diff --git a/README b/README
index bb750e7..a3f9f43 100644
--- a/README
+++ b/README
@@ -1,2 +1,3 @@
-firmware template for ST STM8S micro-controller.
-includes register definitions using macros and structures.
+this is the firmware for the dehumidifier sensor module.
+
+a STM8S003 micro-controller reads the temperature and relative humidity measurements from an AHT20 humidity sensor using I²C, and sends it to the Aplatec DH 10 dehumidifier using a proprietary unidirectional IR-like pulse-width modulated protocol.
diff --git a/main.c b/main.c
index 442a1e5..f04f4a2 100644
--- a/main.c
+++ b/main.c
@@ -1,7 +1,8 @@
-/* firmware template for STM8S microcontroller
- * Copyright (C) 2019-2020 King Kévin <kingkevin@cuvoodoo.info>
+/* firmware for the ALPATEC  DH-10 0101E dehumidifier humidity sensor board
+ * Copyright (C) 2020 King Kévin <kingkevin@cuvoodoo.info>
  * SPDX-License-Identifier: GPL-3.0-or-later
  */
+/* firmware to read out temperature and relative humidity from AHT20 sensor and send it to Aplatec DH 10 dehumidifier */
 #include <stdint.h>
 #include <stdbool.h>
 #include "stm8s.h"
@@ -9,11 +10,196 @@
 // get length of array
 #define ARRAY_LENGTH(x) (sizeof(x) / sizeof((x)[0]))
 
-// blocking wait (in 10 us steps, up to UINT32_MAX / 10)
-static void wait_10us(uint32_t us10)
+// pin to send the humidity value to the humidifier
+// WHTM-03 board
+//#define DEHUMIDIFIER_PORT GPIO_PD
+//#define DEHUMIDIFIER_PIN PD5
+// DH10 board
+#define DEHUMIDIFIER_PORT GPIO_PD
+#define DEHUMIDIFIER_PIN PD2
+
+// the I²C address of the AHT20
+#define SENSOR_ADDR 0x38
+#define AHT20_CMD_RESET 0xBA
+#define AHT20_CMD_INIT 0xBE
+#define AHT20_CMD_TRIGGER 0xAC
+
+// flag set when value should be sent
+volatile bool periodic_flag = false;
+
+// blocking wait, in us
+static void wait_us(uint16_t time)
 {
-	us10 = ((us10 / (1 << CLK->CKDIVR.fields.HSIDIV)) * 1000) / 206; // calibrated for 1 ms
-	for (volatile uint32_t t = 0; t < us10; t++); // burn energy
+	TIM1->ARRH.reg = (time >> 8) & 0xff; // set timeout
+	TIM1->ARRL.reg = (time >> 0) & 0xff; // set timeout
+	TIM1->CR1.fields.CEN = 1; // enable timer
+	while (TIM1->CR1.fields.CEN); // wait until time passed
+}
+
+// send bit to dehumidifier
+static void send_bit(bool bit)
+{
+	// pulse low
+	DEHUMIDIFIER_PORT->ODR.reg &= ~DEHUMIDIFIER_PIN; // set low
+	wait_us(500 - 5); // time with function call delay
+	// pulse high
+	DEHUMIDIFIER_PORT->ODR.reg |= DEHUMIDIFIER_PIN; // set high
+	if (bit) {
+		wait_us(1500 - 7); // long high pulse
+	} else {
+		wait_us(500 - 5); // short high pulse
+	}
+}
+
+// send byte to dehumidifier (LSb first)
+static void send_byte(uint8_t byte)
+{
+	for (int8_t i = 7; i >= 0; i--) {
+		send_bit((byte >> i) & 0x01);
+	}
+}
+
+// send break condition to dehumidifier (after sending the 4 data bytes)
+static void send_break(void)
+{
+	// pulse low
+	DEHUMIDIFIER_PORT->ODR.reg &= ~DEHUMIDIFIER_PIN; // set low
+	wait_us(12250 - 759); // long low pulse
+	DEHUMIDIFIER_PORT->ODR.reg |= DEHUMIDIFIER_PIN; // set high
+}
+
+static uint8_t i2c_transmit(uint8_t addr, const uint8_t* data, uint8_t len)
+{
+	if (0 == len || 0 == data) {
+		return 0;
+	}
+	I2C_SR2 = 0; // clear errors
+	I2C_CR2 |= I2C_CR2_START; // send start condition
+	while (!(I2C_SR1 & I2C_SR1_SB)) { // wait until start bit is sent
