diff --git a/src/gba/gba-gpio.c b/src/gba/gba-gpio.c
index 8cd74a2e9..53c138e82 100644
--- a/src/gba/gba-gpio.c
+++ b/src/gba/gba-gpio.c
@@ -24,6 +24,8 @@ static void _gyroReadPins(struct GBACartridgeGPIO* gpio);
 
 static void _rumbleReadPins(struct GBACartridgeGPIO* gpio);
 
+static void _lightReadPins(struct GBACartridgeGPIO* gpio);
+
 static const int RTC_BYTES[8] = {
 	0, // Force reset
 	0, // Empty
@@ -46,7 +48,13 @@ void GBAGPIOInit(struct GBACartridgeGPIO* gpio, uint16_t* base) {
 void GBAGPIOWrite(struct GBACartridgeGPIO* gpio, uint32_t address, uint16_t value) {
 	switch (address) {
 	case GPIO_REG_DATA:
-		gpio->pinState = value;
+		gpio->pinState &= ~gpio->direction;
+		gpio->pinState |= value;
+		if (gpio->readWrite) {
+			uint16_t old = gpio->gpioBase[0];
+			old &= ~gpio->direction;
+			gpio->gpioBase[0] = old | gpio->pinState;
+		}
 		_readPins(gpio);
 		break;
 	case GPIO_REG_DIRECTION:
@@ -58,12 +66,6 @@ void GBAGPIOWrite(struct GBACartridgeGPIO* gpio, uint32_t address, uint16_t valu
 	default:
 		GBALog(gpio->p, GBA_LOG_WARN, "Invalid GPIO address");
 	}
-
-	if (gpio->readWrite) {
-		uint16_t old = gpio->gpioBase[0];
-		old &= ~gpio->direction;
-		gpio->gpioBase[0] = old | (value & gpio->direction);
-	}
 }
 
 void GBAGPIOInitRTC(struct GBACartridgeGPIO* gpio) {
@@ -92,13 +94,18 @@ void _readPins(struct GBACartridgeGPIO* gpio) {
 	if (gpio->gpioDevices & GPIO_RUMBLE) {
 		_rumbleReadPins(gpio);
 	}
+
+	if (gpio->gpioDevices & GPIO_LIGHT_SENSOR) {
+		_lightReadPins(gpio);
+	}
 }
 
 void _outputPins(struct GBACartridgeGPIO* gpio, unsigned pins) {
 	if (gpio->readWrite) {
 		uint16_t old = gpio->gpioBase[0];
 		old &= gpio->direction;
-		gpio->gpioBase[0] = old | (pins & ~gpio->direction & 0xF);
+		gpio->pinState = old | (pins & ~gpio->direction & 0xF);
+		gpio->gpioBase[0] = gpio->pinState;
 	}
 }
 
@@ -309,6 +316,33 @@ void _rumbleReadPins(struct GBACartridgeGPIO* gpio) {
 	rumble->setRumble(rumble, gpio->p3);
 }
 
+// == Light sensor
+
+void GBAGPIOInitLightSensor(struct GBACartridgeGPIO* gpio) {
+	gpio->gpioDevices |= GPIO_LIGHT_SENSOR;
+	gpio->lightCounter = 0;
+	gpio->lightEdge = false;
+}
+
+void _lightReadPins(struct GBACartridgeGPIO* gpio) {
+	if (gpio->p2) {
+		// Boktai chip select
+		return;
+	}
+	if (gpio->p1) {
+		GBALog(0, GBA_LOG_DEBUG, "[SOLAR] Got reset");
+		gpio->lightCounter = 0;
+	}
+	if (gpio->p0 && gpio->lightEdge) {
+		++gpio->lightCounter;
+	}
+	gpio->lightEdge = !gpio->p0;
+
+	bool sendBit = gpio->lightCounter >= 0x80;
+	_outputPins(gpio, sendBit << 3);
+	GBALog(0, GBA_LOG_DEBUG, "[SOLAR] Output %u with pins %u", gpio->lightCounter, gpio->pinState);
+}
+
 // == Serialization
 
 void GBAGPIOSerialize(struct GBACartridgeGPIO* gpio, struct GBASerializedState* state) {
diff --git a/src/gba/gba-gpio.h b/src/gba/gba-gpio.h
index 0af768116..1664cba43 100644
--- a/src/gba/gba-gpio.h
+++ b/src/gba/gba-gpio.h
@@ -103,6 +103,9 @@ struct GBACartridgeGPIO {
 
 	uint16_t gyroSample;
 	bool gyroEdge;
+
+	int lightCounter : 12;
+	bool lightEdge;
 };
 
 void GBAGPIOInit(struct GBACartridgeGPIO* gpio, uint16_t* gpioBase);
@@ -114,6 +117,8 @@ void GBAGPIOInitGyro(struct GBACartridgeGPIO* gpio);
 
 void GBAGPIOInitRumble(struct GBACartridgeGPIO* gpio);
 
+void GBAGPIOInitLightSensor(struct GBACartridgeGPIO* gpio);
+
 struct GBASerializedState;
 void GBAGPIOSerialize(struct GBACartridgeGPIO* gpio, struct GBASerializedState* state);
 void GBAGPIODeserialize(struct GBACartridgeGPIO* gpio, struct GBASerializedState* state);