avr: add support for the digispark

Blinking the on-board LED works. Nothing else has been tested yet.

14 files changed, 421 insertions, 291 deletions

diff --git a/.travis.yml b/.travis.yml
index 7a684902c..4ede432f0 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -25,4 +25,5 @@ script:
   - tinygo build -o blinky1.nrf51.elf -target=microbit     examples/echo
   - tinygo build -o test.nrf.elf      -target=nrf52840-mdk examples/blinky1
   - tinygo build -o blinky1.stm32.elf -target=bluepill     examples/blinky1
-  - tinygo build -o blinky1.avr.o     -target=arduino      examples/blinky1 # TODO: avr-as/avr-gcc doesn't work
+  - tinygo build -o blinky1.o         -target=arduino      examples/blinky1 # TODO: avr-as/avr-gcc doesn't work
+  - tinygo build -o blinky1.o         -target=digispark    examples/blinky1
diff --git a/Makefile b/Makefile
index 283a1414f..01d0f6252 100644
--- a/Makefile
+++ b/Makefile
@@ -72,6 +72,7 @@ gen-device: gen-device-avr gen-device-nrf gen-device-stm32
 
 gen-device-avr:
 	./tools/gen-device-avr.py lib/avr/packs/atmega src/device/avr/
+	./tools/gen-device-avr.py lib/avr/packs/tiny src/device/avr/
 	go fmt ./src/device/avr
 
 gen-device-nrf:
diff --git a/docs/targets.rst b/docs/targets.rst
index 6f7c6d96a..741dc3ec1 100644
--- a/docs/targets.rst
+++ b/docs/targets.rst
@@ -47,6 +47,9 @@ Note: the AVR backend of LLVM is still experimental so you may encounter bugs.
 
   * `Arduino Uno <https://store.arduino.cc/arduino-uno-rev3>`_ (`ATmega328p
     <https://www.microchip.com/wwwproducts/en/ATmega328p>`_)
+  * `Digispark <http://digistump.com/products/1>`_ (`ATtiny85
+    <https://www.microchip.com/wwwproducts/en/ATtiny85>`_) |br|
+    Very limited support at the moment.
 
