HTU21D(F) sensor support

Crate for reading temperature and humidity values from a HTU21D(F) sensor over I²C.

The implementation is based on the datasheet provided by MEAS [^0] and was tested with a GY-21 sensor board. The API is heavily inspired by the shtcx crate.

[^0] https://cdn-shop.adafruit.com/datasheets/1899_HTU21D.pdf

lib.rs:

• About

• This is a Rust driver for the HTU21D(F) temperature / humidity sensor
• by TE MEAS found in widely available sensor components. It was tested on
• GY-21 sensor but should work with different HTU21 based sensors too.
• The implementation is based on [the official datasheet by TE
• MEAS](https://cdn-shop.adafruit.com/datasheets/1899_HTU21D.pdf).

• Example performing measurements with the HTU21D on the ESP32

• In this example only the HTU21 (GY-21) is accessed over I²C but since
• in a real-world application that is unlikely to be the only slave
• device on the bus, the I²C struct is wrapped with the
• shared-bus crate.
• use esp_println::{print, println};
• use hal::{clock::ClockControl, gpio::IO, i2c::I2C, peripherals::Peripherals,

•        prelude::*, Delay};
  

• use htu21df_sensor::Sensor;
• // set up clock and delay handle to ensure the proper duration between
• // measurement initiation and reading the result
• let clocks = ClockControl::max(system.clock_control).freeze();
• let mut delay = Delay::new(&clocks);
• // initialize I²C with the default GPIO pins on ESP32
• let i2c = I2C::new(

•  peripherals.I2C0,
  

•  io.pins.gpio21,
  

•  io.pins.gpio22,
  

•  100u32.kHz(),
  

•  /* TODO: this will go away soon