08 - SHT20 Temperature & Humidity Application

This chapter describes the SHT20 temperature & humidity sensor application example — sht20_display — on the Pico-G1 expansion board. The application demonstrates how to read temperature and humidity data from an SHT20 sensor over the I2C interface and display the ambient information in real time on a TFT screen. It is a fundamental example for learning I2C environmental monitoring sensors.

The application source code is located in the SDK directory source/app/08_sht20_display/ and provides a complete implementation of reading an I2C temperature & humidity sensor.

1 Application Overview

1.1 Features

• I2C temperature & humidity reading: reads SHT20 temperature and humidity data over the I2C interface
• Environmental monitoring: displays the current ambient temperature and humidity in real time
• High-precision display: supports 0.1°C temperature and 1% humidity resolution
• Multi-chip compatibility: supports SHT20/SI7021/HTU21 and other compatible chips

1.2 Technical Specifications

Parameter	Value
I2C interface	I2C3 (/dev/i2c-3)
Slave address	0x40
Temperature range	-40~125°C
Humidity range	0~100% RH
Temperature accuracy	±0.3°C
Humidity accuracy	±2% RH
Refresh interval	2 s (configurable)

1.3 Test Case List

index	Name	Test command	Expected result (success)	Possible causes of failure
1	Basic reading	./sht20_display	TFT shows temperature/humidity, console prints data	I2C connection failed, wrong address
2	Temperature change	Warm the sensor with a hand	Temperature rises by 2~5°C	Slow sensor response, poor contact
3	Humidity change	Breathe on the sensor	Humidity rises by 10~20%	Contaminated sensor, humidity response lag

1.4 Directory Structure

source/app/08_sht20_display/
├── Makefile              # Build script
├── main.c                # Main program
├── sht20.c               # SHT20 driver implementation
├── sht20.h               # SHT20 driver header
├── i2c_hal.c             # I2C HAL layer implementation
├── i2c_hal.h             # I2C HAL layer header
├── spi_hal.c             # SPI HAL layer implementation
├── spi_hal.h             # SPI HAL layer header
├── st7789.c              # ST7789 driver implementation
├── st7789.h              # ST7789 driver header
├── font8x16.h            # 8×16 ASCII bitmap font
└── README.md             # Documentation

2 Hardware Connection

2.1 Pin Definitions

Signal	On-board GPIO	Controller / node	Description
SCL	GPIO4_1	I2C3 → /dev/i2c-3	Shares the I2C3 bus with OLED/MPU6050
SDA	GPIO4_2	I2C3 → /dev/i2c-3	Shared bus
VCC	3.3V	—	SHT20 supply 2.1~3.6V
GND	GND	—	Ground

2.2 Hardware Circuit

SHT20 wiring diagram:

     Pico-G1                   SHT20 Module
  ┌───────────┐              ┌──────────────┐
  │           │              │              │
  │ GPIO4_1 ──┼────── SCL ───┤ SCL          │
  │           │              │              │
  │ GPIO4_2 ──┼────── SDA ───┤ SDA          │
  │           │              │              │
  │    3.3V ───┼─────────────┤ VCC          │
  │           │              │              │
  │     GND ───┼─────────────┤ GND          │
  └───────────┘              └──────────────┘

Chip compatibility

SHT20, SI7021, and HTU21 share compatible I2C addresses and commands, so they can be used interchangeably.

