Hello Nordic team,
I've recently upgraded my application from nRF Connect SDK v2.7.0 to v2.8.0, and the previous BLE Disconnect issue has been resolved — thanks again for your support!
However, I am now facing a new issue during DFU over BLE using MCUMgr. While DFU via mobile app works fine, using MCUMgr CLI often leads to the following errors:
E: No available ACL buffers!
E: Unexpected first L2CAP frame
These typically appear right after the log: 'Connection parameters updated' During each iteration.
After several iterations, at a random point, the device stops responding and the Disconnect event is not triggered, leading to failed reconnection attempts from MCUMgr.
Key Differences I observed are:
-
Mobile App DFU: Single connection during file transfer.
-
MCUMgr DFU: Multiple rapid connection/disconnection cycles per session.
This issue did not occur with SDK v2.7.0, and only started after migrating to v2.8.0.
I’m following this MCUMgr DFU process:
https://docs.nordicsemi.com/bundle/ncs-3.0.1/page/nrf/app_dev/bootloaders_dfu/dfu_tools_mcumgr_cli.html
Questions:
-
Are there any changes in buffer handling or controller-side behavior in SDK v2.8.0 that might explain this?
-
Should I tune
CONFIG_BT_BUF_ACL_RX_COUNT,TX_COUNT, or related L2CAP parameters? -
Is there a recommended way to handle these frequent connections/disconnections more robustly?
Logs for reference:
Following is one of the log for the reference: [2025-07-15 18:32:55.416] I: 711689 [SWU]SMP BLE advertising started [2025-07-15 18:32:55.416] I: 711693 [DL]DFU over SMP started [2025-07-15 18:33:10.193] I: 726498 [DL]Current number of connections: 1/1 [2025-07-15 18:33:10.215] I: BLE Connected [2025-07-15 18:33:10.422] I: 726726 [DL]Current number of connections: 0/1 [2025-07-15 18:33:10.438] I: BLE Disconnected: 13 [2025-07-15 18:33:16.322] I: 732607 [DL]Current number of connections: 1/1 [2025-07-15 18:33:16.323] I: BLE Connected [2025-07-15 18:33:16.546] I: 732837 [DL]DFU Image upload started [2025-07-15 18:33:17.370] I: Connection parameters updated. [2025-07-15 18:33:17.370] interval: 9, latency: 0, timeout: 42 [2025-07-15 18:33:25.985] I: 742273 [DL]Image 0 download completed successfully [2025-07-15 18:33:26.012] I: 742296 [DL]Current number of connections: 0/1 [2025-07-15 18:33:26.012] I: 742301 [DL]DFU-BLE-Disconnected : 19 [2025-07-15 18:33:26.012] I: BLE Disconnected: 13 [2025-07-15 18:33:38.259] I: 754544 [DL]Current number of connections: 1/1 [2025-07-15 18:33:38.259] I: BLE Connected [2025-07-15 18:33:38.483] I: 754775 [DL]DFU Image upload started [2025-07-15 18:33:39.308] I: Connection parameters updated. [2025-07-15 18:33:39.308] interval: 9, latency: 0, timeout: 42 [2025-07-15 18:34:20.727] I: 797016 [DL]Image 1 download completed successfully [2025-07-15 18:34:20.749] I: 797035 [DL]Current number of connections: 0/1 [2025-07-15 18:34:20.750] I: 797039 [DL]DFU-BLE-Disconnected : 19 [2025-07-15 18:34:20.750] I: BLE Disconnected: 13 [2025-07-15 18:34:27.892] I: 804177 [DL]Current number of connections: 1/1 [2025-07-15 18:34:27.892] I: BLE Connected [2025-07-15 18:34:28.115] I: 804417 [DL]DFU Image upload started [2025-07-15 18:34:28.957] I: Connection parameters updated. [2025-07-15 18:34:28.957] interval: 9, latency: 0, timeout: 42 [2025-07-15 18:34:28.957] E: No available ACL buffers! [2025-07-15 18:34:28.973] E: Unexpected first L2CAP frame [2025-07-15 18:38:53.691] I: 1069979 [DL]Image 2 download completed successfully [2025-07-15 18:38:53.691] I: 1069997 [DL]Current number of connections: 0/1 [2025-07-15 18:38:53.711] I: 1070001 [DL]DFU-BLE-Disconnected : 19 [2025-07-15 18:38:53.711] I: BLE Disconnected: 13 [2025-07-15 18:39:01.356] I: 1077664 [DL]Current number of connections: 1/1 [2025-07-15 18:39:01.372] I: BLE Connected [2025-07-15 18:39:01.612] I: 1077899 [DL]Current number of connections: 0/1 [2025-07-15 18:39:01.612] I: BLE Disconnected: 13 [2025-07-15 18:39:05.960] I: 1082249 [DL]Current number of connections: 1/1 [2025-07-15 18:39:05.960] I: BLE Connected [2025-07-15 18:39:06.174] I: 1082463 [DL]DFU_ImgConfirmHandler: Nordic Image Confirmed by MCUMgr [2025-07-15 18:39:06.190] I: 1082497 [DL]Current number of connections: 0/1 [2025-07-15 18:39:06.206] I: BLE Disconnected: 13 [2025-07-15 18:39:13.392] I: 1089681 [DL]Current number of connections: 1/1 [2025-07-15 18:39:13.392] I: BLE Connected [2025-07-15 18:39:13.606] I: 1089896 [DL]DFU_ImgConfirmHandler: Nordic Image Confirmed by MCUMgr [2025-07-15 18:39:13.623] I: 1089929 [DL]Current number of connections: 0/1 [2025-07-15 18:39:13.638] I: BLE Disconnected: 13 [2025-07-15 18:39:21.267] I: 1097576 [DL]Current number of connections: 1/1 [2025-07-15 18:39:21.289] I: BLE Connected [2025-07-15 18:39:21.576] I: dfu_header_data.image_count: 4 [2025-07-15 18:39:22.341] I: Connection parameters updated. [2025-07-15 18:39:22.341] interval: 9, latency: 0, timeout: 42
Best regards,
Prashant
