fix: warn of radio pins that can stop the board booting (#34)

GPIO12 is a strapping pin on the original ESP32: high at reset, it powers
the flash at 1.8 V. A CC1101 can hold GDO2 high at power-up, so a board
with RX on 12 bootloops after a power cut until EN is pressed. It was the
default RX pin.

- RX defaults to GPIO35 on the original ESP32. Saved settings keep their
  pins.
- The log warns each time the radio starts, and the radio settings show a
  warning, while TX, RX or MISO is on a pin whose level at reset can stop
  the chip booting (src/BootPins.h, data-src/js/pins.js).

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

src/BootPins.h

@@ -0,0 +1,34 @@
+#ifndef BOOT_PINS_H
+#define BOOT_PINS_H
+#include <stdint.h>
+#if __has_include(<sdkconfig.h>)
+#include <sdkconfig.h>
+#endif
+
+// The strapping pins whose level at reset can keep the chip from starting
+// the firmware.  The ROM reads them before any code runs, so a radio that
+// drives one at power-up stops the board booting until EN is pressed (issue
+// #34).  The strapping pins that only pick a download mode, quiet the boot
+// log or set the SDIO timing aren't listed.  web UI: data-src/js/pins.js.
+//
+// Returns what the pin does at reset, or nullptr for a pin that can't stop
+// the boot.
+inline const char *bootPinEffect(uint8_t pin) {
+#if defined(CONFIG_IDF_TARGET_ESP32S2) || defined(CONFIG_IDF_TARGET_ESP32S3)
+  if(pin == 0) return "low at reset starts the download mode";
+  if(pin == 45) return "high at reset powers the flash at 1.8 V";
+#elif defined(CONFIG_IDF_TARGET_ESP32C3) || defined(CONFIG_IDF_TARGET_ESP32C6)
+  if(pin == 9) return "low at reset starts the download mode";
+#elif defined(CONFIG_IDF_TARGET_ESP32C5)
+  if(pin == 28) return "low at reset starts the download mode";
+#elif defined(CONFIG_IDF_TARGET_ESP32P4)
+  if(pin == 35) return "low at reset starts the download mode";
+#else
+  // The original ESP32.  MTDI picks the flash voltage unless the
+  // VDD_SDIO efuses are burned.
+  if(pin == 0) return "low at reset starts the download mode";
+  if(pin == 12) return "high at reset powers the flash at 1.8 V";
+#endif
+  return nullptr;
+}
+#endif

src/Transceiver.cpp

@@ -8,6 +8,7 @@
 #include "driver/gpio.h"
 #include "esp_log.h"
 #include "Utils.h"
+#include "BootPins.h"
 #include "ConfigSettings.h"
 #include "Somfy.h"
 #include "outputs/ProtoSocket.h"
@@ -523,8 +524,11 @@ void transceiver_config_t::load() {
         break;
 #endif
       default:
+        // RX was on 12, a strapping pin (issue #34).  A board that saved its
+        // radio settings keeps its pins.  35 is input only, which RX is; 36
+        // and 39 take false interrupts.
         this->TXPin = 13;
-        this->RXPin = 12;
+        this->RXPin = 35;
         this->MOSIPin = 23;
         this->MISOPin = 19;
         this->SCKPin = 18;
@@ -571,6 +575,10 @@ void transceiver_config_t::load() {
     pref.end();
     //this->printBuffer = somfy.transceiver.printBuffer;
 }
+static void warnBootPin(const char *name, uint8_t pin) {
+  const char *effect = bootPinEffect(pin);
+  if(effect) ESP_LOGW(TAG, "Radio %s is on GPIO%u, a strapping pin: %s.  The radio can hold it there and stop the board booting after a power cut, so move it to another pin.", name, pin, effect);
+}
 void transceiver_config_t::apply() {
     Preferences pref;
     // A transmission in flight stops after its current frame so the radio can
@@ -595,6 +603,12 @@ void transceiver_config_t::apply() {
       pref.end();
       if(!radioInit) return;
       ESP_LOGD(TAG, "Applying radio settings - Setting Data Pins RX:%u TX:%u", this->RXPin, this->TXPin);
+      // The pins the CC1101 drives at power-up, before it is set up: GDO0
+      // puts out a clock, GDO2 CHIP_RDYn, and SO follows CHIP_RDYn while CSN
+      // is low.
+      warnBootPin("TX", this->TXPin);
+      if(this->RXPin != this->TXPin) warnBootPin("RX", this->RXPin);
+      warnBootPin("MISO", this->MISOPin);
       //if(this->TXPin != this->RXPin)
       //  pinMode(this->TXPin, OUTPUT);
       //pinMode(this->RXPin, INPUT);

src/Transceiver.h

@@ -30,7 +30,7 @@ struct transceiver_config_t {
     radio_proto proto = radio_proto::RTS;
     uint8_t SCKPin = 18;
     uint8_t TXPin = 13;
-    uint8_t RXPin = 12;
+    uint8_t RXPin = 35;
     uint8_t MOSIPin = 23;
     uint8_t MISOPin = 19;
     uint8_t CSNPin = 5;