Summary :
We are using the nRF54L15DK as an external HCI controller (running the hci_uart sample from NCS v3.3.0) connected to a custom BLE host on a
separate MCU via HCI H4 UART. The host implements the Channel Sounding Reflector role. When a Pixel 10 (running Android native ranging via nRF
Toolbox) connects, pairing succeeds and CS procedures start, but the Android session eventually terminates with "Session disappeared. Try
reconnecting" (REASON_UNKNOWN). The same Pixel 10 paired against a standalone nRF54L15DK running the ras_reflector sample works correctly and
displays distance.
We want to:
1. Confirm our hci_uart configuration is correct for this use case.
2. Understand whether there are known limitations or additional settings required when the controller is driven by an external host over
HCI-UART for Channel Sounding.
---
Hardware
- nRF54L15DK (PCA10156) as the BLE controller
- Separate MCU as the BLE host, connected via HCI H4 UART
- Pixel 10 as the Channel Sounding initiator (via Android native ranging)
---
NCS Version
NCS v3.3.0
---
Build command
source ~/ncs/tmp/ncs_v33_activate.sh
cd ~/ncs/v3.3.0/zephyr/samples/bluetooth/hci_uart
west build -p -b nrf54l15dk/nrf54l15/cpuapp -- -DEXTRA_CONF_FILE="cs.conf"
west flash
---
Files used
1. Base config — samples/bluetooth/hci_uart/prj.conf (unmodified from the sample)
2. Extra CS overlay — samples/bluetooth/hci_uart/cs.conf (custom file placed in sample root):
CONFIG_BT_CTLR_CHANNEL_SOUNDING=y
CONFIG_BT_CTLR_SDC_CS_ROLE_BOTH=y
CONFIG_BT_CTLR_ADV_EXT=y
CONFIG_BT_CTLR_SDC_CS_MAX_ANTENNA_PATHS=2
CONFIG_BT_CTLR_SDC_CS_NUM_ANTENNAS=1
CONFIG_BT_CTLR_SDC_CS_STEP_MODE3=n
CONFIG_BT_CTLR_DTM_HCI=y
CONFIG_BT_TRANSMIT_POWER_CONTROL=y
CONFIG_BT_CTLR_DATA_LENGTH_MAX=251
CONFIG_BT_BUF_EVT_RX_COUNT=16
CONFIG_BT_BUF_EVT_RX_SIZE=255
CONFIG_BT_BUF_ACL_RX_SIZE=255
CONFIG_BT_BUF_ACL_TX_SIZE=251
CONFIG_BT_BUF_CMD_TX_SIZE=255
3. Board overlay — samples/bluetooth/hci_uart/boards/nrf54l15dk_nrf54l15_cpuapp.overlay (default from sample, unmodified):
&uart20 {
compatible = "nordic,nrf-uarte";
current-speed = <115200>;
status = "okay";
};
&uart20_default {
group1 {
psels = <NRF_PSEL(UART_TX, 1, 11)>,
<NRF_PSEL(UART_RTS, 1, 14)>;
};
group2 {
psels = <NRF_PSEL(UART_RX, 1, 10)>,
<NRF_PSEL(UART_CTS, 1, 12)>;
bias-pull-up;
};
};
The external host is configured to match: UART baud 115200, hardware flow control enabled (RTS/CTS), H4 framing.
---
Observed behaviour
- nRF54L15DK (as HCI controller) enumerates correctly over HCI-UART; the host receives valid HCI events.
- Extended advertising works; the Pixel 10 connects and completes LE Secure Connections pairing.
- CS security (LE_CS_Security_Enable) completes successfully.
- CS configuration (LE_CS_Set_Default_Settings, LE_CS_Create_Config) is accepted by the controller.
- Reconnecting does not recover the session.
- Erasing flash and re-flashing the controller does not help.
- The identical Pixel 10 against the standalone ras_reflector sample on nRF54L15DK works flawlessly.
---
Questions
1. Is the above cs.conf configuration correct and sufficient to enable Channel Sounding support in hci_uart mode when driven by an external
host? Specifically:
- Are CONFIG_BT_CTLR_SDC_CS_MAX_ANTENNA_PATHS=2 and CONFIG_BT_CTLR_SDC_CS_NUM_ANTENNAS=1 the right values to match the nRF54L15DK hardware
(single antenna, 2 paths) so that a Pixel 10 with 2 antennas can negotiate antenna config index 1?
- Is CONFIG_BT_CTLR_SDC_CS_ROLE_BOTH=y required, or is _ROLE_REFLECTOR sufficient?
2. Are there HCI buffer sizes (BT_BUF_EVT_RX_SIZE, BT_BUF_EVT_RX_COUNT, etc.) or other SDC Kconfig knobs specific to CS over hci_uart that
differ from the standalone ras_reflector build?
3. Are there any known issues with the SoftDevice Controller's CS subevent result HCI events being dropped or truncated when relayed over
HCI-UART to an external host, particularly for large step-result payloads?
4. Does the SoftDevice Controller require the external host to acknowledge RAS data (via GATT RAS characteristics) within a specific timeout,
failing which it terminates the CS session? If so, what is that timeout?
---
What we have already ruled out
- UART wiring: physical connection and hardware flow control are verified working (HCI commands and events exchange correctly).
- Controller capability: the standalone ras_reflector with the same board and Pixel 10 works, confirming the hardware supports CS with this
phone.
- Baud rate mismatch: both sides are at 115200 with flow control.
---
Reference
- Standalone reflector build that works: ncs/v3.3.0/nrf/samples/bluetooth/ras_reflector built with android_ranging.conf for
nrf54l15dk/nrf54l15/cpuapp.
- hci_uart sample location: ncs/v3.3.0/zephyr/samples/bluetooth/hci_uart.