+		if (I2C_SR2 & (I2C_SR2_ARLO | I2C_SR2_BERR)) { // bus error occurred
+			return 1;
+		}
+	}
+	if (!(I2C_SR3 & I2C_SR3_MSL)) { // ensure we are in master mode
+		return 2;
+	}
+	I2C_SR2 = 0; // clear errors
+	I2C_DR = (addr << 1) | 0; // select device in transmit
+	while (!(I2C_SR1 & I2C_SR1_ADDR)) { // wait until address has been transmitted
+		if (I2C_SR2 & I2C_SR2_BERR) { // bus error occurred
+			return 3;
+		}
+	}
+	if (I2C_SR2 & I2C_SR2_AF) { // no ACK received
+		return 4;
+	}
+	if (!(I2C_SR3 & I2C_SR3_TRA)) { // ensure we are in transmit mode
+		return 5;
+	}
+	for (uint8_t i = 0; i < len; i++) {
+		while (!(I2C_SR1 & I2C_SR1_TXE)) { // wait until transmit register is empty
+			if (I2C_SR2 & I2C_SR2_BERR) { // bus error occurred
+				return 6;
+			}
+		}
+		if (I2C_SR2 & I2C_SR2_AF) { // no ACK received
+			return 7;
+		}
+		I2C_DR = data[i]; // send data byte
+	}
+	while (!(I2C_SR1 & I2C_SR1_TXE)) { // wait until transmit register is empty
+		if (I2C_SR2 & I2C_SR2_BERR) { // bus error occurred
+			return 8;
+		}
+	}
+	while (!(I2C_SR1 & I2C_SR1_BTF)) { // wait until transmission completed
+		if (I2C_SR2 & I2C_SR2_BERR) { // bus error occurred
+			return 9;
+		}
+	}
+	I2C_CR2 |= I2C_CR2_STOP; // send stop
+	while (I2C_SR3 & I2C_SR3_BUSY) { // wait until transmission ended
+		if (I2C_SR2 & I2C_SR2_BERR) { // bus error occurred
+			return 10;
+		}
+	}
+	if (I2C_SR3 & I2C_SR3_MSL) { // ensure we are back in slave mode
+		return 11;
+	}
+	return 0;
+}
+
+static uint8_t i2c_receive(uint8_t addr, uint8_t* data, uint8_t len)
+{
+	if (0 == len || 0 == data) {
+		return 0;
+	}
+	I2C_SR2 = 0; // clear errors
+	I2C_CR2 |= I2C_CR2_START; // send start condition
+	while (!(I2C_SR1 & I2C_SR1_SB)) { // wait until start bit is sent
+		if (I2C_SR2 & (I2C_SR2_ARLO | I2C_SR2_BERR)) { // bus error occurred
+			return 1;
+		}
+	}
+	if (!(I2C_SR3 & I2C_SR3_MSL)) { // ensure we are in master mode
+		return 2;
+	}
+	I2C_SR2 = 0; // clear errors
+	I2C_DR = (addr << 1) | 1; // select device in receive
+	while (!(I2C_SR1 & I2C_SR1_ADDR)) { // wait until address has been transmitted
+		if (I2C_SR2 & I2C_SR2_BERR) { // bus error occurred
+			return 3;
+		}
+	}
+	if (I2C_SR2 & I2C_SR2_AF) { // no ACK received
+		return 4;
+	}
+	if (I2C_SR3 & I2C_SR3_TRA) { // ensure we are in receive mode
+		return 5;
+	}
+	data[0] = I2C_DR; // just to clear the flag
+	if (1 == len) {
+		I2C_CR2 &= ~I2C_CR2_ACK; // already send NACK
+		I2C_CR2 |= I2C_CR2_STOP; // already program stop
+		while (!(I2C_SR1 & I2C_SR1_RXNE)) { // wait to receive data
+			if (I2C_SR2 & I2C_SR2_BERR) { // bus error occurred
+				return 6;
+			}
+		}
+		data[0] = I2C_DR; // save receive data
+	} else if (2 == len) {
+		I2C_CR2 &= ~I2C_CR2_ACK; // already send NACK
+		while (!(I2C_SR1 & I2C_SR1_BTF)) { // wait until 2 bytes received
+			if (I2C_SR2 & I2C_SR2_BERR) { // bus error occurred
+				return 7;
+			}
+		}
+		I2C_CR2 |= I2C_CR2_STOP; // send stop
+	} else {
+		I2C_CR2 |= I2C_CR2_ACK; // send ACK
+		for (uint8_t i = 0; i < len; i++) {
+			while (!(I2C_SR1 & I2C_SR1_RXNE)) { // wait to receive data
+				if (I2C_SR2 & I2C_SR2_BERR) { // bus error occurred
+					return 8;
+				}
+			}
+			data[i] = I2C_DR; // save receive data
+			if (i + 1 == len) {
+				I2C_CR2 &= ~I2C_CR2_ACK; // send NACK
+				I2C_CR2 |= I2C_CR2_STOP; // send stop
+			}
+		}
+	}
