Overview

The HTU21D (and HTU21DF variant with PTFE filter membrane) is a digital relative humidity and temperature sensor manufactured by TE Connectivity (formerly Measurement Specialties). Housed in a tiny $3.0 \times 3.0\text{ mm}$ 6-pin DFN package, it is widely used in weather stations, home automation HVAC controllers, and medical equipment.

Delivering typical accuracies of $\pm 2\%\ \text{RH}$ and $\pm 0.3^\circ\text{C}$, user-programmable resolution (12-bit RH / 14-bit Temp down to 8-bit / 11-bit), CRC-8 checksum verification, an onboard heating element, and an $I^2C$ interface (0x40), the HTU21D draws an ultra-low standby current of $0.14\ \mu\text{A}$.

Quick reference

Pinout

Breakout module 4-pin header:

Specifications

$I^2C$ Command Opcodes & Math

• Trigger Temperature Measure (Hold Master): 0xE3
• Trigger Humidity Measure (Hold Master): 0xE5
• Trigger Temperature Measure (No Hold): 0xF3
• Trigger Humidity Measure (No Hold): 0xF5

Temperature Calculation ($^\circ\text{C}$)

$\text{Temperature } (^\circ\text{C}) = -46.85 + 175.72 \times \left( \frac{\text{Raw 16-bit Temp Register \& 0xFFFC}}{65536} \right)$

Humidity Calculation ($\%\text{ RH}$)

$\text{Relative Humidity } (\%\text{ RH}) = -6.0 + 125.0 \times \left( \frac{\text{Raw 16-bit RH Register \& 0xFFFC}}{65536} \right)$

Wiring

Example (Adafruit_HTU21DF Library)

#include <Wire.h>
#include "Adafruit_HTU21DF.h"

Adafruit_HTU21DF htu = Adafruit_HTU21DF();

void setup() {
  Serial.begin(115200);
  Serial.println("Adafruit HTU21D-F Humidity & Temp Test");

  if (!htu.begin()) {
    Serial.println("Couldn't find HTU21D-F sensor! Check I2C wiring.");
    while (1);
  }
}

void loop() {
  float temp = htu.readTemperature();
  float rel_hum = htu.readHumidity();

  Serial.print("Temp: "); Serial.print(temp); Serial.print(" °C | ");
  Serial.print("Humidity: "); Serial.print(rel_hum); Serial.println(" % RH");

  delay(1000);
}

Common mistakes

• Holding finger near sensor element: Human skin radiates moisture ($\ge 70\%\text{ RH}$) and thermal heat. Keep hands clear of the small white PTFE sensor filter while taking ambient room measurements.
• Forgetting I2C bus address collision: HTU21D uses fixed address 0x40, which collides with HDC1080 and Si7021 sensors if placed on the same $I^2C$ bus.

Notes

• HTU21D vs Si7021 vs SHT31: HTU21D is fully command-compatible with the Si7021; SHT31 offers higher accuracy ($\pm 1.5\%\text{ RH}$) and configurable $I^2C$ addresses (0x44/0x45).