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

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.

Author: Wessel Nieboer

src/mesh/SX126xInterface.cpp

@@ -93,6 +93,18 @@ template <typename T> bool SX126xInterface<T>::init()
     digitalWrite(LORA_PA_TX_EN, LOW); // Start in RX-ready state
 #endif
 
+#if defined(USE_SKY66122_FEM)
+    // SKY66122-11 FEM initialization — start in RX mode, boost off
+    // See variant.h for full pin mapping and control logic documentation
+    pinMode(SKY66122_CSD, OUTPUT);
+    digitalWrite(SKY66122_CSD, HIGH); // Enable FEM
+    pinMode(SKY66122_CPS, OUTPUT);
+    digitalWrite(SKY66122_CPS, HIGH); // Active path (required for both TX and RX)
+    pinMode(SKY66122_CTX, OUTPUT);
+    digitalWrite(SKY66122_CTX, LOW); // Boost off, RX mode
+    delay(1);                        // Settling time
+#endif
+
 #ifdef RF95_FAN_EN
     digitalWrite(RF95_FAN_EN, HIGH);
     pinMode(RF95_FAN_EN, OUTPUT);
@@ -431,6 +443,13 @@ template <typename T> bool SX126xInterface<T>::sleep()
     digitalWrite(LORA_PA_EN, LOW);
     digitalWrite(LORA_PA_TX_EN, LOW);
 #endif
+
+#if defined(USE_SKY66122_FEM)
+    // Full shutdown — all pins LOW, FEM draws <1 μA
+    digitalWrite(SKY66122_CTX, LOW);
+    digitalWrite(SKY66122_CPS, LOW);
+    digitalWrite(SKY66122_CSD, LOW);
+#endif
     return true;
 }
 
@@ -489,14 +508,20 @@ template <typename T> void SX126xInterface<T>::resetAGC()
     startReceive();
 }
 
-/** Control PA mode for GC1109 FEM - CPS pin selects full PA (txon=true) or bypass mode (txon=false) */
+/** Control PA mode for GC1109 / SKY66122 FEM - selects TX path (txon=true) or RX path (txon=false) */
 template <typename T> void SX126xInterface<T>::setTransmitEnable(bool txon)
 {
 #if defined(USE_GC1109_PA)
     digitalWrite(LORA_PA_POWER, HIGH);         // Ensure LDO is on
     digitalWrite(LORA_PA_EN, HIGH);            // CSD=1: Chip enabled
     digitalWrite(LORA_PA_TX_EN, txon ? 1 : 0); // CPS: 1=full PA, 0=bypass (for RX, CPS is don't care)
 #endif
+
+#if defined(USE_SKY66122_FEM)
+    digitalWrite(SKY66122_CSD, HIGH);         // Ensure FEM is enabled
+    digitalWrite(SKY66122_CPS, HIGH);         // Enable active mode (TX and RX)
+    digitalWrite(SKY66122_CTX, txon ? 1 : 0); // HIGH=TX (boost on, PA), LOW=RX (boost off, LNA)
+#endif
 }
 
 #endif

variants/nrf52840/rak3401_1watt/variant.h

@@ -164,8 +164,23 @@ static const uint8_t SCK = PIN_SPI_SCK;
 #define SX126X_BUSY (9)
 #define SX126X_RESET (4)
 
-#define SX126X_POWER_EN (21)
-// DIO2 controlls an antenna switch and the TCXO voltage is controlled by DIO3
+/*
+ * SKY66122-11 Front-End Module control (RAK13302 1W module)
+ *
+ * CSD (P0.24) — Chip enable: HIGH to power up the FEM, LOW for shutdown (<1 μA)
+ * CPS (P0.21) — Path select: HIGH = active (required for TX and RX), LOW = shutdown
+ * CTX (P0.31) — TX/RX select + 5 V boost enable:
+ *                HIGH = TX mode (boost on, PA energized at 5 V)
+ *                LOW  = RX mode (boost off, LNA on 3.3 V)
+ *
+ * R25 (DIO2→CTX) is NC by default on the RAK13302, so the MCU must drive CTX directly.
+ */
+#define USE_SKY66122_FEM
+#define SKY66122_CSD (24)
+#define SKY66122_CPS (21)
+#define SKY66122_CTX (31)
+
+// DIO2 configures the SX1262 internal RF switch (harmless with R25 NC)
 #define SX126X_DIO2_AS_RF_SWITCH
 #define SX126X_DIO3_TCXO_VOLTAGE 1.8