feature: add PWM pin transmitter to the Morse code example

README.md

@@ -112,3 +112,4 @@ https://en.wikipedia.org/wiki/WSPR_(amateur_radio_software)
 
 - si5351
 - SX126X
+- PWM pin (limited range, should be used with a appropriate low-pass filter)

examples/morse/featherwing.go

@@ -40,7 +40,7 @@ func initRadio() *morse.Morse {
 	println("setting OOK modulation")
 	dev.SetModulationType(sx127x.SX127X_OPMODE_MODULATION_OOK)
 
-	m := morse.NewMorse(&sx127xRadio{device: dev}, 11_400, 20)
+	m := morse.NewMorse(&sx127xRadio{device: dev}, 540_000, 20)
 	m.Configure()
 
 	return m

examples/morse/go.mod

@@ -7,6 +7,7 @@ replace tinygo.org/x/wireless => ../..
 replace tinygo.org/x/wireless/examples/audio => ../audio
 
 require (
+	github.com/chewxy/math32 v1.11.1
 	tinygo.org/x/drivers v0.34.1-0.20260107195827-c21cd39813be
 	tinygo.org/x/wireless v0.0.0-20260121153201-f0d8647de68c
 	tinygo.org/x/wireless/examples/audio v0.0.0-00010101000000-000000000000

examples/morse/go.sum

@@ -1,3 +1,5 @@
+github.com/chewxy/math32 v1.11.1 h1:b7PGHlp8KjylDoU8RrcEsRuGZhJuz8haxnKfuMMRqy8=
+github.com/chewxy/math32 v1.11.1/go.mod h1:dOB2rcuFrCn6UHrze36WSLVPKtzPMRAQvBvUwkSsLqs=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/ebitengine/oto/v3 v3.1.0 h1:9tChG6rizyeR2w3vsygTTTVVJ9QMMyu00m2yBOCch6U=

examples/morse/main.go

@@ -1,8 +1,9 @@
 // FSK4 modem example
 //
-// tinygo flash -size short -tags=si5351 -target=pico -monitor ./examples/morse
-// tinygo flash -size short -tags=featherwing -target=pybadge -monitor ./examples/morse
-// go run ./examples/morse
+// tinygo flash -size short -tags=si5351 -target=pico -monitor .
+// tinygo flash -size short -tags=featherwing -target=pybadge -monitor .
+// tinygo flash -size short -tags=pwm -target=pico -monitor .
+// go run .
 package main
 
 import (
@@ -27,7 +28,7 @@ func main() {
 		println("Transmitting data:", message)
 		radio.Write(message)
 
-		time.Sleep(10 * time.Second)
+		time.Sleep(5 * time.Second)
 	}
 
 	time.Sleep(2 * time.Second)

examples/morse/noradio.go

@@ -1,4 +1,4 @@
-//go:build !si5351 && !featherwing
+//go:build !si5351 && !featherwing && !pwm
 
 package main
 

examples/morse/pwm.go

@@ -0,0 +1,107 @@
+//go:build pwm
+
+package main
+
+import (
+	"machine"
+	"time"
+
+	"github.com/chewxy/math32"
+	"tinygo.org/x/wireless/morse"
+)
+
+var (
+	pwm                = machine.PWM0
+	pin                = machine.GPIO16
+	sampleRate float32 = 11_025.0 // suitable for AM radio
+)
+
+func initRadio() *morse.Morse {
+	err := pwm.Configure(machine.PWMConfig{
+		Period: 1854, // 1854 ns, approx 540 kHz
+	})
+
+	transmitChannel, err := pwm.Channel(pin)
+	if err != nil {
+		println("failed to configure channel")
+		return nil
+	}
+
+	samples := make([]uint32, 512)
+	generateSineWave(samples, 440, sampleRate)
+
+	m := morse.NewMorse(&PinRadio{transmitChannel: transmitChannel, samples: samples}, 540_000, 5)
+	m.Configure()
+
+	return m
+}
+
+// PinRadio implements a simple radio using PWM on a GPIO pin.
+//
+// It uses PWM to generate an audio tone that is transmitted using amplitude
+// modulation (AM).
+//
+// NOTE: You should use a low-pass filter connected to the output pin to get
+// a cleaner signal and avoid harmonics that will cause interference for both
+// yourself and others.
+//
+// See https://github.com/tudbut/picoAM for the original inspiration.
+type PinRadio struct {
+	transmitChannel uint8
+	samples         []uint32
+	stopChan        chan struct{}
+}
+
+func (r *PinRadio) Transmit(freq uint64) error {
+	r.stop()
+	r.stopChan = make(chan struct{})
+
+	go func(stop <-chan struct{}) {
+		for {
+			select {
+			case <-stop:
+				return
+			default:
+			}
+			for _, v := range r.samples {
+				pwm.Set(r.transmitChannel, v)
+				time.Sleep(time.Duration(1e9/sampleRate) * time.Nanosecond)
+			}
+		}
+	}(r.stopChan)
+
+	return nil
+}
+
+func (r *PinRadio) Standby() error {
+	r.stop()
+	pwm.Set(r.transmitChannel, 0)
+
+	return nil
+}
+
+func (r *PinRadio) Close() error {
+	r.stop()
+	pwm.Set(r.transmitChannel, 0)
+
+	return nil
+}
+
+func (r *PinRadio) stop() {
+	if r.stopChan != nil {
+		close(r.stopChan)
+		r.stopChan = nil
+	}
+}
+
+func generateSineWave(samples []uint32, freq uint64, sampleRate float32) {
+	top := pwm.Top()
+	min := top / 4
+	max := top
+
+	for i := range samples {
+		v := math32.Sin(2.0 * math32.Pi * float32(i) * float32(freq) / sampleRate)
+		v = (v + 1.0) / 2.0 // shift to [0, 1]
+		samples[i] = uint32(float32(min) + v*float32(max-min))
+	}
+}

examples/morse/si5351.go

@@ -18,7 +18,7 @@ func initRadio() *morse.Morse {
 		panic(err)
 	}
 
-	m := morse.NewMorse(&Si5351Radio{device: dev}, 11_400, 20)
+	m := morse.NewMorse(&Si5351Radio{device: dev}, 540_000, 5)
 	m.Configure()
 
 	return m