Skyworks FEM control with 802.15.4 with NCS/Zephyr 2.0.0

Question

Hi,

I'm having trouble getting the FEM in my Fanstel BT840X module working (the module with the nRF52840 processor and Skyworks 66112 amplifier).

I'm using nRF Connect version 2.0.0, though I can upgrade if it helps.

The first issue I've had is conflicting documentation on how to do this. I found that the recommended path of adding CONFIG_MPSL and CONFIG_MPSL_FEM does not build if you have CONFIG_NRF_802154_SOURCE_HAL_NORDIC in the project. I started this project from radio_test originally, and that sample uses the _NORDIC version.

The radio_test sample puts references to fem.c and generic_fem.c from the /nrf/samples/bluetooth/direct_test_mode/src/fem/ directory into CMakeLists.txt and Kconfig and it seems to expect you to use CONFIG_FEM and CONFIG_GENERIC_FEM in prj.conf. That scheme will at least build along with CONFIG_NRF_802154_SOURCE_HAL_NORDIC so I've been trying to use that.

Based on other support tickets I found, and information at this link,: https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.9.0/nrf/ug_radio_fem.html#ug-radio-fem-skyworks

I modified my .dts file to add this code in the / { section:

nrf_radio_fem: skyworks_fem {
compatible = "skyworks,sky66112-11", "generic-fem-two-ctrl-pins";
ctx-gpios = <&gpio0 17 GPIO_ACTIVE_HIGH>;
crx-gpios = <&gpio0 19 GPIO_ACTIVE_HIGH>;
cps-gpios = <&gpio0 6 GPIO_ACTIVE_HIGH>;
chl-gpios = <&gpio0 8 GPIO_ACTIVE_HIGH>;
};

and this code outside of that section:

&radio {
fem = <&nrf_radio_fem>;
};

For my testing, I turned down the transmit power to -20dBm, and with the amplifier off my two boards can see each other at about -59dBm across my desk. At this level my boards can communicate but I'd definitely see a jump in RSSI if the amplifier were working.

If I put in the modified .dts file, my two devices can no longer talk to each other - regardless of the presence or absence of any CONFIG values in the prj.conf. So it seems like the .dts changes are making a difference, but not in the direction I expected.

Please help! This is urgent!

Thanks,

Glen

Glen M · Accepted Answer

I think I got it working! I need to do more testing to make sure I don't still have a problem, but redefining the GPIO to be on pins I could get to with an oscilloscope helped immensely. The big thing that made it start working was that in order for the MPSL FEM code to work, MPSL_FEM_GENERIC_TWO_CTRL_PINS_SUPPORT has to be defined. I had tried to define that previously in my kconfig.board file but maybe I did it wrong. I ended up putting the following code in the Kconfig in my main project: config BOARD_ENABLE_FEM bool "FEM enabled" default y select MPSL_FEM_GENERIC_TWO_CTRL_PINS_SUPPORT I had previously tried adding that to my kconfig.board in my board directory but it gave me an error about the default y, saying something about defaults not applying to choices, so I'd removed the default and I think that made it not do anything. Maybe there's some difference between Kconfig and kconfig.board that I don't understand... maybe you can suggest the 'right' way to do this, but for now what I have works. ---------------------- So, for anyone else trying to do this, you need the Kconfig modification, you need these two lines in your prj.conf: CONFIG_MPSL =y CONFIG_MPSL_FEM =y and you need to add this to your .dts file: / { nrf_radio_fem: skyFem { compatible = "skyworks,sky66112-11", "generic-fem-two-ctrl-pins"; ctx-gpios = <&gpio0 17 GPIO_ACTIVE_HIGH>; crx-gpios = <&gpio0 19 GPIO_ACTIVE_HIGH>; }; I also have this line in my .dts file which may or may not be necessary... I'll do more experimenting later to see if I can remove it: &radio { fem = <&nrf_radio_fem>; }; A final piece of information I just got answered in a different support request: The MPSL FEM library takes into account the 22dB amplification you get from the PA, so when you set the power level, either through the metadata or through a call to nrf_802154_tx_power_set, don't set it to 2dBm like the Skyworks/Fanstel documentation says to - set it to the final output power you want. For my project I'm now setting it to 22dBm. Somewhere along the way I found a thing you can add to the dts file to tell it what the PA gain is, but I'm leaving it at the default because it should be correct for my amplifier.

Skyworks FEM control with 802.15.4 with NCS/Zephyr 2.0.0

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