Why is the event length of the BLE Central HRS demo set to such a high value of 320*1.25ms = 400ms?
The demo seems to work ok with the BLE Peripheral HRS demo using the default value (BLE_GAP_EVENT_LENGTH_DEFAULT) of 3*1.25ms = 4.75ms.
What is the ATT MTU size's dependency on event length?
// Configure the maximum event length.
memset(&ble_cfg, 0x00, sizeof(ble_cfg));
ble_cfg.conn_cfg.conn_cfg_tag = CONN_CFG_TAG;
ble_cfg.conn_cfg.params.gap_conn_cfg.event_length = 320;
ble_cfg.conn_cfg.params.gap_conn_cfg.conn_count = BLE_GAP_CONN_COUNT_DEFAULT;
err_code = sd_ble_cfg_set(BLE_CONN_CFG_GAP, &ble_cfg, ram_start);
APP_ERROR_CHECK(err_code);