touch: add capacitive touch sensing on normal GPIO pins

Tested on the following chips/boards:

  * RP2040 (Raspberry Pi Pico)
  * ATSAMD21 (Adafruit PyBadge)
  * NRF52840 (PCA10056 developer board)
  * ESP8266 (NodeMCU)
  * ATmega328p (Arduino Uno)
  * ESP32C3 (WaveShare ESP-C3-32S-Kit)
  * FE310 (SiFive HiFive1 rev B)

The sensitivity threshold in the example may need to be adjusted per
board though, the default value of 100 typically recognizes when a cable
is being touched but the RP2040 for example is capable of doing much
more precise measurements if the power supply is sufficiently
noise-free.

examples/touch/capacitive/main.go

@@ -0,0 +1,69 @@
+// Capacitive touch sensing example.
+//
+// This capacitive touch sensor works by charging a normal GPIO pin, then slowly
+// discharging it through a 1MΩ resistor and seeing how long it takes to go from
+// high to low.
+//
+// Use as follows:
+//   - Change touchPin below as needed.
+//   - Connect this pin to some metal surface, like a piece of aluminimum foil.
+//     Make sure this surface is covered (using paper, Scotch tape, etc).
+//   - Also connect this same pin to ground through a 1MΩ resistor.
+//
+// This sensor is very sensitive to noise on the power source, so you should
+// probably try to limit it by running from a battery for example. Especially
+// phone chargers can produce a lot of noise.
+package main
+
+import (
+	"machine"
+	"time"
+
+	"tinygo.org/x/drivers/touch/capacitive"
+)
+
+const touchPin = machine.GP16 // Raspberry Pi Pico
+
+func main() {
+	time.Sleep(time.Second * 2)
+	println("start")
+
+	led := machine.LED
+	led.Configure(machine.PinConfig{Mode: machine.PinOutput})
+	led.Low()
+
+	// Configure the array of GPIO pins used for capacitive touch sensing.
+	// We're using only one pin.
+	array := capacitive.NewArray([]machine.Pin{touchPin})
+
+	// Use a dynamic threshold, meaning the GPIO pin is automatically calibrated
+	// and re-calibrated to adjust for varying environments (e.g. changing
+	// humidity).
+	array.SetDynamicThreshold(100)
+
+	wasTouching := false
+	for i := uint32(0); ; i++ {
+		// Update the GPIO pin. This must be called very often.
+		array.Update()
+		touching := array.Touching(0)
+
+		// Indicate whether the pin is touched via the LED.
+		led.Set(touching)
+
+		// Print something when the touch state changed.
+		if wasTouching != touching {
+			wasTouching = touching
+			if touching {
+				println("  touch!")
+			} else {
+				println("  release!")
+			}
+		}
+
+		// Print the current value, as a debugging aid. It's not really meant to
+		// be used directly.
+		if i%128 == 32 {
+			println("touch value:", array.RawValue(0))
+		}
+	}
+}

smoketest.sh

@@ -73,6 +73,7 @@ tinygo build -size short -o ./build/test.hex -target=microbit ./examples/st7789/
 tinygo build -size short -o ./build/test.hex -target=circuitplay-express ./examples/thermistor/main.go
 tinygo build -size short -o ./build/test.hex -target=circuitplay-bluefruit ./examples/tone
 tinygo build -size short -o ./build/test.hex -target=arduino-nano33 ./examples/tm1637/main.go
+tinygo build -size short -o ./build/test.hex -target=pico ./examples/touch/capacitive
 tinygo build -size short -o ./build/test.hex -target=pyportal ./examples/touch/resistive/fourwire/main.go
 tinygo build -size short -o ./build/test.hex -target=pyportal ./examples/touch/resistive/pyportal_touchpaint/main.go
 tinygo build -size short -o ./build/test.hex -target=itsybitsy-m0 ./examples/vl53l1x/main.go

