avr: initial implementation for PWM

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

Edited slightly by Ayke van Laethem

src/examples/pwm/pwm.go

@@ -0,0 +1,54 @@
+package main
+
+import (
+	"machine"
+	"time"
+)
+
+// This example assumes that an RGB LED is connected to pins 3, 5 and 6 on an Arduino.
+// Change the values below to use different pins.
+const (
+	redPin   = 3
+	greenPin = 5
+	bluePin  = 6
+)
+
+// cycleColor is just a placeholder until math/rand or some equivalent is working.
+func cycleColor(color uint8) uint8 {
+	if color < 10 {
+		return color + 1
+	} else if color < 200 {
+		return color + 10
+	} else {
+		return 0
+	}
+}
+
+func main() {
+	machine.InitPWM()
+
+	red := machine.PWM{redPin}
+	red.Configure()
+
+	green := machine.PWM{greenPin}
+	green.Configure()
+
+	blue := machine.PWM{bluePin}
+	blue.Configure()
+
+	var rc uint8
+	var gc uint8 = 20
+	var bc uint8 = 30
+
+	for {
+		rc = cycleColor(rc)
+		gc = cycleColor(gc)
+		bc = cycleColor(bc)
+
+		red.Set(uint16(rc) << 8)
+		green.Set(uint16(gc) << 8)
+		blue.Set(uint16(bc) << 8)
+
+		time.Sleep(time.Millisecond * 500)
+	}
+}

src/machine/machine.go

@@ -15,3 +15,7 @@ func (p GPIO) High() {
 func (p GPIO) Low() {
 	p.Set(false)
 }
+
+type PWM struct {
+	Pin uint8
+}

src/machine/machine_avr.go

@@ -58,3 +58,69 @@ func (p GPIO) Get() bool {
 		return (val > 0)
 	}
 }
+
+// InitPWM initializes the registers needed for PWM.
+func InitPWM() {
+	// use waveform generation
+	*avr.TCCR0A |= avr.TCCR0A_WGM00
+
+	// set timer 0 prescale factor to 64
+	*avr.TCCR0B |= avr.TCCR0B_CS01 | avr.TCCR0B_CS00
+
+	// set timer 1 prescale factor to 64
+	*avr.TCCR1B |= avr.TCCR1B_CS11
+
+	// put timer 1 in 8-bit phase correct pwm mode
+	*avr.TCCR1A |= avr.TCCR1A_WGM10
+
+	// set timer 2 prescale factor to 64
+	*avr.TCCR2B |= avr.TCCR2B_CS22
+
+	// configure timer 2 for phase correct pwm (8-bit)
+	*avr.TCCR2A |= avr.TCCR2A_WGM20
+}
+
+// Configure configures a PWM pin for output.
+func (pwm PWM) Configure() {
+	if pwm.Pin < 8 {
+		*avr.DDRD |= 1 << pwm.Pin
+	} else {
+		*avr.DDRB |= 1 << (pwm.Pin - 8)
+	}
+}
+
+// Set turns on the duty cycle for a PWM pin using the provided value. On the AVR this is normally a
+// 8-bit value ranging from 0 to 255.
+func (pwm PWM) Set(value uint16) {
+	value8 := value >> 8
+	switch pwm.Pin {
+	case 3:
+		// connect pwm to pin on timer 2, channel B
+		*avr.TCCR2A |= avr.TCCR2A_COM2B1
+		*avr.OCR2B = avr.RegValue(value8) // set pwm duty
+	case 5:
+		// connect pwm to pin on timer 0, channel B
+		*avr.TCCR0A |= avr.TCCR0A_COM0B1
+		*avr.OCR0B = avr.RegValue(value8) // set pwm duty
+	case 6:
+		// connect pwm to pin on timer 0, channel A
+		*avr.TCCR0A |= avr.TCCR0A_COM0A1
+		*avr.OCR0A = avr.RegValue(value8) // set pwm duty
+	case 9:
+		// connect pwm to pin on timer 1, channel A
+		*avr.TCCR1A |= avr.TCCR1A_COM1A1
+		// this is a 16-bit value, but we only currently allow the low order bits to be set
+		*avr.OCR1AL = avr.RegValue(value8) // set pwm duty
+	case 10:
+		// connect pwm to pin on timer 1, channel B
+		*avr.TCCR1A |= avr.TCCR1A_COM1B1
+		// this is a 16-bit value, but we only currently allow the low order bits to be set
+		*avr.OCR1BL = avr.RegValue(value8) // set pwm duty
+	case 11:
+		// connect pwm to pin on timer 2, channel A
+		*avr.TCCR2A |= avr.TCCR2A_COM2A1
+		*avr.OCR2A = avr.RegValue(value8) // set pwm duty
+	default:
+		panic("Invalid PWM pin")
+	}
+}

src/runtime/runtime_avr.go

@@ -57,7 +57,7 @@ func initUART() {
 	*avr.UBRR0H = 0
 	*avr.UBRR0L = 8
 	*avr.UCSR0B = avr.UCSR0B_RXEN0 | avr.UCSR0B_TXEN0   // enable RX and TX
-	*avr.UCSR0C = avr.UCSR0C_UCSZ0                    // 8-bits data
+	*avr.UCSR0C = avr.UCSR0C_UCSZ01 | avr.UCSR0C_UCSZ00 // 8-bits data
 }
 
 func putchar(c byte) {

tools/gen-device-avr.py

@@ -183,10 +183,22 @@ const (
                     out.write(': {description}'.format(**register))
                 out.write('\n')
             for bitfield in register['bitfields']:
+                name = bitfield['name']
+                value = bitfield['value']
+                if '{:08b}'.format(value).count('1') == 1:
                     out.write('\t{name} = 0x{value:x}'.format(**bitfield))
                     if bitfield['description']:
                         out.write(' // {description}'.format(**bitfield))
                     out.write('\n')
+                else:
+                    n = 0
+                    for i in range(8):
+                        if (value >> i) & 1 == 0: continue
+                        out.write('\t{}{} = 0x{:x}'.format(name, n, 1 << i))
+                        if bitfield['description']:
+                            out.write(' // {description}'.format(**bitfield))
+                        n += 1
+                        out.write('\n')
         out.write(')\n')
 
 def writeLD(outdir, device):