HTS221

An Elixir library for working with the HTS221 sensor. This sensor reports relative humidity and temperature.

For more information about the HTS221 sensor please see the resources section below.

Installation

The package can be installed by adding hts221 to your list of dependencies in mix.exs:

def deps do
  [
    {:hts221, "~> 1.0"}
  ]
end

Usage

Quick Start

This is assuming the HTS221 is using I2C.

{:ok, _pid} = HTS221.Server.start_link(transport: {HTS221.Transport.I2C, [bus_name: "i2c-1"]})

temp = HTS221.Server.temperature(HTS221.Server)

hum  = HTS221.Server.humidity(HTS221.Server)

Transports

To use the HTS221 you will need to provide a module that implements the HTS221.Transport behaviour. This library provides the HTS221.Transport.I2C module if you are using the HTS221 in I2C mode.

{:ok, transport} = HTS221.Transport.init(HTS221.Transport.I2C, bus_name: "i2c-1")

Calibration

The HTS221 calibration contains read-only data about the coefficients for calculating the temperature in celsius and the humidity in percent. These values are calibrated at the factory for each HTS221 sensor, so they are unique to each sensor. In order to calcuate the temperature and the humidity you will need to read the calibration and use that with the HTS221.calculate_temperature/2 and HTS221.calculate_humidity/2 functions.

To read the calibration use the HTS221.read_calibration/1 function:

{:ok, %HTS221.Calibration{} = calibration} = HTS221.read_calibration(transport)

Power Mode

By default the HTS221 is in "power-down" mode, which means the sensor will not any new readings. In order to enable the sensor to take new readings you can set the HTS221.CTRLReg1 register to use :active for its power mode.

ctrl_reg1 = %HTS221.CTRLReg1{
  power_mode: :active
}

:ok = HTS221.write_register(transport, ctrl_reg1)

However, by default the sensor's output data rate (ORD) is set to :one_shot. This means that you will need to trigger a new reading via the CTRL_REG2. If you want the sensor to read without needing to trigger a new reading you can set the CTRL_REG2 up like this:

ctrl_reg1 = %HTS221.CTRLReg1{
  power_mode: :active,
  output_data_rate: :one_Hz
}

:ok = HTS221.write_register(transport, ctrl_reg1)

For best data consistency though you will want to set the :block_data_update field to :wait_for_reading. The recommend base CTRL_REG1 configuration should look like:

ctrl_reg1 = %HTS221.CTRLReg1{
  power_mode: :active,
  output_data_rate: :one_Hz,
  block_data_update: :wait_for_reading
}

:ok = HTS221.write_register(transport, ctrl_reg1)

Read and Calculate Temperature

{:ok, %HTS221.Calibration{} = calibration} = HTS221.read_calibration(transport)
{:ok, %HTS221.Temperature{} = temp} = HTS221.read_temperature(transport)

HTS221.calculate_temperature(temp, calibration)

Read and Calculate Humidity

{:ok, %HTS221.Calibration{} = calibration} = HTS221.read_calibration(transport)
{:ok, %HTS221.Humidity{} = hum} = HTS221.read_humidity(transport)

HTS221.calculate_temperature(hum, calibration)

Resources

Docs

• HTS221 datasheet
• Interpreting humidity and temperature readings

Recommended Hardware

The hardware below is what I have used to develop, however this should would with any supported hardware that Nerves supports.

• PMOD HTS221
• Raspberry PI PMOD Hat
• Raspberry PI zero