touch/capacitive/gpio.go

@@ -0,0 +1,313 @@
+package capacitive
+
+import (
+	"machine"
+	"runtime/interrupt"
+	"time"
+)
+
+const (
+	// How often to measure.
+	// The Update function will wait until this amount of time has passed.
+	measurementFrequency       = 200
+	minTimeBetweenMeasurements = time.Second / measurementFrequency
+
+	// How much to multiply values before averaging. A value higher than 1 will
+	// help to avoid integer rounding errors and may improve accuracy slightly.
+	oversampling = 8
+
+	// How many samples to use for the moving average.
+	movingAverageWindow = 16
+
+	// After how many samples should the touch sensor be recalibrated?
+	// This should be a power of two (for efficient division) and be a multiple
+	// of movingAverageWindow. Ideally it should cause a recalibration every 5s
+	// or so.
+	recalibrationSamples = 1024
+)
+
+type Array struct {
+	// Time when the last update finished. This is used to make sure we call
+	// Update() the expected number of times per second.
+	lastUpdate time.Time
+
+	// List of pins to measure each time.
+	pins []machine.Pin
+
+	// Raw values (non-smoothed) from the last read.
+	values []uint16
+
+	hasFirstMeasurement bool
+
+	// Static threshold. Zero if using a dynamic threshold.
+	staticThreshold uint16
+
+	// How long to measure.
+	measureCycles uint16
+
+	// Sensitivity (in promille) for the dynamic threshold.
+	sensitivity uint16
+
+	// Capacitance trackers for dynamic capacitance measurement.
+	trackers []capacitanceTracker
+}
+
+// Create a new array of pins to be used as touch sensors.
+// The pins do not need to be initialized. The array is immediately ready to
+// use.
+//
+// By default, NewArray configures a static threshold that is not very
+// sensitive. If you want the touch inputs to be more sensitive, use
+// SetDynamicThreshold.
+func NewArray(pins []machine.Pin) *Array {
+	for _, pin := range pins {
+		pin.Configure(machine.PinConfig{Mode: machine.PinOutput})
+		pin.High()
+	}
+	array := &Array{
+		pins:          pins,
+		values:        make([]uint16, len(pins)),
+		measureCycles: uint16(machine.CPUFrequency() / 125000), // 1000 on the RP2040 (which is 125MHz)
+		lastUpdate:    time.Now(),
+	}
+
+	// A threshold of 500 works well on the RP2040. Scale this number to
+	// something similar on other chips.
+	array.SetStaticThreshold(int(machine.CPUFrequency() / 250000))
+
+	return array
+}
+
+// Use a static threshold. This works well on simple touch surfaces where you'll
+// directly touch the metal.
+func (a *Array) SetStaticThreshold(threshold int) {
+	if threshold > 0xffff {
+		threshold = 0xffff
+	}
+	a.staticThreshold = uint16(threshold)
+	a.trackers = nil
+}
+
+// Use a dynamic threshold (as promille), that will calibrate automatically.
+// This is needed when you want to be able to detect touches through a
+// non-conducting surface for example. Something like 100‰ (10%) will probably
+// work in many cases, though you may need to try different value to reliably
+// detect touches.
+func (a *Array) SetDynamicThreshold(sensitivity int) {
+	a.sensitivity = uint16(sensitivity)
+	a.staticThreshold = 0
+	a.trackers = make([]capacitanceTracker, len(a.pins))
+}
+
+// Measure all GPIO pins. This function must be called very often, ideally about
+// 100-200 times per second (it will delay a bit when called more than 200 times
+// per second).
+func (a *Array) Update() {
+	// Wait until enough time has passed to charge all pins.
+	now := time.Now()
+	timeSinceLastUpdate := now.Sub(a.lastUpdate)
+	sleepTime := minTimeBetweenMeasurements - timeSinceLastUpdate
+	time.Sleep(sleepTime)
+	a.lastUpdate = now.Add(sleepTime) // should be ~equivalent to time.Now()
+
+	// Measure each pin in turn.
+	for i, pin := range a.pins {
+		// Interrupts must be disabled during measuring for accurate results.
+		mask := interrupt.Disable()
+
+		// Switch to input. This will stop the charging, and let it discharge
+		// through the resistor.
+		pin.Configure(machine.PinConfig{Mode: machine.PinInput})
+
+		// Wait for the pin to go low again.
+		// A longer duration means more capacitance, which means something is
+		// touching it (finger, banana, etc).
+		count := uint32(i)
+		for i := 0; i < int(a.measureCycles); i++ {
+			if !pin.Get() {
+				break
+			}
+			count++
+		}
+
+		interrupt.Restore(mask)
+
+		a.values[i] = uint16(count)
+
+		// Set the pin to high, to charge it for the next measurement.
+		pin.Configure(machine.PinConfig{Mode: machine.PinOutput})
+		pin.High()
+	}
+
+	// The first measurement tends to be slightly off (too low value) so ignore
+	// that one.
+	if !a.hasFirstMeasurement {
+		a.hasFirstMeasurement = true
+		return
+	}
+
+	for i := 0; i < len(a.trackers); i++ {
+		a.trackers[i].addValue(int(a.values[i]), int(a.sensitivity))
+	}
+}
+
+// Return the raw value of the given pin index of the most recent call to
+// Update. This value is not smoothed in any way.
+func (a *Array) RawValue(index int) int {
+	return int(a.values[index])
+}
+
+// Return the value from the moving average. This value is only available when a