3 Build and Deployment

3.1 Build the Application

export PATH=$PATH:<SDK>/tools/linux/toolchains/arm-gcc12.2.0-linux-uclibceabi/bin
cd <SDK>/source/app/08_sht20_display
make

3.2 Run the Application

scp sht20_display root@<board_ip>:/usr/bin/
ssh root@<board_ip> '/usr/bin/sht20_display'

3.3 Expected Output

Console output

/mnt # ./sht20_display
[sht20] pad 复用：I2C3(4_1/4_2) -> func2
[sht20] pad 0x100C0010 -> 0x00001002
[sht20] pad 0x100C0014 -> 0x00001002
[sht20] 初始化 SPI 屏（/dev/spidev2.0）...
[spi] pad 0x100C0028 -> 0x00001004
[spi] pad 0x100C002C -> 0x00001004
[spi] pad 0x100C0030 -> 0x00001000
[spi] pad 0x100C0020 -> 0x00001005
[spi] pad 0x100C001C -> 0x00001005
[spi] opening /dev/spidev2.0 ...
[spi] spidev opened, fd=3
[spi] spidev mode/bits/speed set (MODE3/8b/24MHz)
[spi] chardev request DC  @ /dev/gpiochip4 line 5 ...
[spi] chardev request RES @ /dev/gpiochip4 line 4 ...
[spi] chardev request CS  @ /dev/gpiochip5 line 1 ...
[spi] chardev-verify: DC=0(expect0) RES=1(expect1) CS=1(expect1)  ==> OK(chardev 真驱动了引脚)
[spi] spi_hal_init done
[tft] init: SLPOUT
[tft] init: SLPOUT +120ms ok
[tft] init: config cmds ok
[tft] init: DISPON
[tft] init: DISPON ok
[tft] init: clear-flush start
[tft] flush #1 start
[tft] flush #1 done
[tft] init: clear-flush done
[sht20] 初始化 SHT20（/dev/i2c-3 addr 0x40）...
[sht20] @ /dev/i2c-3 addr 0x40 就绪 (user_reg=0x3A)
[sht20] 采集中，每 2s 刷新，Ctrl+C 退出。
[sht20] T=23.7 C  RH=60.5 %

TFT screen display

4 SHT20 Temperature & Humidity Principles

4.1 I2C Communication Protocol

The SHT20 communicates using the standard I2C protocol:

• Slave address: 0x40 (write address 0x80, read address 0x81)
• Supported speeds: standard mode (100 kHz) and fast mode (400 kHz)
• Data format: 16-bit measurement data + 8-bit CRC

4.2 Measurement Commands

Command name	Command code	Description
Temperature (hold)	0xE3	Hold Master mode
Temperature (no hold)	0xF3	No Hold Master mode
Humidity (hold)	0xE5	Hold Master mode
Humidity (no hold)	0xF5	No Hold Master mode

4.3 Data Conversion Formulas

Temperature calculation:

float sht20_convert_temperature(uint16_t raw_temp)
{
    return -46.85f + 175.72f * (raw_temp / 65536.0f);
}

Humidity calculation:

float sht20_convert_humidity(uint16_t raw_humi)
{
    return -6.0f + 125.0f * (raw_humi / 65536.0f);
}

5 Key Programming Points

5.1 I2C Device Operations

Opening the I2C device:

int i2c_open(const char *i2c_bus, uint8_t device_addr)
{
    int fd = open(i2c_bus, O_RDWR);
    if (fd < 0) return -1;

    if (ioctl(fd, I2C_SLAVE, device_addr) < 0) {
        close(fd);
        return -1;
    }

    return fd;
}

5.2 Temperature & Humidity Reading

Reading temperature data:

float sht20_read_temperature(int i2c_fd)
{
    uint8_t cmd = 0xE3;  // Temperature hold command
    uint8_t data[3];

    i2c_write_byte(i2c_fd, SHT20_ADDR, &cmd, 1);
    usleep(85000);  // Wait for the measurement to finish

    i2c_read_bytes(i2c_fd, SHT20_ADDR, data, 3);

    uint16_t raw_temp = (data[0] << 8) | data[1];
    return sht20_convert_temperature(raw_temp);
}

Reading humidity data:

float sht20_read_humidity(int i2c_fd)
{
    uint8_t cmd = 0xE5;  // Humidity hold command
    uint8_t data[3];

    i2c_write_byte(i2c_fd, SHT20_ADDR, &cmd, 1);
    usleep(30000);  // Wait for the measurement to finish

    i2c_read_bytes(i2c_fd, SHT20_ADDR, data, 3);

    uint16_t raw_humi = (data[0] << 8) | data[1];
    return sht20_convert_humidity(raw_humi);
}

