time of the BLE_GAP_EVT_CONN_PARAM_UPDATE event. The time difference between these events is about 600 ms. I can not understand why such a difference in time. Maybe I did not correctly understand the meaning of the parameters MIN_CONN_INTERVAL and MAX_CONN_INTERVAL? If so, then how can you reduce the time for a full connection, so that the BLE_GAP_EVT_CONN_PARAM_UPDATE event occurs as soon as possible.

To check what the actual value is transmitted by the central device, you can make a breakpoint during the execution of the program at the time of the BLE_GAP_EVT_CONN_PARAM_UPDATE event, which I actually did.

According to the received data, the central device set the value of the maximum and minimum interval of 7.5 ms.In the peripheral device, I have the following parameters:

MIN_CONN_INTERVAL    MSEC_TO_UNITS (7.5, UNIT_1_25_MS)

MAX_CONN_INTERVAL   MSEC_TO_UNITS (30, UNIT_1_25_MS)

SLAVE_LATENCY            1

CONN_SUP_TIMEOUT   MSEC_TO_UNITS (250, UNIT_10_MS)

I also read that the connection interval can vary from 7.5 ms to 4 s, maybe this value is the connection interval for my central device?

Chip: BT840F (analog nRF52840);

SDK: nRF5_SDK_15.2.0;

Softdevice: s132_nrf52_6.1.0_softdevice;

Phone: Xiaomi Redmi 5+.

Thanks in advance for your help.

[Simonr]

Hi

If the device needs 11.25 ms to complete the connection event you should set MIN_CONN_INTERVAL = 11.25 ms so that the device doesn't try connecting at times lower than possible. This should make your application connect on the first try.

Next, I'm still struggling to understand what this 500-800 ms delay is. Do you have any screenshot or sniffer log you can upload that might be helpful?

Best regards,

Simon

[Filya]

Hi

I set MIN_CONN_INTERVAL = 11.25 ms and it did not give results. Also, the first time it hits the BLE_GAP_EVT_CONN_PARAM_UPDATE event, it shows 7.5 ms.

As for the demonstration, the following is the event handler code:

static void ble_evt_handler(ble_evt_t const * p_ble_evt, void * p_context)
{
  ret_code_t err_code = NRF_SUCCESS;
  
  gap_evt_1 = p_ble_evt->header.evt_id;
  
  switch (p_ble_evt->header.evt_id)
  {
  case BLE_GAP_EVT_DISCONNECTED:
    {
      NRF_LOG_INFO("Disconnected.");
      app_timer_stop(m_our_char_timer_id);
      time_conn[3] = time_conn[0] - (time_conn[1]+time_conn[2]);
      time_conn[0] = 0;
      NRF_LOG_INFO("CONNECTED = %d, CONN_PARAM_UPDATE = %d, DISCONNECTED = %d", time_conn[1], time_conn[2], time_conn[3]);
    } break;
    
  case BLE_GAP_EVT_CONNECTED:
    {
      NRF_LOG_INFO("Connected.");
      err_code = bsp_indication_set(BSP_INDICATE_CONNECTED);
      APP_ERROR_CHECK(err_code);
      
      min_int = p_ble_evt->evt.gap_evt.params.connected.conn_params.min_conn_interval;
      max_int = p_ble_evt->evt.gap_evt.params.connected.conn_params.max_conn_interval;
      NRF_LOG_INFO("Min int = %d, Max int = %d", min_int, max_int);     
      
      m_conn_handle = p_ble_evt->evt.gap_evt.conn_handle;
      err_code = nrf_ble_qwr_conn_handle_assign(&m_qwr, m_conn_handle);
      APP_ERROR_CHECK(err_code);
      
      time_conn[1] = time_conn[0];
    } break;
    
  case BLE_GAP_EVT_CONN_PARAM_UPDATE:
    {  
      
      min_int_upd = p_ble_evt->evt.gap_evt.params.conn_param_update.conn_params.min_conn_interval;
      max_int_upd = p_ble_evt->evt.gap_evt.params.conn_param_update.conn_params.max_conn_interval;
      slave_conn_upd = p_ble_evt->evt.gap_evt.params.conn_param_update.conn_params.slave_latency;
      conn_sup_upd = p_ble_evt->evt.gap_evt.params.conn_param_update.conn_params.conn_sup_timeout;
      
