🔧 Fix LNA/PA power control for Heltec v4, wireless tracker v2 (#9029)

* Fix LNA/PA power control for Heltec v4, wireless tracker v2

* Stop using pin 46 as RF switch, just let DIO2 switch handle the RF path

---------

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/mesh/SX126xInterface.cpp

@@ -53,13 +53,21 @@ template <typename T> bool SX126xInterface<T>::init()
 #endif
 
 #if defined(USE_GC1109_PA)
+    // GC1109 FEM chip initialization
+    // See variant.h for full pin mapping and control logic documentation
+
+    // VFEM_Ctrl (LORA_PA_POWER): Power enable for GC1109 LDO (always on)
     pinMode(LORA_PA_POWER, OUTPUT);
     digitalWrite(LORA_PA_POWER, HIGH);
 
+    // CSD (LORA_PA_EN): Chip enable - must be HIGH to enable GC1109 for both RX and TX
     pinMode(LORA_PA_EN, OUTPUT);
-    digitalWrite(LORA_PA_EN, LOW);
+    digitalWrite(LORA_PA_EN, HIGH);
+
+    // CPS (LORA_PA_TX_EN): PA mode select - HIGH enables full PA during TX, LOW for RX (don't care)
+    // Note: TX/RX path switching (CTX) is handled by DIO2 via SX126X_DIO2_AS_RF_SWITCH
     pinMode(LORA_PA_TX_EN, OUTPUT);
-    digitalWrite(LORA_PA_TX_EN, LOW);
+    digitalWrite(LORA_PA_TX_EN, LOW); // Start in RX-ready state
 #endif
 
 #ifdef RF95_FAN_EN
@@ -377,13 +385,13 @@ template <typename T> bool SX126xInterface<T>::sleep()
     return true;
 }
 
-/** Some boards require GPIO control of tx vs rx paths */
+/** Control PA mode for GC1109 FEM - CPS pin selects full PA (txon=true) or bypass mode (txon=false) */
 template <typename T> void SX126xInterface<T>::setTransmitEnable(bool txon)
 {
 #if defined(USE_GC1109_PA)
-    digitalWrite(LORA_PA_POWER, HIGH);
-    digitalWrite(LORA_PA_EN, HIGH);
-    digitalWrite(LORA_PA_TX_EN, txon ? 1 : 0);
+    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
 }
 

variants/esp32s3/heltec_v4/variant.h

@@ -29,10 +29,32 @@
 #define SX126X_DIO2_AS_RF_SWITCH
 #define SX126X_DIO3_TCXO_VOLTAGE 1.8
 
-#define USE_GC1109_PA   // We have a GC1109 power amplifier+attenuator
-#define LORA_PA_POWER 7 // power en
-#define LORA_PA_EN 2
-#define LORA_PA_TX_EN 46 // enable tx
+// ---- GC1109 RF FRONT END CONFIGURATION ----
+// The Heltec V4 uses a GC1109 FEM chip with integrated PA and LNA
+// RF path: SX1262 -> GC1109 PA -> Pi attenuator -> Antenna
+// Measured net TX gain (non-linear due to PA compression):
+//   +11dB at 0-15dBm input  (e.g., 10dBm in -> 21dBm out)
+//   +10dB at 16-17dBm input
+//   +9dB  at 18-19dBm input
+//   +7dB  at 21dBm input    (e.g., 21dBm in -> 28dBm out max)
+// Control logic (from GC1109 datasheet):
+//   Shutdown:        CSD=0, CTX=X, CPS=X
+//   Receive LNA:     CSD=1, CTX=0, CPS=X  (17dB gain, 2dB NF)
+//   Transmit bypass: CSD=1, CTX=1, CPS=0  (~1dB loss, no PA)
+//   Transmit PA:     CSD=1, CTX=1, CPS=1  (full PA enabled)
+// Pin mapping:
+//   CTX (pin 6)  -> SX1262 DIO2: TX/RX path select (automatic via SX126X_DIO2_AS_RF_SWITCH)
+//   CSD (pin 4)  -> GPIO2: Chip enable (HIGH=on, LOW=shutdown)
+//   CPS (pin 5)  -> GPIO46: PA mode select (HIGH=full PA, LOW=bypass)
+//   VCC0/VCC1    -> Vfem via U3 LDO, controlled by GPIO7
+#define USE_GC1109_PA
+#define LORA_PA_POWER 7  // VFEM_Ctrl - GC1109 LDO power enable
+#define LORA_PA_EN 2     // CSD - GC1109 chip enable (HIGH=on)
+#define LORA_PA_TX_EN 46 // CPS - GC1109 PA mode (HIGH=full PA, LOW=bypass)
+
+// GC1109 FEM: TX/RX path switching is handled by DIO2 -> CTX pin (via SX126X_DIO2_AS_RF_SWITCH)
+// GPIO46 is CPS (PA mode), not TX control - setTransmitEnable() handles it in SX126xInterface.cpp
+// Do NOT use SX126X_TXEN/RXEN as that would cause double-control of GPIO46
 
