AHT20 with Arduino SoftWire software I2C

Created June 2026

I've been working on a project using an ESP32-C3 board that only has one available I²C interface broken out. For this project I needed to connect two AHT20 temperature sensors to the same board. Since the AHT20 modules have a fixed I²C address of 0x38, this meant I had to put them on separate pins.

To provide I²C on arbitrary pins, I turned to the SoftWire library from the Arduino library manager. Unfortunately I had some trouble getting existing AHT20 libraries to play nice with SoftWire instances where they expected TwoWire (hardware I²C) instances. I don't like to modify existing libraries included with IDEs in case it messes something else up later when I forget about my modifications, so I gave our AI overlords a try with a few simple steps:

I asked ChatGPT to generate standalone code to get the temperature from an AHT20 sensor using SoftWire.
I corrected the hallucinated function calls for SoftWire.
I tested the code with an actual sensor.
I corrected some sensor initialisation sequence errors by looking an existing AHT20 library initialisation sequences.
I modified the functions to let me pass in a SoftWire instance.
I wrapped it up in a quick example to get temperatures from two sensors.

To test this code, I uploaded to one of the ESP32-C3 boards with a tiny display (using Arduino board profile ESP32-C3 dev board) after enabling the USB CDC on boot option to make the serial terminal available. With this working as expected, I was able to continue my project past this example.

As usual, this is available on my Github repository.


/*
	Example code to read two I2C AHT20 sensors connected to different pins
	and print their temperature to the serial terminal using the SoftWire
	software I2C library.
	
	Based on ChatGPT generated code, but modified to allow two sensors.
	
	Tested with ESP32-C3 board and two AHT20 breakouts from AliExpress.
*/

#include "SoftWire.h"

// sensor 1 pins
#define SENSOR1_SDA_PIN 4
#define SENSOR1_SCL_PIN 3

// sensor 2 pins
#define SENSOR2_SDA_PIN 6
#define SENSOR2_SCL_PIN 5

// AHT20 address. should be 0x38 normally
#define AHT20_ADDR 0x38


SoftWire i2c_1(SENSOR1_SDA_PIN, SENSOR1_SCL_PIN);
SoftWire i2c_2(SENSOR2_SDA_PIN, SENSOR2_SCL_PIN);

// Required by SoftWire high-level API
uint8_t txBuffer_1[16];
uint8_t rxBuffer_1[16];
uint8_t txBuffer_2[16];
uint8_t rxBuffer_2[16];

bool aht20Init();
bool aht20ReadTemperature(float &tempC);

void setup() {
    Serial.begin(115200);

    i2c_1.setTxBuffer(txBuffer_1, sizeof(txBuffer_1));
    i2c_1.setRxBuffer(rxBuffer_1, sizeof(rxBuffer_1));
    i2c_2.setTxBuffer(txBuffer_2, sizeof(txBuffer_2));
    i2c_2.setRxBuffer(rxBuffer_2, sizeof(rxBuffer_2));

    i2c_1.begin();
	i2c_2.begin();

    delay(100);

    if (!aht20Init(i2c_1)) {
        Serial.println("1 AHT20 init failed");
        while (1);
    }
	
	if (!aht20Init(i2c_2)) {
        Serial.println("2 AHT20 init failed");
        while (1);
    }

    Serial.println("AHT20 ready");
}

void loop() {
    float tempC;

    if (aht20ReadTemperature(i2c_1, tempC)) {
        Serial.print("1 Temperature: ");
        Serial.print(tempC, 2);
        Serial.println(" C");
    } else {
        Serial.println("1 Read failed");
    }
	
	if (aht20ReadTemperature(i2c_2, tempC)) {
        Serial.print("2 Temperature: ");
        Serial.print(tempC, 2);
        Serial.println(" C");
    } else {
        Serial.println("2 Read failed");
    }

    delay(2000);
}

bool aht20Init(SoftWire &i2c) {
    // Optional soft reset
    i2c.beginTransmission(AHT20_ADDR);
    i2c.write(0xBA);

    if (i2c.endTransmission() != 0)
        return false;

    delay(50);

    // Calibration / initialization command
    i2c.beginTransmission(AHT20_ADDR);
    i2c.write(0xBE);
    i2c.write(0x08);
    i2c.write(0x00);

    if (i2c.endTransmission() != 0)
        return false;

    delay(20);

    // Read status byte
    uint8_t count = i2c.requestFrom(AHT20_ADDR, (uint8_t)1);

    if (count != 1)
        return false;

    uint8_t status = i2c.read();

    // Bit 3 = calibrated
    if (!(status & 0x08)) {
        Serial.print("Status = 0x");
        Serial.println(status, HEX);
        return false;
    }

    return true;
}

bool aht20ReadTemperature(SoftWire &i2c, float &tempC) {
    // Trigger measurement
    i2c.beginTransmission(AHT20_ADDR);
    i2c.write(0xAC);
    i2c.write(0x33);
    i2c.write(0x00);

    if (i2c.endTransmission() != 0)
        return false;

    delay(100);

    uint8_t data[7];

    uint8_t count = i2c.requestFrom(AHT20_ADDR, (uint8_t)7);

    if (count != 7)
        return false;

    for (uint8_t i = 0; i < 7; i++) {
        if (!i2c.available())
            return false;

        data[i] = i2c.read();
    }

    uint8_t status = data[0];

    // Busy bit
    if (status & 0x80)
        return false;

    uint32_t rawTemp =
        ((uint32_t)(data[3] & 0x0F) << 16) |
        ((uint32_t)data[4] << 8) |
        ((uint32_t)data[5]);

    tempC =
        ((float)rawTemp * 200.0f / 1048576.0f) - 50.0f;

    return true;
}