+	while (I2C_SR3 & I2C_SR3_BUSY) { // wait until transmission ended
+		if (I2C_SR2 & I2C_SR2_BERR) { // bus error occurred
+			return 10;
+		}
+	}
+	if (I2C_SR3 & I2C_SR3_MSL) { // ensure we are back in slave mode
+		return 11;
+	}
+
+	return 0;
 }
 
 void main(void)
@@ -24,12 +210,45 @@ void main(void)
 	CLK->CKDIVR.fields.CPUDIV = CLK_CKDIVR_CPUDIV_DIV0; // don't divide CPU frequency to 16 MHz
 	while (!CLK->ICKR.fields.HSIRDY); // wait for internal oscillator to be ready
 
-	// configure auto-wakeup (AWU) to be able to refresh the watchdog
-	// 128 kHz LSI used by default in option bytes CKAWUSEL
-	// we skip measuring the LS clock frequency since there is no need to be precise
-	AWU->TBR.fields.AWUTB = 10; // interval range: 128-256 ms
-	AWU->APR.fields.APR = 0x3e; // set time to 256 ms
-	AWU_CSR |= AWU_CSR_AWUEN; // enable AWU (start only when entering wait or active halt mode)
+	// configure dehumidifier output pin
+	DEHUMIDIFIER_PORT->DDR.reg |= DEHUMIDIFIER_PIN; // switch pin to output
+	DEHUMIDIFIER_PORT->CR1.reg &= ~DEHUMIDIFIER_PIN; // switch output to open-drain
+	//DEHUMIDIFIER_PORT->CR1.reg |= DEHUMIDIFIER_PIN; // switch output to push/pull
+	DEHUMIDIFIER_PORT->ODR.reg |= DEHUMIDIFIER_PIN; // set idle high
+
+	// configure I²C to communicate with ATH20 humidity sensor
+	CLK_PCKENR1 |= CLK_PCKENR1_I2C; // enable I²C peripheral (should be on per default)
+	GPIO_PB->CR1.reg |= (PB4 | PB5); // enable internal pull-up on SCL/SDA (there should also be external pull-up resistors)
+	GPIO_PB->DDR.reg &= ~(PB4 | PB5); // set SCL/SDA as input before it is used as alternate function by the peripheral
+	I2C_CR1 |= I2C_CR1_PE; // enable I²C peripheral (must be done before any other register is written)
+	I2C_CR2 |= I2C_CR2_STOP; // release lines
+	I2C_CR2 |= I2C_CR2_SWRST; // reset peripheral, in case we got stuck and the dog bit
+	while (!(GPIO_PB->IDR.reg & PB4)); // wait for SCL line to be released
+	while (!(GPIO_PB->IDR.reg & PB5)); // wait for SDA line to be released
+	I2C_CR2 &= ~I2C_CR2_SWRST; // release reset
+	I2C_CR1 &= ~I2C_CR1_PE; // disable I²C peripheral to configure it
+	I2C_FREQR = 16; // the peripheral frequency is 16 MHz (must match CPU frequency)
+	I2C_CCRH = (uint8_t)(8000 >> 8); // set control clock to 100 kHz (16 MHz / 2 * 8000), standard mode
+	I2C_CCRL = (uint8_t)(8000 >> 0); // set control clock to 100 kHz (16 MHz / 2 * 8000)
+	I2C_TRISER = 0x09; // set maximum rise time for SCL (100 ns in standard mode)
+	I2C_CR1 |= I2C_CR1_PE; // re-enable I²C peripheral
+
+	// configure timer 2 for periodic 250 ms value sending
+	TIM2->CR1.fields.URS = 1; // only interrupt on overflow
+	TIM2->PSCR.fields.PSC = 6; // best prescaler for most precise timer for up to 0.262 second (1.0/(16E6/(2**6)) * 2**16)
+	TIM2->ARRH.reg = 62500 >> 8; // set the timeout to 250 ms
+	TIM2->ARRL.reg = 62500 & 0xff; // set the timeout to 250 ms
+	TIM2->IER.fields.UIE = 1; // enable update/overflow interrupt
+	TIM2->CR1.fields.CEN = 1; // enable timer for periodic value output
+
+	// configure timer 1 for pulse sending (500 us)
+	TIM1->CR1.fields.OPM = 1; // only run once
+	TIM1->CR1.fields.URS = 1; // only interrupt on overflow
+	TIM1->PSCRH.reg = ((16 - 1) >> 8); // set prescaler to have us
+	TIM1->PSCRL.reg = ((16 - 1) >> 0); // set prescaler to have us
