Adding driver for four-wire resistive touchscreen (#118)

* resistive: Adding driver for four-wire resistive touchscreen, as used on the Adafruit PyPortal.

touch/point.go

@@ -0,0 +1,17 @@
+package touch
+
+// Pointer is a device that is capable of reading a single touch point
+type Pointer interface {
+	ReadTouchPoint() Point
+}
+
+// Point represents the result of reading a single touch point from a screen.
+// X and Y are the horizontal and vertical coordinates of the touch, while Z
+// represents the touch pressure.  In general, client code will want to inspect
+// the value of Z to see if it is above some threshold to determine if a touch
+// is detected at all.
+type Point struct {
+	X int
+	Y int
+	Z int
+}

touch/resistive/fourwire.go

@@ -0,0 +1,124 @@
+package resistive
+
+import (
+	"machine"
+
+	"tinygo.org/x/drivers/touch"
+)
+
+// FourWire represents a resistive touchscreen with a four-wire interface as
+// described in http://ww1.microchip.com/downloads/en/Appnotes/doc8091.pdf
+type FourWire struct {
+	yp machine.ADC
+	ym machine.ADC
+	xp machine.ADC
+	xm machine.ADC
+
+	readSamples int
+}
+
+// FourWireConfig is passed to the Configure method. All of the pins must be
+// specified for this to be a valid configuration. ReadSamples is optional, and
+// if not set with default to 2.
+type FourWireConfig struct {
+
+	// Y+ pin, must be capable of analog reads
+	YP machine.Pin
+
+	// Y- pin, must be capable of analog reads
+	YM machine.Pin
+
+	// X+ pin, must be capable of analog reads
+	XP machine.Pin
+
+	// X- pin, must be capable of analog reads
+	XM machine.Pin
+
+	// If set, each call to ReadTouchPoint() will sample the X, Y, and Z values
+	// and average them.  This can help smooth out spurious readings, for example
+	// ones that result from the capacitance of a TFT under the touchscreen
+	ReadSamples int
+}
+
+// Configure should be called once before starting to read the device
+func (res *FourWire) Configure(config *FourWireConfig) error {
+
+	res.yp = machine.ADC{Pin: config.YP}
+	res.ym = machine.ADC{Pin: config.YM}
+	res.xp = machine.ADC{Pin: config.XP}
+	res.xm = machine.ADC{Pin: config.XM}
+
+	if config.ReadSamples < 1 {
+		res.readSamples = 2
+	} else {
+		res.readSamples = config.ReadSamples
+	}
+
+	return nil
+}
+
+// ReadTouchPoint reads a single touch.Point from the device.  If the device
+// was configured with ReadSamples > 1, each value will be sampled that many
+// times and averaged to smooth over spurious results of the analog reads.
+func (res *FourWire) ReadTouchPoint() (p touch.Point) {
+	p.X = int(sample(res.ReadX, res.readSamples))
+	p.Y = int(sample(res.ReadY, res.readSamples))
+	p.Z = int(sample(res.ReadZ, res.readSamples))
+	return
+}
+
+// sample the results of the provided function and average the results
+func sample(fn func() uint16, numSamples int) (v uint16) {
+	sum := 0
+	for n := 0; n < numSamples; n++ {
+		sum += int(fn())
+	}
+	return uint16(sum / numSamples)
+}
+
+// ReadX reads the "raw" X-value on a 16-bit scale without multiple sampling
+func (res *FourWire) ReadX() uint16 {
+	res.ym.Pin.Configure(machine.PinConfig{Mode: machine.PinInputPulldown})
+
+	res.xp.Pin.Configure(machine.PinConfig{Mode: machine.PinOutput})
+	res.xp.Pin.High()
+
+	res.xm.Pin.Configure(machine.PinConfig{Mode: machine.PinOutput})
+	res.xm.Pin.Low()
+
+	res.yp.Configure()
+
+	return 0xFFFF - res.yp.Get()
+}
+
+// ReadY reads the "raw" Y-value on a 16-bit scale without multiple sampling
+func (res *FourWire) ReadY() uint16 {
+	res.xm.Pin.Configure(machine.PinConfig{Mode: machine.PinInputPulldown})
+
+	res.yp.Pin.Configure(machine.PinConfig{Mode: machine.PinOutput})
+	res.yp.Pin.High()
+
+	res.ym.Pin.Configure(machine.PinConfig{Mode: machine.PinOutput})
+	res.ym.Pin.Low()
+
+	res.xp.Configure()
+
+	return 0xFFFF - res.xp.Get()
+}
+
+// ReadZ reads the "raw" Z-value on a 16-bit scale without multiple sampling
+func (res *FourWire) ReadZ() uint16 {
+	res.xp.Pin.Configure(machine.PinConfig{Mode: machine.PinOutput})
+	res.xp.Pin.Low()
+
+	res.ym.Pin.Configure(machine.PinConfig{Mode: machine.PinOutput})
+	res.ym.Pin.High()
+
+	res.xm.Configure()
+	res.yp.Configure()
+
+	z1 := res.xm.Get()
+	z2 := res.yp.Get()
+
+	return 0xFFFF - (z2 - z1)
+}