I2C SHT21 Digital Temperature and Humidity Sensor

• Product Name: SHT21 Digital Temperature and Humidity Sensor

• Sensor Type: Combined digital humidity and temperature sensor

• Interface: I²C (Inter-Integrated Circuit)

• Manufacturer: Sensirion AG (original design)

• Package Type: Surface Mount (DFN – Dual Flat No-leads), breakout board available

Electrical Characteristics

• Supply Voltage Range: 2.1V to 3.6V DC

• Typical Operating Voltage: 3.0V DC

• Maximum Supply Current: <1.5 mA (measuring)

• Sleep Mode Current: <0.15 µA

• Communication Interface: I²C (2-wire: SDA & SCL)

• I²C Address: 0x40 (7-bit address)

• Logic Voltage Levels: Compatible with 3.3V and 5V microcontrollers (via level shifting)

• Max Clock Frequency: 400 kHz (Fast Mode I²C)

Measurement Specifications

Humidity Measurement

• Measurement Range: 0% RH to 100% RH

• Typical Accuracy: ±2% RH (20% – 80% RH), ±3% RH (full range)

• Repeatability: ±0.1% RH

• Resolution:

• 12-bit (default), 8-bit (configurable)

• Response Time: ~8 seconds (tau63%)

• Hysteresis: ±1% RH

Temperature Measurement

• Measurement Range: –40°C to +125°C

• Typical Accuracy: ±0.3°C (0°C to 60°C), up to ±0.9°C over full range

• Repeatability: ±0.1°C

• Resolution:

• 14-bit (default), 12-bit (configurable)

• Response Time: ~5 seconds (tau63%)

Sensor Features

• Digital Output: Calibrated and linearized digital I²C output

• Factory Calibrated: Yes

• Checksum Support: CRC-8 for data integrity verification

• Status Bits: End-of-conversion, heater status, etc.

• Integrated Heater: For self-test and dehumidification (controllable)

• Low Power Consumption: Suitable for battery-powered applications

Physical & Mechanical Properties

• Sensor Dimensions (chip): ~3 × 3 × 1.1 mm (DFN package)

• Breakout Board Size (if applicable): ~15mm × 10mm

• Pinout (4-pin):

• VCC – Power Supply (2.1V – 3.6V)

• GND – Ground

• SCL – Serial Clock Line

• SDA – Serial Data Line

Environmental & Protection

• Operating Temperature Range: –40°C to +125°C

• Operating Humidity Range: 0% RH to 100% RH (non-condensing)

• Long-Term Stability:

• Humidity Drift: <0.5% RH/year

• Temperature Drift: <0.04°C/year

• Robustness: Fully calibrated, no need for external components

• Optional Protective Filter Cap: Available for harsh environments

Software & Communication

• Data Protocol: I²C with master-slave architecture

• Typical Commands:

• Trigger Temperature Measurement (Hold/No Hold Master Mode)

• Trigger Humidity Measurement (Hold/No Hold Master Mode)

• Read the user register

• Write the user register

• 8-bit CRC Checksum on each transmission

• Driver Support:

• Libraries available for Arduino, Raspberry Pi, ESP32, STM32

• Compatible with Linux, Windows, RTOS, and embedded systems