DLE doesn't seem to be working in hci_uart sample

[Matthias Ringwald]

Hi there

I'm using the HCI UART bridge from NCS v3.2.1 in zephyr/samples/bluetooth/hci_uart with the provided overlay-all-bt_ll_sw_split.conf on an nRF54L15Dk and our Bluetooth Host stack: https://github.com/bluekitchen/btstack/)

While doing throughput tests with a 200 bytes MTU over GATT against the PTS / BL654 with PacketCraft Controller,  I've wondered about the lower-than-expected data-rate.

Inspection of the HCI log of the nRF54l15 as Peripheral shows that the DLE is enabled in general as the SoftDevice reports TX Len 251 and TX Time 2704 us for the HCI Read Maximum Data Length.

We then use the LE Write Suggested Default Data Length with these values to set them as default. However, there's no HCI Event Data Length Change event later when the connection gets established.

Following the https://academy.nordicsemi.com/courses/bluetooth-low-energy-fundamentals/lessons/lesson-3-bluetooth-le-connections/topic/blefund-lesson-3-exercise-2/ I was also sending the HCI LE Set Data Length Command although that should not be necessary due to the earlier LE Write Suggested Default Data Length.

As a comparison, I've replaced the nRF54L15 with a second PTS dongle, where DLE gets activated as expected.

I've attached two zips with logs: one with PTS -> PTS and one with PTS -> nRF54L15. In the zips are the HCI trace as .pklg for Wireshark as well as the console log which also shows how to compile and run our examples. I've also include the .config File from the west built. 

Why is DLE not working here? Do you have an HCI trace that shows a working DLE setup for comparison?

Thanks & best regards,

 Matthias

2026-01-26-DLE-PTS-nRF54L15-SoftDevice.zip

2026-01-26-DLE-PTS-PTS.zip

[Hung Bui]

Hi Matthias, 

Could you explain why you use overlay-all-bt_ll_sw_split.conf when you want to use softdevice controller? My understanding is that CONFIG_BT_LL_SW_SPLIT=y will select Zephyr controller, not softdevice controller. 

My suggestion is to test the nRF54L as a stand alone BLE device first and verify that it can do DLE before you move to the hci_uart. 
Have you tried the sample \nrf\samples\bluetooth\throughput ? I believe it should have the DLE covered. 

[Matthias Ringwald]

Thanks for your quick reply.

---

I have tested the Throughput example and made two observations:

1. DLE works as expected (at least according to throughput numbers and the callback)
2.  the le_data_len_updated callback is not called on the peripheral side.

I see this on central

==== Starting throughput test ====
PHY update pending
LE PHY updated: TX PHY LE 1M, RX PHY LE 1M
LE Data length update pending
LE data len updated: TX (len: 27 time: 2120) RX (len: 27 time: 2120)

And this on peripheral

LE PHY updated: TX PHY LE 1M, RX PHY LE 1M

==============...

While this doesn't have an effect on the throughput, it makes debugging the DLE setup harder.
It looks like the missing callback might be an issue in the SoftDevice Controller.

Please check attached log files. Do you agree that the callback should have been called?

2026-01-29-Throughput.zip

---

Back to the HCI UART example. I did use overlay-all-bt_ll_sw_split.conf in the hope to enable all features. Let's ignore this configuration for now.

We use an extended version of the HCI UART example that supports an additional command for time sync between the Host and the Controller. The project is here: https://github.com/bluekitchen/hci_uart_iso_timesync

I've attached new log files as well as the .config file from build/hci_uart_iso_timesync/.config which shows that 

CONFIG_BT_CTLR_DATA_LENGTH_MAX=251
CONFIG_BT_CTLR_DATA_LENGTH=y

2026-01-29-DLE-PTS-nRF54L15-SoftDevice.zip

During stack startup, DLE seems to be supported, but nothing happens. What's wrong / missing?
Any suggestions to get more insight or how to find the difference between your Throughput example and our tests using HCI UART?

