Fix RAK3401 SKY66122-11 FEM control: enable CSD/CPS for proper PA and LNA operation

The RAK13302 1W module uses a Skyworks SKY66122-11 front-end module with
three digital control pins (CSD, CTX, CPS) that must be actively driven
by the host MCU. The previous code only managed CTX (GPIO 31) — toggling
it for TX/RX — but never initialized CSD (GPIO 24) or CPS (GPIO 21),
leaving them floating with no pull-up/pull-down resistors on the PCB.

With floating CSD and CPS, the SKY66122 was in an undefined operating
mode:
- The 30 dB TX PA may not have been reliably engaging
- The 16 dB RX LNA was never reliably active, degrading receive
sensitivity

variants/rak3401/RAK3401Board.cpp

@@ -23,10 +23,33 @@ void RAK3401Board::begin() {
   pinMode(PIN_3V3_EN, OUTPUT);
   digitalWrite(PIN_3V3_EN, HIGH);
 
-#ifdef P_LORA_PA_EN
-  // Initialize RAK13302 1W LoRa transceiver module PA control pin
+  // Initialize SKY66122-11 FEM on the RAK13302 module.
+  // CSD (P0.24) and CPS (P0.21) must be HIGH for both TX and RX modes.
+  // CTX (P0.31) selects TX(HIGH) vs RX(LOW) and also enables the 5V boost
+  // converter that powers the PA section (VCC1/VCC2).
+  // The LNA section (VSUP1/VCC0) runs on 3.3V and works with boost off.
+  pinMode(P_LORA_PA_CSD, OUTPUT);
+  digitalWrite(P_LORA_PA_CSD, HIGH);       // CSD=1: enable FEM
+
+  pinMode(SX126X_POWER_EN, OUTPUT);
+  digitalWrite(SX126X_POWER_EN, HIGH);     // CPS=1: enable TX/RX paths
+
   pinMode(P_LORA_PA_EN, OUTPUT);
-  digitalWrite(P_LORA_PA_EN, LOW);  // Start with PA disabled
-  delay(10);  // Allow PA module to initialize
+  digitalWrite(P_LORA_PA_EN, LOW);         // CTX=0: RX mode, boost off
+
+  delay(1);  // SKY66122 turn-on settling time
+}
+
+#ifdef NRF52_POWER_MANAGEMENT
+void RAK3401Board::initiateShutdown(uint8_t reason) {
+  // Put SKY66122 in guaranteed <1 uA shutdown (Mode 4: CSD=0, CTX=0, CPS=0)
+  digitalWrite(P_LORA_PA_EN, LOW);         // CTX=0, boost off
+  digitalWrite(SX126X_POWER_EN, LOW);      // CPS=0
+  digitalWrite(P_LORA_PA_CSD, LOW);        // CSD=0
+
+  // Disable 3V3 switched peripherals
+  digitalWrite(PIN_3V3_EN, LOW);
+
+  enterSystemOff(reason);
+}
 #endif
-}

variants/rak3401/RAK3401Board.h

@@ -38,13 +38,14 @@ public:
     return "RAK 3401";
   }
 
-#ifdef P_LORA_PA_EN
+  // SKY66122 FEM TX/RX switching via CTX pin.
+  // CTX=HIGH: TX mode + 5V boost ON (PA powered from VCC1/VCC2)
+  // CTX=LOW:  RX mode + 5V boost OFF (LNA powered from VSUP1 at 3.3V)
   void onBeforeTransmit() override {
-    digitalWrite(P_LORA_PA_EN, HIGH);  // Enable PA before transmission
+    digitalWrite(P_LORA_PA_EN, HIGH);  // CTX=1: TX mode, boost on
   }
 
   void onAfterTransmit() override {
-    digitalWrite(P_LORA_PA_EN, LOW);   // Disable PA after transmission to save power
+    digitalWrite(P_LORA_PA_EN, LOW);   // CTX=0: RX mode, boost off
   }
-#endif
 };

variants/rak3401/variant.h

@@ -147,8 +147,14 @@ static const uint8_t AREF = PIN_AREF;
 #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 FEM control pins (active HIGH, active LOW = shutdown <1uA)
+// CSD+CPS must be HIGH for TX and RX; CTX selects TX(HIGH) vs RX(LOW)
+// CTX also enables the 5V boost converter for the PA during TX
+#define P_LORA_PA_CSD (24)          // P0.24 -> SKY66122 CSD (pin 11) - FEM enable
+#define SX126X_POWER_EN (21)        // P0.21 -> SKY66122 CPS (pin 1) - path select
+#define P_LORA_PA_EN  (31)          // P0.31 -> SKY66122 CTX (pin 2) - TX/RX + boost EN
+
+// DIO2 has a NC 0R footprint (R25) to CTX; not connected by default
 #define SX126X_DIO2_AS_RF_SWITCH
 #define SX126X_DIO3_TCXO_VOLTAGE 1.8
 
@@ -159,7 +165,6 @@ static const uint8_t AREF = PIN_AREF;
 #define P_LORA_DIO_1 SX126X_DIO1
 #define P_LORA_BUSY SX126X_BUSY
 #define P_LORA_RESET SX126X_RESET
-#define P_LORA_PA_EN  31
 
 // enables 3.3V periphery like GPS or IO Module
 // Do not toggle this for GPS power savings