fix(wio-tracker-l2): stop the I2S clock between sounds; GPS and TCXO fixes

The speaker's I2S channel was only disabled when idle, which leaves MCLK
running. Its fractional divider (160 MHz / 39 1/16 = 4.096 MHz) makes a
comb of lines every 256 kHz: one at 869.632 MHz, in the EU narrow mesh
channel, lifted the LoRa noise floor from ~-102 to ~-85 dBm, and others
land near GPS L1 and BeiDou B1. The channel is now deleted and its pins
held low after each sound, and made again for the next one. Measured:
noise floor -102 dBm, outdoor first fix 87 s (was 620 s and more).

Also: TCXO at 3.0 V (the L2's part, as Meshtastic has it); GLONASS
enabled on the L76K after each power-up; begin() on a running receiver
no longer resets it (a reset threw away the acquisition in progress).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/helpers/ui/buzzer.cpp

@@ -426,11 +426,22 @@ static bool i2sInstall() {
 }
 static void i2sUninstall() { i2s_channel_disable(s_tx); i2s_del_channel(s_tx); s_tx = nullptr; }
 static void i2sWrite(const void* src, size_t n, uint32_t ms) {
+  if (!s_tx) return;
   size_t w;
   i2s_channel_write(s_tx, src, n, &w, ms);
 }
-static void i2sStop()    { i2s_channel_disable(s_tx); }   // clocks off
-static void i2sRestart() { i2s_channel_enable(s_tx); }    // auto-clear left the buffers silent
+// Clocks off between sounds: a disabled channel still drives MCLK, and its
+// fractional divider (160 MHz / 39 1/16) makes a comb of lines every 256 kHz
+// right up the LoRa band -- 869.632 MHz sat in the EU narrow channel, ~15 dB
+// over the Wio Tracker L2's noise floor. So the channel goes, the pins held
+// low, and a sound makes it again.
+static void i2sStop() {
+  if (!s_tx) return;
+  i2sUninstall();
+  const int pins[] = { PIN_I2S_MCLK, PIN_I2S_BCK, PIN_I2S_WS, PIN_I2S_DOUT };
+  for (int p : pins) { gpio_reset_pin((gpio_num_t)p); pinMode(p, OUTPUT); digitalWrite(p, LOW); }
+}
+static void i2sRestart() { if (!s_tx) i2sInstall(); }
 #else
 static const i2s_port_t I2S_PORT = I2S_NUM_0;
 static bool i2sInstall() {

variants/wio-tracker-l2/platformio.ini

@@ -29,7 +29,7 @@ build_flags = ${esp32_base.build_flags}
   -D P_LORA_MISO=5
   -D P_LORA_MOSI=6
   -D SX126X_DIO2_AS_RF_SWITCH=true
-  -D SX126X_DIO3_TCXO_VOLTAGE=1.8
+  -D SX126X_DIO3_TCXO_VOLTAGE=3.0 ; the L2 TCXO is a 3 V part (Meshtastic PR #11960)
   -D SX126X_CURRENT_LIMIT=140
   -D SX126X_RX_BOOSTED_GAIN=1
   -D ENV_INCLUDE_GPS=1

variants/wio-tracker-l2/target.cpp

@@ -23,6 +23,7 @@ void buzzerAmpPower(bool on) { board.setSpeakerAmp(on); }
 #endif
 
 void L2GpsProvider::begin() {
+  if (_started && board.gnssPowered() && !_uart_off) return;   // already running
   board.setGnssPower(true);   // with the power-up reset
   if (_uart_off) {            // back from stop(): the UART again
     Serial1.setRxBufferSize(1024);
@@ -30,6 +31,20 @@ void L2GpsProvider::begin() {
     _uart_off = false;
   }
   MicroNMEALocationProvider::begin();
+  _started = true;
+  _cfg_pending = true;
+  _cfg_rx = rxChars();
+}
+
+// The L76K starts with GPS + BeiDou only; GLONASS as well gives it a third
+// more satellites. Sent once it has booted (NMEA arriving), again after each
+// power-up (not saved in the module).
+void L2GpsProvider::loop() {
+  MicroNMEALocationProvider::loop();
+  if (_cfg_pending && rxChars() - _cfg_rx > 200) {
+    _cfg_pending = false;
+    sendSentence("$PCAS04,7");   // GPS + BeiDou + GLONASS
+  }
 }
 
 void L2GpsProvider::stop() {
@@ -41,7 +56,9 @@ void L2GpsProvider::stop() {
   board.setGnssPower(false);
 }
 
-void L2GpsProvider::reset() { board.gnssReset(); }
+// The sensor manager resets right after every begin(); begin() already did
+// the power-up one, and a second would restart a running receiver.
+void L2GpsProvider::reset() { }
 bool L2GpsProvider::isEnabled() { return board.gnssPowered(); }
 
 bool radio_init() {

variants/wio-tracker-l2/target.h

@@ -16,15 +16,21 @@
 
 // The L76K's power and reset lines are on the IO expander, out of the NMEA
 // provider's reach: begin() / stop() / reset() drive them here, so GPS off
-// (Settings, duty cycling) really powers the receiver down.
+// (Settings, duty cycling) really powers the receiver down. begin() on a
+// running receiver does nothing: a reset there (the app setting "gps" again)
+// threw away the acquisition in progress.
 class L2GpsProvider : public MicroNMEALocationProvider {
   bool _uart_off = false;
+  bool _started = false;       // begin() ran (the boot one does the reset)
+  bool _cfg_pending = false;   // constellations to set once the module talks
+  uint32_t _cfg_rx = 0;
 public:
   using MicroNMEALocationProvider::MicroNMEALocationProvider;
   void begin() override;
   void stop() override;
   void reset() override;
   bool isEnabled() override;
+  void loop() override;
 };
 
 extern WioTrackerL2Board board;