+	TIM1->CNTRH.reg = 0; // reset counter value (should be per default)
+	TIM1->CNTRL.reg = 0; // reset counter value (should be per default)
+	wait_us(2); // test it once
 
 	// configure independent watchdog (very loose, just it case the firmware hangs)
 	IWDG->KR.fields.KEY = IWDG_KR_KEY_REFRESH; // reset watchdog
@@ -40,18 +259,74 @@ void main(void)
 
 	rim(); // re-enable interrupts
 	bool action = false; // if an action has been performed
+
+	
+	wait_us(40000); // wait 40 ms for device to start up
+	const uint8_t aht20_reset[] = {AHT20_CMD_RESET};
+	uint8_t rc = i2c_transmit(SENSOR_ADDR, aht20_reset, ARRAY_LENGTH(aht20_reset)); // send software reset
+	if (rc) {
+		while(true); // let the dog bite
+	}
+	wait_us(20000); // wait 20 ms for reset to complete
+	uint8_t rx_buffer[7]; // to receive the measurements
+	rc = i2c_receive(SENSOR_ADDR, rx_buffer, 1); // read the status
+	if (rc) {
+		while(true); // let the dog bite
+	}
+	if (!(rx_buffer[0] & 0x08)) { // ensure it is calibrated
+		const uint8_t aht20_init[] = {AHT20_CMD_INIT};
+		rc = i2c_transmit(SENSOR_ADDR, aht20_init, ARRAY_LENGTH(aht20_init)); // initialise device (not sure what it does)
+		if (rc) {
+			while(true); // let the dog bite
+		}
+		wait_us(10000); // wait 10 ms for calibration to complete
+	}
+	// trigger measurement
+	const uint8_t aht20_trigger[] = {AHT20_CMD_TRIGGER, 0x33, 0x00};
+	rc = i2c_transmit(SENSOR_ADDR, aht20_trigger, ARRAY_LENGTH(aht20_trigger));
+	if (rc) {
+		while(true); // let the dog bite
+	}
+	wait_us(80000); // wait 80 ms for measurement to complete
+
+	uint8_t humidity = 99;
+	uint8_t temp = 0;
 	while (true) {
 		IWDG_KR = IWDG_KR_KEY_REFRESH; // reset watchdog
+		if (periodic_flag) { // time to get and send measurements
+			periodic_flag = false; // clear flag
+			rc = i2c_receive(SENSOR_ADDR, rx_buffer, ARRAY_LENGTH(rx_buffer)); // read measurement
+			if (rc) {
+				while(true); // let the dog bite
+			}
+			if (!(rx_buffer[0] & 0x80)) { // not busy doing the measurement
+				humidity = (rx_buffer[1] * (uint16_t)100) / 256; // save new humidity value, only use the 8 MSb out of 20
+				temp = (((uint8_t)(rx_buffer[3] << 4) + (uint8_t)(rx_buffer[4] >> 4)) * (uint16_t)200) / 256 - 50; // save new temperature, only use the 8 MSB (out of 20)
+				// trigger next measurement
+				rc = i2c_transmit(SENSOR_ADDR, aht20_trigger, ARRAY_LENGTH(aht20_trigger));
+				if (rc) {
+					while(true); // let the dog bite
+				}
+			}
+			send_byte(temp);
+			send_byte(humidity);
+			send_byte(temp ^ 0xff);
+			send_byte(humidity ^ 0xff);
+			send_break();
+		}
 		if (action) { // something has been performed, check if other flags have been set meanwhile
 			action = false; // clear flag
 		} else { // nothing down
-			wfi(); // go to sleep (wait for any interrupt, including periodic AWU)
+			wfi(); // go to sleep (wait for periodic ping)
 		}
 	}
 }
 
-void awu(void) __interrupt(IRQ_AWU) // auto wakeup
+void tim2_isr(void) __interrupt(IRQ_TIM2_UO) // timer 2 update interrupt service routine
 {
-	volatile uint8_t awuf = AWU_CSR; // clear interrupt flag by reading it (reading is required, and volatile prevents compiler optimization)
-	// let the main loop kick the dog
+	if (TIM2->SR1.fields.UIF) { // 250 ms passed, time to send data
+		TIM2->SR1.fields.UIF = 0; // clear flag
+		periodic_flag = true; // notify main loop
+	}
 }
+