Add MAX6675 device

Author: jmhobbs

max6675/max6675.go

@@ -0,0 +1,51 @@
+// Datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/max6675.pdf
+package max6675
+
+import (
+	"errors"
+	"machine"
+)
+
+// ErrThermocoupleOpen is returned when the thermocouple input is open.
+// i.e. not attached  or faulty
+var ErrThermocoupleOpen = errors.New("thermocouple input open")
+
+type Device struct {
+	bus machine.SPI
+	cs  machine.Pin
+}
+
+// Create a new Device to read from a MAX6675 thermocouple.
+// Pins must be configured before use.  Frequency for SPI
+// should be 4.3MHz maximum.
+func NewDevice(bus machine.SPI, cs machine.Pin) *Device {
+	return &Device{
+		bus: bus,
+		cs:  cs,
+	}
+}
+
+// Read and return the temperature in celsius
+func (d *Device) Read() (float32, error) {
+	var (
+		read  []byte = []byte{0, 0}
+		value uint16
+	)
+
+	d.cs.Low()
+	if err := d.bus.Tx([]byte{0, 0}, read); err != nil {
+		return 0, err
+	}
+	d.cs.High()
+
+	// datasheet: Bit D2 is normally low and goes high if the thermocouple input is open.
+	if read[1]&0x04 == 0x04 {
+		return 0, ErrThermocoupleOpen
+	}
+
+	// data is 12 bits, split across the two bytes
+	// -XXXXXXX XXXXX---
+	value = (uint16(read[0]) << 5) | (uint16(read[1]) >> 3)
+
+	return float32(value) * 0.25, nil
+}