      NRF_LOG_INFO("Min int upd = %d, Max int upd = %d, Slave latency = %d, Conn sup time = %d", min_int_upd, max_int_upd, slave_conn_upd, conn_sup_upd);
      app_timer_start(m_our_char_timer_id, OUR_CHAR_TIMER_INTERVAL, NULL);
      if (disc_evt == 0)
      {
        NRF_LOG_INFO("start conn"); 
        time_conn[2] = time_conn[0] - time_conn[1];
      }
    } break;
    
  case BLE_GAP_EVT_PHY_UPDATE_REQUEST:
    {
      NRF_LOG_DEBUG("PHY update request.");
      ble_gap_phys_t const phys =
      {
        .rx_phys = BLE_GAP_PHY_AUTO,
        .tx_phys = BLE_GAP_PHY_AUTO,
      };
      err_code = sd_ble_gap_phy_update(p_ble_evt->evt.gap_evt.conn_handle, &phys);
      APP_ERROR_CHECK(err_code);
    } break;
    
  case BLE_GATTC_EVT_TIMEOUT:
    {
      // Disconnect on GATT Client timeout event.
      NRF_LOG_DEBUG("GATT Client Timeout.");
      err_code = sd_ble_gap_disconnect(p_ble_evt->evt.gattc_evt.conn_handle,
                                       BLE_HCI_REMOTE_USER_TERMINATED_CONNECTION);
      APP_ERROR_CHECK(err_code);
      break;
      
    case BLE_GATTS_EVT_TIMEOUT:
      // Disconnect on GATT Server timeout event.
      NRF_LOG_DEBUG("GATT Server Timeout.");
      err_code = sd_ble_gap_disconnect(p_ble_evt->evt.gatts_evt.conn_handle,
                                       BLE_HCI_REMOTE_USER_TERMINATED_CONNECTION);
      APP_ERROR_CHECK(err_code);
    } break;
    
  default:
    // No implementation needed.
    break;
  }
}

And a screenshot from the terminal:

1) The parameters of the minimum and maximum interval of the connection event recorded at the time of contact with the event BLE_GAP_EVT_CONNECTED.
2) The parameters of the minimum and maximum interval of the connection event recorded at the time of contact with the event BLE_GAP_EVT_CONN_PARAM_UPDATE. Here it is clear that there were 2 hits in this event with different parameters.
3) Here are the times in ms. The time is shown from the moment the device woke up and hit every event (BLE_GAP_EVT_CONNECTED, BLE_GAP_EVT_CONN_PARAM_UPDATE, BLE_GAP_EVT_DISCONNECTED).

The parameters of the minimum and maximum interval, which I set:

#define MIN_CONN_INTERVAL   MSEC_TO_UNITS(11.25, UNIT_1_25_MS)
#define MAX_CONN_INTERVAL   MSEC_TO_UNITS(12, UNIT_1_25_MS)
#define SLAVE_LATENCY       1
#define CONN_SUP_TIMEOUT    MSEC_TO_UNITS(350, UNIT_10_MS)

I also decided to check on the source code of the example, which I took as a basis. The results are similar, just a few occurrences.

1 option:

#define MIN_CONN_INTERVAL               MSEC_TO_UNITS(500, UNIT_1_25_MS)  
#define MAX_CONN_INTERVAL               MSEC_TO_UNITS(1000, UNIT_1_25_MS)           
#define SLAVE_LATENCY                   0                                          
#define CONN_SUP_TIMEOUT                MSEC_TO_UNITS(4000, UNIT_10_MS)

2 option:

#define MIN_CONN_INTERVAL               MSEC_TO_UNITS(48.75, UNIT_1_25_MS)           
#define MAX_CONN_INTERVAL               MSEC_TO_UNITS(1000, UNIT_1_25_MS)           
#define SLAVE_LATENCY                   0                                          
#define CONN_SUP_TIMEOUT                MSEC_TO_UNITS(4000, UNIT_10_MS)