Fix RAK13302 1W FEM control: drive SKY66122-11 CSD/CPS/CTX directly

[weebl2000]

The RAK13302's SKY66122-11 FEM has three control pins (CSD, CPS, CTX) that must be actively driven by the MCU. The existing code only managed CPS via SX126X_POWER_EN and relied on DIO2 for TX/RX switching, but R25 (DIO2→CTX) is NC by default, leaving CSD floating and CTX undriven. This put the FEM in an undefined state — the PA wouldn't reliably engage during TX and the 16 dB RX LNA was never active.

Add USE_SKY66122_FEM support (following the existing USE_GC1109_PA pattern) to explicitly initialize, sleep, and toggle all three FEM pins for correct TX/RX/shutdown modes.

See also meshcore-dev/MeshCore#1836.

I don't have a RAK13302 device so I cannot test this myself.

References:

• RAK13302 docs
• SKY66122-11 FEM datasheet
• SX1262 datasheet

[weebl2000]

Fixed formatting

[caveman99]

That's more or less identical patterns for both PA's Why don't aggregate them user USE_LORA_PA or something similar and only differentiate in the sleep call to not switch off the power for one of these?

I really don't like duplicate code blocks that do almost the same.

[Copilot]

Pull request overview

This PR fixes the SKY66122-11 Front-End Module (FEM) control for the RAK13302 1W module on the RAK3401 nRF52840 variant. The existing code only managed the CPS pin via SX126X_POWER_EN and relied on DIO2 for TX/RX switching, but the RAK13302 hardware has R25 (DIO2→CTX) as NC (not connected) by default, leaving the FEM in an undefined state where the PA and LNA were not properly controlled.

Changes:

• Add explicit MCU control of all three SKY66122-11 FEM pins (CSD, CPS, CTX) following the existing USE_GC1109_PA pattern
• Remove SX126X_POWER_EN define (pin 21) and repurpose it for SKY66122_CPS control
• Implement proper initialization, TX/RX switching, and sleep mode shutdown for the FEM

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
variants/nrf52840/rak3401_1watt/variant.h	Adds USE_SKY66122_FEM support with pin definitions for CSD (24), CPS (21), and CTX (31), and comprehensive documentation of the control logic
src/mesh/SX126xInterface.cpp	Implements FEM initialization in init(), TX/RX path switching in setTransmitEnable(), and power-down in sleep()

[weebl2000]

I'm not sure if it would be clearer if we unified the FEM logic: https://github.com/weebl2000/meshtastic-firmware/pull/1/changes

Thoughts?

[thebentern]

I'm not sure if it would be clearer if we unified the FEM logic: https://github.com/weebl2000/meshtastic-firmware/pull/1/changes

Thoughts?

Looks cleaner to me

[weebl2000]

Looks cleaner to me

updated PR to include the unifying

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

[weebl2000]

That's more or less identical patterns for both PA's Why don't aggregate them user USE_LORA_PA or something similar and only differentiate in the sleep call to not switch off the power for one of these?

I really don't like duplicate code blocks that do almost the same.

Addressed this, please review again! @caveman99

[weebl2000]

Actually, I think it's already correct, closing this!