5.3 CRC Check

CRC8 algorithm:

bool sht20_check_crc(const uint8_t *data, uint8_t len, uint8_t checksum)
{
    uint8_t crc = 0xFF;

    for (uint8_t i = 0; i < len; i++) {
        crc ^= data[i];
        for (uint8_t bit = 8; bit > 0; bit--) {
            if (crc & 0x80) {
                crc = (crc << 1) ^ 0x131;
            } else {
                crc = crc << 1;
            }
        }
    }

    return (crc == checksum);
}

6 Troubleshooting

Problem	Possible cause	Solution
Reading failed	I2C connection failed, wrong address	Check the wiring, confirm address 0x40, use i2cdetect -y 3 to check visibility
Abnormal temperature	Damaged sensor, poor contact	Reconnect the sensor, replace the sensor
Humidity stuck	Calibration issue, contaminated sensor	Recalibrate the sensor, clean the sensor
CRC errors	I2C interference, electrical noise	Check the I2C pull-up resistors, add filter capacitors
Jumpy values	Rapid environment changes, unstable airflow	Stabilize the measurement environment, apply data filtering

Measurement tips

For best measurement results:

• Warm-up time: let the sensor warm up for 1~2 minutes after power-on before measuring
• Placement: avoid direct sunlight, air-conditioner outlets, and heat sources
• Regular calibration: calibrate every 6 months against a standard temperature/humidity source

7 Advanced Features

7.1 Data Filtering

Moving-average filter:

#define FILTER_WINDOW 5

void sht20_filter_add(float temp, float humi)
{
    static float temp_buf[FILTER_WINDOW] = {0};
    static float humi_buf[FILTER_WINDOW] = {0};
    static int index = 0;

    temp_buf[index] = temp;
    humi_buf[index] = humi;
    index = (index + 1) % FILTER_WINDOW;

    // Compute the averages
    float temp_avg = 0, humi_avg = 0;
    for (int i = 0; i < FILTER_WINDOW; i++) {
        temp_avg += temp_buf[i];
        humi_avg += humi_buf[i];
    }
    temp_avg /= FILTER_WINDOW;
    humi_avg /= FILTER_WINDOW;

    printf("[SHT20] 温度: %.1f°C 湿度: %.1f%% (滤波后)\n", temp_avg, humi_avg);
}

7.2 Temperature & Humidity Alarms

Alarm detection logic:

typedef struct {
    float temp_warning_high;   // High temperature warning threshold
    float temp_warning_low;    // Low temperature warning threshold
    float humi_warning_high;   // High humidity warning threshold
    float humi_warning_low;    // Low humidity warning threshold
} sht20_alarm_threshold_t;

sht20_alarm_threshold_t g_alarm_threshold = {
    .temp_warning_high = 35.0f,
    .temp_warning_low = 15.0f,
    .humi_warning_high = 70.0f,
    .humi_warning_low = 30.0f,
};

void sht20_check_alarm(float temp, float humi)
{
    if (temp > g_alarm_threshold.temp_warning_high) {
        printf("[SHT20] 温度过高警告: %.2f°C\n", temp);
    }
    if (temp < g_alarm_threshold.temp_warning_low) {
        printf("[SHT20] 温度过低警告: %.2f°C\n", temp);
    }
    if (humi > g_alarm_threshold.humi_warning_high) {
        printf("[SHT20] 湿度过高警告: %.2f%%\n", humi);
    }
    if (humi < g_alarm_threshold.humi_warning_low) {
        printf("[SHT20] 湿度过低警告: %.2f%%\n", humi);
    }
}

8 References

• I2C Interface in Detail
• OLED Display Application
• TFT Display Application
• Development Environment Setup