Thanks, 
Matthias

  

[Hung Bui]

Hi Matthias, 
Could you try with CONFIG_BT_USER_DATA_LEN_UPDATE=y  ? 
I have made a blog with example here: Building a Bluetooth application on nRF Connect SDK - Part 3 Optimizing the connection 
In the example we request data length update from the device's side. 

It's a bit out of date (SDKv2.5.2) but if you have a nRF52 DK you can test with that. Or you can port it to NCS v3.x to test. 

[Matthias Ringwald]

Hi Hung

1. I've tried adding CONFIG_BT_USER_DATA_LEN_UPDATE=y and got the following error/warning:

warning: BT_USER_DATA_LEN_UPDATE (defined at subsys/bluetooth/host/Kconfig:407,
subsys/bluetooth/host/Kconfig:407) was assigned the value 'y' but got the value 'n'. Check these
unsatisfied dependencies: ((BT_DATA_LEN_UPDATE && BT_CONN && BT_HCI_HOST && BT_RPC_STACK) ||
(BT_DATA_LEN_UPDATE && BT_CONN && BT_HCI_HOST && BT_HCI && BT)) (=n). See
docs.zephyrproject.org/.../kconfig.html and/or look up
BT_USER_DATA_LEN_UPDATE in the menuconfig/guiconfig interface. 

This matches my expectation as we don't use Zephyr Bluetooth Host. Instead, in the HCI UART bridge,  BT_HCI_RAW is defined for direct HCI access..

--

2. Thanks for the link to your older blog post. It's also from the view of an application using Zephyr's host stack like the one I've linked to Nordic academy.  As reported, DLE works with the Throughput example, but not with similar KConfig flags in the HCI UART project.

Going back to my previous question: is my observation correct that there's no DLE update callback when the device in peripheral role?

--

3. How can we debug the SoftDevice controller to find out why it isn't starting and/or responding to DLE updates when used in the HCI UART or our "hci_uart_iso_timesync" project?

Thanks,
 Matthias

[Hung Bui]

Hi Matthias, 
I found that I had a very similar ticket exactly a year ago. 
Could you take a look at this: https://devzone.nordicsemi.com/support-private/support/338248#permalink=

Do you have CONFIG_BT_ISO_* enabled ?  In the ticket, if CONFIG_BT_ISO is not enabled DLE worked just fine. If CONFIG_BT_ISO_* is enabled, the DLE max length is set to 27 and the following configuration needed: 

CONFIG_BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT_OVERRIDE=y

CONFIG_BT_CTLR_SDC_MAX_CONN_EVENT_LEN_DEFAULT=5000 

I have provided samples in the ticket that you can try (NCS v2.9.0). 

The ticket was initially private, and some info has been redacted before it's converted to public ticket, I would suggest to go straight to the last replies. 

[Adrian Eggenberger]

Hi Matthias

I followed this issue and now an old ticked of me is referenced...  :-)

For us the two mentioned CONFIG_BT_CTLR_SDC_MAX_CONN_EVENT_LEN... kconfig options solved the problems with data length extension. We still use these settings within our solution (uses NCS v3.1.1).

Regards Adrian

[Matthias Ringwald]

Hi Hung.

Thanks for the answer and pointing to the older ticket.

After adding these two KConfig options, the throughput for bi-directional data at 1M-Phy jumped from 50 kB/s to 80 kB/s matching our expectations! 

As I'm not the only one struggling with this, I think it would be helpful to promote a "All batteries included" KConfig setup which enables all features of the SoftDevice for the HCI UART example/use-case as there's no need to conserve memory to save.

I find it much easier to disable a feature I don't need then to figure out what's needed to enable it. 

Anyways thanks again for helping us getting this sorted out. We'll try to keep our HCI UART example up-to-date.

Best,
  Matthias 

[Matthias Ringwald]

Hi Adrian

Well, then thanks for bringing this up last year. I've just verified that it works on NCS v3.2.1.

Best regards,
 Matthias