mcp3008: add implementation for MCP3008 ADC with SPI interface

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

mcp3008/mcp3008.go

@@ -0,0 +1,92 @@
+// Package mcp3008 implements a driver for the MCP3008 Analog to Digital Converter.
+//
+// Datasheet: http://ww1.microchip.com/downloads/en/DeviceDoc/21295d.pdf
+//
+package mcp3008 // import "tinygo.org/x/drivers/mcp3008"
+
+import (
+	"errors"
+	"machine"
+)
+
+// Device wraps MCP3008 SPI ADC.
+type Device struct {
+	bus machine.SPI
+	cs  machine.Pin
+	tx  []byte
+	rx  []byte
+	CH0 ADCPin
+	CH1 ADCPin
+	CH2 ADCPin
+	CH3 ADCPin
+	CH4 ADCPin
+	CH5 ADCPin
+	CH6 ADCPin
+	CH7 ADCPin
+}
+
+// ADCPin is the implementation of the ADConverter interface.
+type ADCPin struct {
+	machine.Pin
+	d *Device
+}
+
+// New returns a new MCP3008 driver. Pass in a fully configured SPI bus.
+func New(b machine.SPI, csPin machine.Pin) *Device {
+	d := &Device{bus: b,
+		cs: csPin,
+		tx: make([]byte, 3),
+		rx: make([]byte, 3),
+	}
+
+	// setup all channels
+	d.CH0 = d.GetADC(0)
+	d.CH1 = d.GetADC(1)
+	d.CH2 = d.GetADC(2)
+	d.CH3 = d.GetADC(3)
+	d.CH4 = d.GetADC(4)
+	d.CH5 = d.GetADC(5)
+	d.CH6 = d.GetADC(6)
+	d.CH7 = d.GetADC(7)
+
+	return d
+}
+
+// Configure sets up the device for communication
+func (d *Device) Configure() {
+	d.cs.Configure(machine.PinConfig{Mode: machine.PinOutput})
+}
+
+// Read analog data from channel
+func (d *Device) Read(ch int) (uint16, error) {
+	if ch < 0 || ch > 7 {
+		return 0, errors.New("invalid channel for MCP3008 Read")
+	}
+
+	return d.GetADC(ch).Get(), nil
+}
+
+// GetADC returns an ADC for a specific channel.
+func (d *Device) GetADC(ch int) ADCPin {
+	return ADCPin{machine.Pin(ch), d}
+}
+
+// Get the current reading for a specific ADCPin.
+func (p ADCPin) Get() uint16 {
+	p.d.tx[0] = 0x01
+	p.d.tx[1] = byte(8+p.Pin) << 4
+	p.d.tx[2] = 0x00
+
+	p.d.cs.Low()
+	p.d.bus.Tx(p.d.tx, p.d.rx)
+
+	// scale result to 16bit value like other ADCs
+	result := uint16((p.d.rx[1]&0x3))<<8 + uint16(p.d.rx[2])<<6
+	p.d.cs.High()
+
+	return result
+}
+
+// Configure here just for interface compatibility.
+func (p ADCPin) Configure() {
+}