connect interval

Question

SDK： nRF5_SDK_11.0.0
Softdevice： s132_nrf52_2.0.0_softdevice 
 #define MIN_CONN_INTERVAL MSEC_TO_UNITS(20, UNIT_1_25_MS) /**< Minimum acceptable connection interval (20 ms), Connection interval uses 1.25 ms units. */
#define MAX_CONN_INTERVAL MSEC_TO_UNITS(20, UNIT_1_25_MS) /**< Maximum acceptable connection interval (75 ms), Connection interval uses 1.25 ms units. */
 
 In the program I set the maximum minimum connection interval of 20ms, and then use the Android to connect the device, but in my capture of the Broadcast packets：
ADV_CONNECT_REQ - LLData - Interval - 0x0028,
Calculate the connection interval is 50ms, by observing the following data packet found that the connection event interval is 50ms. I know the connection interval decision is the phone, but the phone can set the connection interval beyond the setting range of the slave？

Sigurd · Accepted Answer

Hi, 
 When initializing the Connection Parameters module you have the option to decide if a failed connection parameters update shall cause an automatic disconnection or not. This is done by setting the disconnect_on_fail flag. A connection parameters update is here considered failed if the central and peripheral have not agreed upon the connection parameters after MAX_CONN_PARAMS_UPDATE_COUNT number of connection parameter update attempts. 
 Code snippet: 
 /**@brief Function for initializing the Connection Parameters module.
 */
static void conn_params_init(void)
{
 ret_code_t err_code;
 ble_conn_params_init_t cp_init;

 memset(&cp_init, 0, sizeof(cp_init));

 cp_init.p_conn_params = NULL;
 cp_init.first_conn_params_update_delay = FIRST_CONN_PARAMS_UPDATE_DELAY;
 cp_init.next_conn_params_update_delay = NEXT_CONN_PARAMS_UPDATE_DELAY;
 cp_init.max_conn_params_update_count = MAX_CONN_PARAMS_UPDATE_COUNT;
 cp_init.start_on_notify_cccd_handle = m_hrs.hrm_handles.cccd_handle;
 cp_init.disconnect_on_fail = false;
 cp_init.evt_handler = on_conn_params_evt;
 cp_init.error_handler = conn_params_error_handler;

 err_code = ble_conn_params_init(&cp_init);
 APP_ERROR_CHECK(err_code);
}
 
 Note that you will get a BLE_CONN_PARAMS_EVT_FAILED event after a failed connection parameters update. This allows you to directly disconnect, and/or take other actions in the Connection Parameters event handler, even if you have set the disconnect_on_fail flag to false. 
 Code snippet: 
 * @param[in] p_evt Event received from the Connection Parameters Module.
 */
static void on_conn_params_evt(ble_conn_params_evt_t * p_evt)
{
 ret_code_t err_code;

 if (p_evt->evt_type == BLE_CONN_PARAMS_EVT_FAILED)
 {
 err_code = sd_ble_gap_disconnect(m_conn_handle, BLE_HCI_CONN_INTERVAL_UNACCEPTABLE);
 APP_ERROR_CHECK(err_code);
 }
}