thermistor: add support for thermistors such as the NTC 3950

Signed-off-by: Ron Evans <ron@hybridgroup.com>

.circleci/config.yml

@@ -25,5 +25,6 @@ jobs:
       - run: tinygo build -size short -o test.elf -target=itsybitsy-m0 ./examples/mag3110/main.go
       - run: tinygo build -size short -o test.elf -target=itsybitsy-m0 ./examples/mma8653/main.go
       - run: tinygo build -size short -o test.elf -target=itsybitsy-m0 ./examples/mpu6050/main.go
+      - run: tinygo build -size short -o test.elf -target=circuitplay-express ./examples/thermistor/main.go
       - run: tinygo build -size short -o test.elf -target=itsybitsy-m0 ./examples/vl53l1x/main.go
       - run: tinygo build -size short -o test.elf -target=circuitplay-express ./examples/ws2812/main.go

README.md

@@ -66,6 +66,7 @@ func main() {
 | [MAG3110 magnetometer](https://www.nxp.com/docs/en/data-sheet/MAG3110.pdf) | I2C |
 | [MMA8653 accelerometer](https://www.nxp.com/docs/en/data-sheet/MMA8653FC.pdf) | I2C |
 | [MPU6050 accelerometer/gyroscope](https://store.invensense.com/datasheets/invensense/MPU-6050_DataSheet_V3%204.pdf) | I2C |
+| [Thermistor](https://www.farnell.com/datasheets/33552.pdf) | ADC |
 | [VL53L1X time-of-flight distance sensor](https://www.st.com/resource/en/datasheet/vl53l1x.pdf) | I2C |
 | [WS2812 RGB LED](https://cdn-shop.adafruit.com/datasheets/WS2812.pdf) | GPIO |
 

examples/thermistor/main.go

@@ -0,0 +1,26 @@
+// This example uses the settings for the thermistor that is built in to the
+// Adafruit Circuit Playground Express.
+package main
+
+import (
+	"machine"
+	"time"
+
+	"github.com/tinygo-org/drivers/thermistor"
+)
+
+const ADC_PIN = machine.TEMPSENSOR
+
+func main() {
+	machine.InitADC()
+
+	sensor := thermistor.New(ADC_PIN)
+	sensor.Configure()
+
+	for {
+		temp, _ := sensor.ReadTemperature()
+		println("Temperature:", temp/1000, "ºC")
+
+		time.Sleep(2 * time.Second)
+	}
+}

thermistor/thermistor.go

@@ -0,0 +1,86 @@
+// Package thermistor is for temperature sensing using a thermistor
+// such as the NTC 3950.
+//
+// Datasheet: https://www.farnell.com/datasheets/33552.pdf
+//
+// This code is an interpretation of Adafruit Thermistor module in Python:
+// https://github.com/adafruit/Adafruit_CircuitPython_Thermistor
+//
+// It uses the Steinhart–Hart equation to calculate the temperature
+// based on the resistance:
+// https://en.wikipedia.org/wiki/Steinhart%E2%80%93Hart_equation
+//
+// To use with other thermistors adjust the BCoefficient and NominalTemperature
+// values to match the specific thermistor you wish to use.
+//
+//			sensor.NominalTemperature = 25
+//			sensor.BCoefficient = 3950
+//
+// Set the SeriesResistor and NominalResistance based on the microcontroller voltage and
+// circuit that you have in use. Set HighSide based on if the thermistor is connected from
+// the ADC pin to the powered side (true) or to ground (false).
+//
+//			sensor.SeriesResistor = 10000
+//			sensor.NominalResistance = 10000
+//			sensor.HighSide = true
+//
+package thermistor
+
+import (
+	"machine"
+	"math"
+)
+
+// Device holds the ADC pin and the needed settings for calculating the
+// temperature based on the resistance.
+type Device struct {
+	adc                *machine.ADC
+	SeriesResistor     uint32
+	NominalResistance  uint32
+	NominalTemperature uint32
+	BCoefficient       uint32
+	HighSide           bool
+}
+
+// New returns a new thermistor driver given an ADC pin.
+func New(pin uint8) Device {
+	adc := machine.ADC{pin}
+	return Device{
+		adc:                &adc,
+		SeriesResistor:     10000,
+		NominalResistance:  10000,
+		NominalTemperature: 25,
+		BCoefficient:       3950,
+		HighSide:           true,
+	}
+}
+
+// Configure configures the ADC pin used for the thermistor.
+func (d *Device) Configure() {
+	d.adc.Configure()
+}
+
+// ReadTemperature returns the temperature in celsius milli degrees (ºC/1000)
+func (d *Device) ReadTemperature() (temperature int32, err error) {
+	var reading uint32
+	if d.HighSide {
+		// Thermistor connected from analog input to high logic level.
+		val := d.adc.Get()
+		reading = uint32(val) / 64
+		reading = (1023 * d.SeriesResistor) / reading
+		reading -= d.SeriesResistor
+	} else {
+		// Thermistor connected from analog input to ground.
+		reading = d.SeriesResistor / uint32(65535/d.adc.Get()-1)
+	}
+
+	var steinhart float64
+	steinhart = float64(reading) / float64(d.NominalResistance) // (R/Ro)
+	steinhart = math.Log(steinhart)                             // ln(R/Ro)
+	steinhart /= float64(d.BCoefficient)                        // 1/B * ln(R/Ro)
+	steinhart += 1.0 / (float64(d.NominalTemperature) + 273.15) // + (1/To)
+	steinhart = 1.0 / steinhart                                 // Invert
+	steinhart -= 273.15                                         // convert to C
+
+	return int32(steinhart * 1000), nil
+}