Sensirion Sht4x Support (#597)

sht4x: implemented driver

sht4x/sht4x.go

@@ -0,0 +1,76 @@
+// Package sht4x provides a driver for the SHT4x digital humidity sensor series by Sensirion.
+// Datasheet: https://www.sensirion.com/media/documents/33FD6951/64D3B030/Sensirion_Datasheet_SHT4x.pdf
+package sht4x
+
+import (
+	"time"
+
+	"tinygo.org/x/drivers"
+)
+
+const DefaultAddress = 0x44
+
+const (
+	// single-shot, high-repeatability measurement
+	commandMeasurement = 0xfd
+)
+
+// Device represents a SHT4x sensor
+type Device struct {
+	bus     drivers.I2C
+	Address uint8
+}
+
+// New creates a new SHT4x connection. The I2C bus must already be
+// configured.
+func New(bus drivers.I2C) Device {
+	return Device{
+		bus:     bus,
+		Address: DefaultAddress,
+	}
+}
+
+// ReadTemperatureHumidity starts a measurement and then reads out the results. This function blocks
+// while the measurement is in progress.
+//
+// Temperature is returned in [degree Celsius], multiplied by 1000,
+// and relative humidity in [percent relative humidity], multiplied by 1000.
+func (d *Device) ReadTemperatureHumidity() (temperatureMilliCelsius int32, relativeHumidityMilliPercent int32, err error) {
+	rawTemp, rawHum, err := d.rawReadings()
+	if err != nil {
+		return 0, 0, err
+	}
+
+	// from the reference driver: https://github.com/Sensirion/embedded-sht/blob/fcc8a523210cc1241a2750899ff6b0f68f3ed212/sht4x/sht4x.c#L81
+	temperatureMilliCelsius = ((21875 * int32(rawTemp)) >> 13) - 45000
+	relativeHumidityMilliPercent = ((15625 * int32(rawHum)) >> 13) - 6000
+
+	return temperatureMilliCelsius, relativeHumidityMilliPercent, err
+}
+
+// rawReadings returns the sensor's raw values of the temperature and humidity
+func (d *Device) rawReadings() (uint16, uint16, error) {
+	err := d.bus.Tx(uint16(d.Address), []byte{commandMeasurement}, nil)
+	if err != nil {
+		return 0, 0, err
+	}
+
+	// max time for measurement according to datasheet
+	time.Sleep(10 * time.Millisecond)
+
+	var data [6]byte
+	err = d.bus.Tx(uint16(d.Address), nil, data[:])
+	if err != nil {
+		return 0, 0, err
+	}
+
+	tTicks := readUint(data[0], data[1])
+	rhTicks := readUint(data[3], data[4])
+
+	return tTicks, rhTicks, nil
+}
+
+// readUint converts two bytes to uint16
+func readUint(msb byte, lsb byte) uint16 {
+	return (uint16(msb) << 8) | uint16(lsb)
+}