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

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

examples/touch/resistive/fourwire/main.go

@@ -0,0 +1,80 @@
+// demo of 4-wire touchscreen as described in app note:
+// http://ww1.microchip.com/downloads/en/Appnotes/doc8091.pdf
+package main
+
+import (
+	"machine"
+	"math"
+
+	"tinygo.org/x/drivers/touch"
+	"tinygo.org/x/drivers/touch/resistive"
+)
+
+var (
+	resistiveTouch = new(resistive.FourWire)
+)
+
+const (
+	Xmin = 750
+	Xmax = 325
+	Ymin = 840
+	Ymax = 240
+)
+
+func main() {
+
+	// configure touchscreen
+	machine.InitADC()
+	resistiveTouch.Configure(&resistive.FourWireConfig{
+		YP: machine.TOUCH_YD, // y+
+		YM: machine.TOUCH_YU, // y-
+		XP: machine.TOUCH_XR, // x+
+		XM: machine.TOUCH_XL, // x-
+	})
+
+	last := touch.Point{}
+
+	// loop and poll for touches, including performing debouncing
+	debounce := 0
+	for {
+
+		point := resistiveTouch.ReadTouchPoint()
+		touch := touch.Point{}
+		if point.Z>>6 > 100 {
+			touch.X = mapval(point.X>>6, Xmin, Xmax, 0, 240)
+			touch.Y = mapval(point.Y>>6, Ymin, Ymax, 0, 320)
+			touch.Z = point.Z >> 6 / 100
+		} else {
+			touch.X = 0
+			touch.Y = 0
+			touch.Z = 0
+		}
+
+		if last.Z != touch.Z {
+			debounce = 0
+			last = touch
+		} else if math.Abs(float64(touch.X-last.X)) > 4 ||
+			math.Abs(float64(touch.Y-last.Y)) > 4 {
+			debounce = 0
+			last = touch
+		} else if debounce > 1 {
+			debounce = 0
+			HandleTouch(last)
+		} else if touch.Z > 0 {
+			debounce++
+		} else {
+			last = touch
+			debounce = 0
+		}
+
+	}
+}
+
+// based on Arduino's "map" function
+func mapval(x int, inMin int, inMax int, outMin int, outMax int) int {
+	return (x-inMin)*(outMax-outMin)/(inMax-inMin) + outMin
+}
+
+func HandleTouch(touch touch.Point) {
+	println("touch point:", touch.X, touch.Y, touch.Z)
+}