NCS how to increase NUS max frame size

[Nicolas Brunner]

nRF5340 with NCS 1.4.0.

I'm using the NUS (code derived from central_uart & peripheral_uart sample) and i'm trying to send longer frame.

I have tried to add the following lines in my proj.conf but I was unsuccessful:

CONFIG_BT_DATA_LEN_UPDATE=y
CONFIG_BT_L2CAP_RX_MTU=247
CONFIG_BT_L2CAP_TX_MTU=247
CONFIG_BT_CTLR_TX_BUFFER_SIZE=251
CONFIG_BT_CTLR_DATA_LENGTH_MAX=251

What should i do to allow frame of 251 bytes ?

Edited subject: NCS how to enable DLE -> NCS how to increase NUS max frame size

[Hung Bui]

Hi Nicolas, 

The data length update should be executed automatically. 
Could you please check in your autoconf.h do you have:
#define CONFIG_BT_AUTO_DATA_LEN_UPDATE 1
#define CONFIG_BT_DATA_LEN_UPDATE 1

I would suggest to try testing with our throughput example. 

You can try capture a sniffer trace to make it easier to investigate. 

If you want to do it manually you would need to define CONFIG_BT_USER_DATA_LEN_UPDATE and call bt_conn_le_data_len_update() to execute. 

[Nicolas Brunner]

My NUS code is now working with longer frame. Here is the modification I have done to make this works:

Using hci_rpmsg with the added conf:

CONFIG_BT_CTLR_TX_BUFFER_SIZE=251
CONFIG_BT_CTLR_DATA_LENGTH_MAX=251
CONFIG_BT_MAX_CONN=8

Added to central and peripheral conf:

CONFIG_BT_L2CAP_TX_MTU=247
CONFIG_BT_L2CAP_RX_MTU=247

Added a call to bt_gatt_exchange_mtu() in the central code:

static void exchange_func(struct bt_conn *conn, uint8_t att_err,
        struct bt_gatt_exchange_params *params)
{
    struct bt_conn_info info = {0};
    int err;

    LOG_INF("MTU exchange %s", att_err == 0 ? "successful" : "failed");

    err = bt_conn_get_info(conn, &info);
    if (err) {
        LOG_ERR("Failed to get connection info %d", err);
    }
}

static void discovery_complete(struct bt_gatt_dm *dm, void *context)
{
    struct bt_nus_client *nus = context;
    LOG_INF("Service discovery completed");

    bt_gatt_dm_data_print(dm);

    bt_nus_handles_assign(dm, nus);
    bt_nus_subscribe_receive(nus);

    bt_gatt_dm_data_release(dm);

    exchange_params.func = exchange_func;

    int err = bt_gatt_exchange_mtu(default_conn, &exchange_params);
    if (err) {
        LOG_ERR("MTU exchange failed (err %d)", err);
    } else {
        LOG_INF("MTU exchange pending");
    }
}

[Nicolas Brunner]

In my autoconf.h the following are defined:
#define CONFIG_BT_AUTO_DATA_LEN_UPDATE 1
#define CONFIG_BT_DATA_LEN_UPDATE 1

But the max data len was only updated by calling bt_gatt_exchange_mtu(). Is this normal ?

[Hung Bui]

Please make sure you don't have CONFIG_BT_USER_DATA_LEN_UPDATE=y defined anywhere. 

[Nicolas Brunner]

CONFIG_BT_USER_DATA_LEN_UPDATE is not defined but I still need to call bt_gatt_exchange_mtu().

[Hung Bui]

Please note that bt_gatt_exchange_mtu() is to exchange the MTU, not to exchange data length support. They are 2 different things :) 

[Nicolas Brunner]

Ok, thanks. So is manually calling bt_gatt_exchange_mtu() the correct thing to do for increasing the MTU ?

[Hung Bui]

Yes, you can find that we called it in the throughput example. 

[Nicolas Brunner]

Ok, thank you for all the help.