+// dynamic threshold has been set, it will panic otherwise.
+func (a *Array) SmoothedValue(index int) int {
+	return int(a.trackers[index].avg) / oversampling
+}
+
+// Return whether the given pin index is currently being touched.
+func (a *Array) Touching(index int) bool {
+	if a.staticThreshold != 0 {
+		// Using a static threshold.
+		return a.values[index] > a.staticThreshold
+	}
+
+	return a.trackers[index].touching
+}
+
+// Separate object to store calibration data and track capacitance over time.
+type capacitanceTracker struct {
+	recentValues [movingAverageWindow]uint16
+	sum          uint32
+	avg          uint16
+
+	baseline   uint16
+	noise      uint16
+	valueCount uint8
+	touching   bool
+
+	recalibrationCount    uint8
+	recalibrationPrevAvg  uint16
+	recalibrationNoiseSum int32
+	recalibrationSum      uint32
+}
+
+func (ct *capacitanceTracker) addValue(value int, sensitivity int) {
+	// Maybe increase the resolution slightly by oversampling. This should
+	// increase the resolution a little bit after averaging and should reduce
+	// rounding errors.
+	// Typical input values on the RP2040 are 100-200 (or up to 1000 or so when
+	// touching the metal) so multiplying by 4-8 should be fine. Other chips
+	// generally have much lower values.
+	value *= oversampling
+	if value > 0xffff {
+		value = 0xffff // unlikely, but make sure we don't overflow
+	}
+
+	// This does a number of things at the same time:
+	//  * Add the new value to the recentValues array.
+	//  * Calculate the moving sum (and average) of recentValues using a
+	//    recursive moving average algorithm:
+	//    https://www.dspguide.com/ch15/5.htm
+	ptr := &ct.recentValues[ct.valueCount%movingAverageWindow]
+	ct.sum -= uint32(*ptr)
+	ct.sum += uint32(value)
+	ct.avg = uint16(ct.sum / movingAverageWindow)
+	*ptr = uint16(value)
+	ct.valueCount++
+
+	// Do an initial calibration once the first values have been read.
+	if ct.baseline == 0 && ct.valueCount == movingAverageWindow {
+		ct.baseline = ct.avg
+
+		// Calculate initial noise as an average absolute deviation:
+		// https://en.wikipedia.org/wiki/Average_absolute_deviation
+		// This is a quick and imprecise way to find the noise, better noise
+		// detection happens during recalibration.
+		var diffSum uint32
+		for _, sample := range ct.recentValues {
+			diff := int(ct.avg) - int(sample)
+			if diff < 0 {
+				diff = -diff
+			}
+			diffSum += uint32(diff)
+		}
+		ct.noise = uint16(diffSum / (movingAverageWindow / 2))
+	}
+
+	// Now determine whether the touch pad is being touched.
+
+	if ct.baseline == 0 {
+		// Not yet calibrated.
+		ct.touching = false
+		return
+	}
+
+	// Calculate the threshold.
+	// Divide by 65536 (instead of 65500) to avoid a potentially expensive
+	// division while still being close enough.
+	threshold := (uint32(ct.baseline) * uint32(sensitivity+1000) * 65) / 65536
+
+	// Add noise to the threshold, to avoid toggling quickly. This mainly
+	// filters out mains noise.
+	threshold += uint32(ct.noise)
+
+	// Implement some hysteresis: if the touch pad was previously touched, lower
+	// the threshold a little to avoid bouncing effects.
+	// TODO: let this hysteresis depend on the amount of noise.
+	if ct.touching {
+		threshold = (threshold*3 + uint32(ct.baseline)) / 4 // lower the threshold by 25%
+	}
+
+	// Is the pad being touched?
+	ct.touching = uint32(ct.avg) > threshold
+
+	// Do a recalibration after the sensor hasn't been touched for ~5s, to
+	// account for drift over time (humidity etc).
+	if ct.touching {
+		// Reset calibration (start from zero).
+		ct.recalibrationCount = 0
+		ct.recalibrationSum = 0
+		ct.recalibrationNoiseSum = 0
+	} else {
+		// Add the last batch of samples to the sum.
+		if ct.valueCount%movingAverageWindow == 0 {
+			ct.recalibrationCount++
+
+			// Wait a few cycles before starting data collection for
+			// calibration.
+			cycle := int(ct.recalibrationCount) - 3
+
+			if cycle < 0 {
+				// Store the previous average, to calculate the noise value.
+				ct.recalibrationPrevAvg = ct.avg
+
+			} else if cycle >= 0 {
+				// Collect data for recalibration.
+				ct.recalibrationSum += ct.sum
+
+				// Add difference between two (averaged) samples as a measure of
+				// the noise.
+				diff := int32(ct.recalibrationPrevAvg) - int32(ct.avg)
+				if diff < 0 {
+					diff = -diff
+				}
+				ct.recalibrationNoiseSum += diff
+				ct.recalibrationPrevAvg = ct.avg
+
+			}
+
+			// Do the recalibration after enough samples have been collected.
+			// Note: the noise is basically the average of absolute differences
+			// between two averaging windows. I don't know whether this
+			// algorithm has a name, but it seems to work here to detect the
+			// amount of noise.
+			const totalRecalibrationCount = recalibrationSamples / movingAverageWindow
+			if cycle == totalRecalibrationCount {
+				ct.baseline = uint16(ct.recalibrationSum / recalibrationSamples)
+				ct.noise = uint16(ct.recalibrationNoiseSum / (totalRecalibrationCount / 2))
+				ct.recalibrationCount = 0
+				ct.recalibrationSum = 0
+				ct.recalibrationNoiseSum = 0
+			}
+		}
+	}
+}