nRF54L15-DK: RSSI reads noise floor (-105 dBm) when using RADIO peripheral directly — RF front-end not receiving

RE: nRF54L15-DK: RSSI reads noise floor (-105 dBm) when using RADIO peripheral directly — RF front-end not receiving

Humphrey0 — Tue, 24 Mar 2026 19:44:25 GMT

Thanks

RE: nRF54L15-DK: RSSI reads noise floor (-105 dBm) when using RADIO peripheral directly — RF front-end not receiving

PaKa — Mon, 23 Mar 2026 08:34:29 GMT

Note, the connector on the DK is a SWF connector and not a uFL. uFL cables can't be used and the fact that you can't see any difference is caused by this. You can buy SWF cables from for instance Digikey.

RE: nRF54L15-DK: RSSI reads noise floor (-105 dBm) when using RADIO peripheral directly — RF front-end not receiving

Hieu — Mon, 23 Mar 2026 08:19:42 GMT

Hi Humphrey0,

The datasheet is a little unclear about this, but I can't think of any other reasons that would explain your observation.

Could you please do a sanity check and setup a BLE advertiser and see if the RSSI test reads the signal strength correctly? You can do that with another nRF DK.
If you don't have another DK, you can also use a phone and the nRF Connect app to setup an advertiser. I am not 100% sure on iOS, but the Android app for sure support this.

Best regards,

Hieu

RE: nRF54L15-DK: RSSI reads noise floor (-105 dBm) when using RADIO peripheral directly — RF front-end not receiving

Humphrey0 — Sat, 21 Mar 2026 02:43:08 GMT

Hi Hieu,

Thank you for the reply. I should clarify — I am not trying to receive or decode LoRa packets on the nRF54L15. I don't need preamble detection or sync word matching at all.

My use case is continuous RSSI measurement only: I put the RADIO in RX mode (BLE 1Mbit PHY, 2440 MHz) and periodically trigger TASKS_RSSISTART to read the ambient RF power level. The LoRa transmitter is simply a known RF source to verify that the RSSI readings respond to signal presence.

The problem is that RSSI reads noise floor (~-105 dBm) regardless of any RF activity, even with a strong signal source (USRP SDR transmitting CW at 30 dB gain, <30 cm distance). This suggests the RF front-end is not properly receiving.

What I have already tried:

HFXO started and tuned (XOSTARTED=1, XOTUNED=1)
All FICR TRIMCNF values for RADIO applied (21 trim registers)
Errata 31 (VREQCTRL) applied
MLTPAN-6 and MLTPAN-56 workarounds applied
Two different nRF54L15-DK boards tested
Both PCB antenna and external uFL antenna on J1

Could there be additional RF front-end initialization steps required when using the RADIO peripheral directly (without softdevice/MPSL)? For example:

Power domain configuration for the radio subsystem
Additional undocumented trim/calibration beyond FICR TRIMCNF
Clock routing requirements beyond HFXO

For reference, the same approach (direct RADIO access + RSSI sampling) works perfectly on TI CC2652R7.

Thanks,

RE: nRF54L15-DK: RSSI reads noise floor (-105 dBm) when using RADIO peripheral directly — RF front-end not receiving

Hieu — Thu, 19 Mar 2026 23:31:48 GMT

Hi Humphrey0,

I suspect that the nRF54L15 is not yet setup to use the correct preamble. Actually, I wonder if it is even possible to do that...

Could you please check the possible preamble configurations of the nRF54L15 against the LoRa device to see if the nRF54L15 can receive such packets in the first place?

Hieu