mirror PR #51 onto V4.3

zephcore/boards/esp32/heltec_wifi_lora32_v43/heltec_wifi_lora32_v43_procpu.dts

@@ -8,7 +8,7 @@
  *   OLED:   SSD1306 128x64 on I2C0
  *   Button: GPIO0
  *   LED:    GPIO35 (PWM via LEDC0)
- *   Vext:   GPIO36 active-HIGH (powers OLED and sensors)
+ *   Vext:   GPIO36 active-LOW PMOS rail switch (powers OLED and sensors)
  *   BatADC: GPIO1 = ADC1_CH0 (&adc0), enable GPIO37 active-HIGH (vbat_enable in overlay)
  *
  * V4.3 vs V4.2 difference — FEM chip changed:
@@ -82,13 +82,20 @@
 	 * zephyr,user { io-channels } (see board.overlay), and a voltage-divider
 	 * on &adc1 ch0 would target GPIO11 = the SX1262 MISO. See &adc0 below. */
 
-	/* Vext — powers OLED and external peripherals (active-HIGH on GPIO36).
-	 * regulator-fixed + SSD1306 vin-supply: powers rail at boot without relying on
-	 * power-domains (CONFIG_PM_DEVICE is disabled in prj.conf). */
+	/* Vext rail switch — PMOS, ACTIVE LOW (GPIO36 low = rail ON).
+	 * Powers the OLED (incl. its I2C pull-ups) and the expansion header.
+	 * Same rail circuit as V4.2 (only the PA differs on V4.3); verified on V4
+	 * hardware that GPIO36 high leaves the rail floating (~1.6 V) and the OLED
+	 * unpowered, while low gives a clean 3.3 V. Confirmed against Arduino
+	 * MeshCore: RefCountedDigitalPin::begin() drives PIN_VEXT_EN to !active
+	 * (LOW) and the V4 OLED build constructs the display with NULL periph_power,
+	 * so GPIO36 is held LOW continuously there too.
+	 * regulator-boot-on + always-on: the rail comes up at regulator init,
+	 * well before the SSD1306 driver init. */
 	vext_enable: vext-enable {
 		compatible = "regulator-fixed";
 		regulator-name = "vext";
-		enable-gpios = <&gpio1 4 GPIO_ACTIVE_HIGH>;  /* GPIO36 = gpio1 pin 4 */
+		enable-gpios = <&gpio1 4 GPIO_ACTIVE_LOW>;  /* GPIO36 = gpio1 pin 4 */
 		regulator-boot-on;
 		regulator-always-on;
 	};