 #if HAS_TFT
 #define USE_TFTDISPLAY 1

variants/esp32s3/heltec_wireless_tracker_v2/variant.h

@@ -73,7 +73,29 @@
 #define SX126X_DIO2_AS_RF_SWITCH
 #define SX126X_DIO3_TCXO_VOLTAGE 1.8
 
-#define USE_GC1109_PA   // We have a GC1109 power amplifier+attenuator
-#define LORA_PA_POWER 7 // power en
-#define LORA_PA_EN 4
-#define LORA_PA_TX_EN 46 // enable tx
+// ---- GC1109 RF FRONT END CONFIGURATION ----
+// The Heltec Wireless Tracker V2 uses a GC1109 FEM chip with integrated PA and LNA
+// RF path: SX1262 -> GC1109 PA -> Pi attenuator -> Antenna
+// Measured net TX gain (non-linear due to PA compression):
+//   +11dB at 0-15dBm input  (e.g., 10dBm in -> 21dBm out)
+//   +10dB at 16-17dBm input
+//   +9dB  at 18-19dBm input
+//   +7dB  at 21dBm input    (e.g., 21dBm in -> 28dBm out max)
+// Control logic (from GC1109 datasheet):
+//   Shutdown:        CSD=0, CTX=X, CPS=X
+//   Receive LNA:     CSD=1, CTX=0, CPS=X  (17dB gain, 2dB NF)
+//   Transmit bypass: CSD=1, CTX=1, CPS=0  (~1dB loss, no PA)
+//   Transmit PA:     CSD=1, CTX=1, CPS=1  (full PA enabled)
+// Pin mapping:
+//   CTX (pin 6)  -> SX1262 DIO2: TX/RX path select (automatic via SX126X_DIO2_AS_RF_SWITCH)
+//   CSD (pin 4)  -> GPIO4: Chip enable (HIGH=on, LOW=shutdown)
+//   CPS (pin 5)  -> GPIO46: PA mode select (HIGH=full PA, LOW=bypass)
+//   VCC0/VCC1    -> Vfem via U3 LDO, controlled by GPIO7
+#define USE_GC1109_PA
+#define LORA_PA_POWER 7  // VFEM_Ctrl - GC1109 LDO power enable
+#define LORA_PA_EN 4     // CSD - GC1109 chip enable (HIGH=on)
+#define LORA_PA_TX_EN 46 // CPS - GC1109 PA mode (HIGH=full PA, LOW=bypass)
+
+// GC1109 FEM: TX/RX path switching is handled by DIO2 -> CTX pin (via SX126X_DIO2_AS_RF_SWITCH)
+// GPIO46 is CPS (PA mode), not TX control - setTransmitEnable() handles it in SX126xInterface.cpp
+// Do NOT use SX126X_TXEN/RXEN as that would cause double-control of GPIO46