最大连接间隔

Hello,
I'm using PRO board and SDK 5.0.4. I'd like to set a large connection interval to reduce power consumption. Based on ble_peripheral project, I change parameters in user _config.h. When I change the interval to around 300ms, it works and I can see the waveform using powerprofiler. However, when I set the connection interval further to around 3.6~4.0s which is the limit according to BLE specification, the actual interval is still 30ms. This is my user.config.h: (I didn't change other places)

/ *
****************************************************************************************
*
* GAPM configuration
*
****************************************************************************************
*/
static const struct gapm_configuration user_gapm_conf = {
/// Device Role: Central, Peripheral, Observer or Broadcaster
.role = GAP_PERIPHERAL_SLV,

/// Device IRK used for resolvable random BD address generation (LSB first)
.irk = {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00},

/// Device Appearance (0x0000 - Unknown appearance)
//根据填写https://developer.bluetooth.org/gatt/characteristics/Pages/Characteristi...
.appearance = 0,

/// Device Appearance write permission requirements for peer device (@see gapm_write_att_perm)
.appearance_write_perm = GAPM_WRITE_DISABLE,

/// Device Name write permission requirements for peer device (@see gapm_write_att_perm)
.name_write_perm = GAPM_WRITE_DISABLE,

/// maximal mtu.
.max_mtu = 23，

///预期的连接间隔Shoud为3.5s
/// Peripheral only: *****************************************************************
/// Slave preferred Minimum of connection interval measured in ble double slots (1.25ms)
///使用宏观MS_TO_DOUBLESLOTS从毫秒（MS）转换为双插槽
.con_intv_min = MS_TO_DOUBLESLOTS(3200),

/// Slave preferred Maximum of connection interval measured in ble double slots (1.25ms)
///使用宏观MS_TO_DOUBLESLOTS从毫秒（MS）转换为双插槽
.con_intv_max = ms_to_doubleslots（3800），

/// Slave preferred Connection latency. It is measured in connection events skipped
.con_latency = 0,

/// Slave preferred Link supervision timeout measured in timer units (10 ms)
/// use the macro MS_TO_TIMERUNITS to convert from milliseconds (ms) to timer units
.superv_to = MS_TO_TIMERUNITS(8000),

///隐私设置位字段（0b1 =启用，0b0 =禁用）
/// - [位0]：隐私支持
/// - [bit 1]: Multiple Bond Support (Peripheral only); If enabled, privacy flag is
/// read only.
/// - [位2]：重新连接地址可见。
.flags = 0
};

/ *
****************************************************************************************
*
*参数更新配置
*
****************************************************************************************
*/
static const struct connection_param_configuration user_connection_param_conf = {
///预期的连接间隔Shoud为3.5s
/// Connection interval minimum measured in ble double slots (1.25ms)
///使用宏观MS_TO_DOUBLESLOTS从毫秒（MS）转换为双插槽
.intv_min = MS_TO_DOUBLESLOTS(3200),

/// Connection interval maximum measured in ble double slots (1.25ms)
///使用宏观MS_TO_DOUBLESLOTS从毫秒（MS）转换为双插槽
.intv_max = MS_TO_DOUBLESLOTS(3800),

/// Latency measured in connection events
.latency = 0,

/// Supervision timeout measured in timer units (10 ms)
/// use the macro MS_TO_TIMERUNITS to convert from milliseconds (ms) to timer units
.time_out = MS_TO_TIMERUNITS(5000),

/// Minimum Connection Event Duration measured in ble double slots (1.25ms)
///使用宏观MS_TO_DOUBLESLOTS从毫秒（MS）转换为双插槽
.ce_len_min = MS_TO_DOUBLESLOTS(0),

/// Maximum Connection Event Duration measured in ble double slots (1.25ms)
///使用宏观MS_TO_DOUBLESLOTS从毫秒（MS）转换为双插槽
.ce_len_max = MS_TO_DOUBLESLOTS(0),
};

My question is 1) What is the longest allowed connection interval of Dialog chip? (4s?) and 2) Do I need to change other parameters to set a longer interval, say, 3.8s? Thanks!