fix: warn of radio pins that block flashing over USB

GPIO2 on the original ESP32, 46 on the S2/S3 and 8 on the C3/C6 don't stop
a normal boot, but held at the wrong level at reset they keep esptool from
starting the download mode.  The WT32-ETH01 wiring puts TX on GPIO2.

Each warning now says what the pin does and what to do: move it for a pin
that stops the boot, disconnect it to flash for one that only blocks USB.

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

test/js/pins.test.js

@@ -18,6 +18,22 @@ test('RX on GPIO12 warns of the flash voltage on the original ESP32', () => {
     assert.match(warnings[0], /1\.8 V/);
 });
 
+test('TX on GPIO2 warns of flashing over USB on the original ESP32', () => {
+    // The WT32-ETH01 wiring in the wiki.
+    const warnings = BootPins.radioWarnings('', Object.assign({}, esp32Defaults, { TXPin: 2 }));
+    assert.equal(warnings.length, 1);
+    assert.match(warnings[0], /^TX is on GPIO-2, .*Held high, it stops flashing over USB, so disconnect it to flash\.$/);
+});
+
+test('GPIO8 held low blocks flashing on the C3 and C6', () => {
+    const config = { SCKPin: 4, CSNPin: 7, MOSIPin: 6, MISOPin: 5, TXPin: 10, RXPin: 8 };
+    for (const chip of ['c3', 'c6']) {
+        const warnings = BootPins.radioWarnings(chip, config);
+        assert.equal(warnings.length, 1);
+        assert.match(warnings[0], /^RX is on GPIO-8, .*Held low, it stops flashing over USB/);
+    }
+});
+
 test('an unknown chip model warns of nothing', () => {
     assert.deepEqual(BootPins.radioWarnings('h2', Object.assign({}, esp32Defaults, { RXPin: 12 })), []);
 });

test/test_bootpins/test_bootpins.cpp

@@ -19,10 +19,18 @@ static void test_gpio0_picks_the_boot_mode() {
   TEST_ASSERT_NOT_NULL(strstr(effect, "download mode"));
 }
 
+static void test_gpio2_blocks_flashing_over_usb() {
+  // It only counts with GPIO0 low, so the board boots, but esptool can't
+  // start the download mode while it is high.
+  const char *effect = bootPinEffect(2);
+  TEST_ASSERT_NOT_NULL(effect);
+  TEST_ASSERT_NOT_NULL(strstr(effect, "high"));
+  TEST_ASSERT_NOT_NULL(strstr(effect, "USB"));
+}
+
 static void test_harmless_strapping_pins_are_not_listed() {
-  // 2 only matters for the download mode, 5 and 15 set the SDIO timing and
-  // the boot log.  5 is the default CSN.
-  TEST_ASSERT_NULL(bootPinEffect(2));
+  // 5 and 15 set the SDIO timing and quiet the boot log.  5 is the default
+  // CSN.
   TEST_ASSERT_NULL(bootPinEffect(5));
   TEST_ASSERT_NULL(bootPinEffect(15));
 }
@@ -34,14 +42,17 @@ static void test_default_radio_pins_are_safe() {
 }
 
 static void test_other_chips_boot_pins_are_safe_here() {
+  TEST_ASSERT_NULL(bootPinEffect(8));
   TEST_ASSERT_NULL(bootPinEffect(9));
   TEST_ASSERT_NULL(bootPinEffect(45));
+  TEST_ASSERT_NULL(bootPinEffect(46));
 }
 
 int main(int, char **) {
   UNITY_BEGIN();
   RUN_TEST(test_gpio12_picks_the_flash_voltage);
   RUN_TEST(test_gpio0_picks_the_boot_mode);
+  RUN_TEST(test_gpio2_blocks_flashing_over_usb);
   RUN_TEST(test_harmless_strapping_pins_are_not_listed);
   RUN_TEST(test_default_radio_pins_are_safe);
   RUN_TEST(test_other_chips_boot_pins_are_safe_here);

test/test_bootpins_s3/test_bootpins_s3.cpp

@@ -17,6 +17,12 @@ static void test_gpio0_picks_the_boot_mode() {
   TEST_ASSERT_NOT_NULL(bootPinEffect(0));
 }
 
+static void test_gpio46_blocks_flashing_over_usb() {
+  const char *effect = bootPinEffect(46);
+  TEST_ASSERT_NOT_NULL(effect);
+  TEST_ASSERT_NOT_NULL(strstr(effect, "USB"));
+}
+
 static void test_gpio12_is_safe() {
   // 12 is the S3's default SCK.
   TEST_ASSERT_NULL(bootPinEffect(12));
@@ -32,6 +38,7 @@ int main(int, char **) {
   UNITY_BEGIN();
   RUN_TEST(test_gpio45_picks_the_flash_voltage);
   RUN_TEST(test_gpio0_picks_the_boot_mode);
+  RUN_TEST(test_gpio46_blocks_flashing_over_usb);
   RUN_TEST(test_gpio12_is_safe);
   RUN_TEST(test_default_radio_pins_are_safe);
   return UNITY_END();