Hold GC1109 FEM power during deep sleep for LNA RX wake (meshtastic#9572)

* Hold GC1109 PA_POWER and PA_EN during deep sleep for LNA RX wake

Use rtc_gpio_hold_en to latch PA_POWER (LDO) and PA_EN (CSD) HIGH
during deep sleep so the GC1109 LNA remains powered for wake-on-packet
RX. Previously these pins used weak pull-ups which could lose state.

On deep sleep wake, skip these pins in the blanket RTC hold release and
instead release them in SX126xInterface::init() after GPIO registers are
set HIGH first, avoiding a power glitch on the GC1109.

Trade-off: ~6.5mA additional deep sleep current for significantly
improved wake-on-packet RX sensitivity (~17dB).

Reference: meshcore-dev/MeshCore#1600

* Add LDO startup delay before GC1109 chip enable

TLV75733P LDO has ~550us startup time (datasheet tSTR). On cold boot,
wait 1ms for VBAT to stabilise before driving CSD/CPS, per GC1109
power-on sequence requirement. On deep sleep wake the LDO is held on
via RTC latch so no delay is needed.

Author: weebl2000

src/mesh/SX126xInterface.cpp

@@ -6,6 +6,10 @@
 #ifdef ARCH_PORTDUINO
 #include "PortduinoGlue.h"
 #endif
+#if defined(USE_GC1109_PA) && defined(ARCH_ESP32)
+#include <driver/rtc_io.h>
+#include <esp_sleep.h>
+#endif
 
 #include "Throttle.h"
 
@@ -55,14 +59,33 @@ template <typename T> bool SX126xInterface<T>::init()
 #if defined(USE_GC1109_PA)
     // GC1109 FEM chip initialization
     // See variant.h for full pin mapping and control logic documentation
+    //
+    // On deep sleep wake, PA_POWER and PA_EN are held HIGH by RTC latch (set in
+    // enableLoraInterrupt). We configure GPIO registers before releasing the hold
+    // so the pad transitions atomically from held-HIGH to register-HIGH with no
+    // power glitch. On cold boot the hold_dis is a harmless no-op.
 
     // VFEM_Ctrl (LORA_PA_POWER): Power enable for GC1109 LDO (always on)
     pinMode(LORA_PA_POWER, OUTPUT);
     digitalWrite(LORA_PA_POWER, HIGH);
+    rtc_gpio_hold_dis((gpio_num_t)LORA_PA_POWER);
+
+    // TLV75733P LDO has ~550us startup time (datasheet tSTR). On cold boot, wait
+    // for VBAT to stabilise before driving CSD/CPS, per GC1109 requirement:
+    // "VBAT must be prior to CSD/CPS/CTX for the power on sequence"
+    // On deep sleep wake the LDO was held on via RTC latch, so no delay needed.
+#if defined(ARCH_ESP32)
+    if (esp_sleep_get_wakeup_cause() == ESP_SLEEP_WAKEUP_UNDEFINED) {
+        delayMicroseconds(1000);
+    }
+#else
+    delayMicroseconds(1000);
+#endif
 
     // 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, HIGH);
+    rtc_gpio_hold_dis((gpio_num_t)LORA_PA_EN);
 
     // 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

src/sleep.cpp

@@ -162,6 +162,13 @@ void initDeepSleep()
     if (wakeCause != ESP_SLEEP_WAKEUP_UNDEFINED) {
         LOG_DEBUG("Disable any holds on RTC IO pads");
         for (uint8_t i = 0; i <= GPIO_NUM_MAX; i++) {
+#if defined(USE_GC1109_PA)
+            // Skip GC1109 FEM power pins - they are held HIGH during deep sleep to keep
+            // the LNA active for RX wake. Released later in SX126xInterface::init() after
+            // GPIO registers are set HIGH first, avoiding a power glitch.
+            if (i == LORA_PA_POWER || i == LORA_PA_EN)
+                continue;
+#endif
             if (rtc_gpio_is_valid_gpio((gpio_num_t)i))
                 rtc_gpio_hold_dis((gpio_num_t)i);
 
@@ -556,8 +563,13 @@ void enableLoraInterrupt()
 #endif
 
 #if defined(USE_GC1109_PA)
-    gpio_pullup_en((gpio_num_t)LORA_PA_POWER);
-    gpio_pullup_en((gpio_num_t)LORA_PA_EN);
+    // Keep GC1109 FEM powered during deep sleep so LNA remains active for RX wake.
+    // Set PA_POWER and PA_EN HIGH (overrides SX126xInterface::sleep() shutdown),
+    // then latch with RTC hold so the state survives deep sleep.
+    digitalWrite(LORA_PA_POWER, HIGH);
+    rtc_gpio_hold_en((gpio_num_t)LORA_PA_POWER);
+    digitalWrite(LORA_PA_EN, HIGH);
+    rtc_gpio_hold_en((gpio_num_t)LORA_PA_EN);
     gpio_pulldown_en((gpio_num_t)LORA_PA_TX_EN);
 #endif