 
 WebAssembly
diff --git a/src/machine/board_digispark.go b/src/machine/board_digispark.go
new file mode 100644
index 000000000..e4065d357
--- /dev/null
+++ b/src/machine/board_digispark.go
@@ -0,0 +1,7 @@
+// +build attiny85,digispark
+
+package machine
+
+const (
+	LED = 1
+)
diff --git a/src/machine/machine_atmega.go b/src/machine/machine_atmega.go
new file mode 100644
index 000000000..ed93fc3eb
--- /dev/null
+++ b/src/machine/machine_atmega.go
@@ -0,0 +1,262 @@
+// +build avr,atmega
+
+package machine
+
+import (
+	"device/avr"
+)
+
+// Configure sets the pin to input or output.
+func (p GPIO) Configure(config GPIOConfig) {
+	if config.Mode == GPIO_OUTPUT { // set output bit
+		if p.Pin < 8 {
+			*avr.DDRD |= 1 << p.Pin
+		} else {
+			*avr.DDRB |= 1 << (p.Pin - 8)
+		}
+	} else { // configure input: clear output bit
+		if p.Pin < 8 {
+			*avr.DDRD &^= 1 << p.Pin
+		} else {
+			*avr.DDRB &^= 1 << (p.Pin - 8)
+		}
+	}
+}
+
+// Set changes the value of the GPIO pin. The pin must be configured as output.
+func (p GPIO) Set(value bool) {
+	if value { // set bits
+		if p.Pin < 8 {
+			*avr.PORTD |= 1 << p.Pin
+		} else {
+			*avr.PORTB |= 1 << (p.Pin - 8)
+		}
+	} else { // clear bits
+		if p.Pin < 8 {
+			*avr.PORTD &^= 1 << p.Pin
+		} else {
+			*avr.PORTB &^= 1 << (p.Pin - 8)
+		}
+	}
+}
+
+// Get returns the current value of a GPIO pin.
+func (p GPIO) Get() bool {
+	if p.Pin < 8 {
+		val := *avr.PIND & (1 << p.Pin)
+		return (val > 0)
+	} else {
+		val := *avr.PINB & (1 << (p.Pin - 8))
+		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")
+	}
+}
+
+// I2CConfig is used to store config info for I2C.
+type I2CConfig struct {
+	Frequency uint32
+}
+
+// Configure is intended to setup the I2C interface.
+func (i2c I2C) Configure(config I2CConfig) {
+	// Default I2C bus speed is 100 kHz.
+	if config.Frequency == 0 {
+		config.Frequency = TWI_FREQ_100KHZ
+	}
+
+	// Activate internal pullups for twi.
+	*avr.PORTC |= (avr.DIDR0_ADC4D | avr.DIDR0_ADC5D)
+
+	// Initialize twi prescaler and bit rate.
+	*avr.TWSR |= (avr.TWSR_TWPS0 | avr.TWSR_TWPS1)
+
+	// twi bit rate formula from atmega128 manual pg. 204:
+	// SCL Frequency = CPU Clock Frequency / (16 + (2 * TWBR))
+	// NOTE: TWBR should be 10 or higher for master mode.
+	// It is 72 for a 16mhz board with 100kHz TWI
+	*avr.TWBR = avr.RegValue(((CPU_FREQUENCY / config.Frequency) - 16) / 2)
+
+	// Enable twi module.
+	*avr.TWCR = avr.TWCR_TWEN
+}
+
+// Tx does a single I2C transaction at the specified address.
+// It clocks out the given address, writes the bytes in w, reads back len(r)
+// bytes and stores them in r, and generates a stop condition on the bus.
+func (i2c I2C) Tx(addr uint16, w, r []byte) error {
+	if len(w) != 0 {
+		i2c.start(uint8(addr), true) // start transmission for writing
+		for _, b := range w {
+			i2c.writeByte(b)
+		}
+	}
+	if len(r) != 0 {
+		i2c.start(uint8(addr), false) // re-start transmission for reading
+		for i := range r {            // read each char
+			r[i] = i2c.readByte()
+		}
+	}
+	if len(w) != 0 || len(r) != 0 {
+		// Stop the transmission after it has been started.
+		i2c.stop()
+	}
+	return nil
+}
+
+// start starts an I2C communication session.
+func (i2c I2C) start(address uint8, write bool) {
+	// Clear TWI interrupt flag, put start condition on SDA, and enable TWI.
+	*avr.TWCR = (avr.TWCR_TWINT | avr.TWCR_TWSTA | avr.TWCR_TWEN)
+
+	// Wait till start condition is transmitted.
+	for (*avr.TWCR & avr.TWCR_TWINT) == 0 {
+	}
+
+	// Write 7-bit shifted peripheral address.
+	address <<= 1
+	if !write {
+		address |= 1 // set read flag
+	}
+	i2c.writeByte(address)
+}
+
+// stop ends an I2C communication session.
+func (i2c I2C) stop() {
+	// Send stop condition.
+	*avr.TWCR = (avr.TWCR_TWEN | avr.TWCR_TWINT | avr.TWCR_TWSTO)
+
+	// Wait for stop condition to be executed on bus.
+	for (*avr.TWCR & avr.TWCR_TWSTO) == 0 {
+	}
+}
+
+// writeByte writes a single byte to the I2C bus.
+func (i2c I2C) writeByte(data byte) {
+	// Write data to register.
+	*avr.TWDR = avr.RegValue(data)
+
+	// Clear TWI interrupt flag and enable TWI.
+	*avr.TWCR = (avr.TWCR_TWEN | avr.TWCR_TWINT)
+
+	// Wait till data is transmitted.
+	for (*avr.TWCR & avr.TWCR_TWINT) == 0 {
+	}
+}
+
+// readByte reads a single byte from the I2C bus.
+func (i2c I2C) readByte() byte {
+	// Clear TWI interrupt flag and enable TWI.
+	*avr.TWCR = (avr.TWCR_TWEN | avr.TWCR_TWINT | avr.TWCR_TWEA)
+
+	// Wait till read request is transmitted.
+	for (*avr.TWCR & avr.TWCR_TWINT) == 0 {
+	}
+
+	return byte(*avr.TWDR)
+}
+
+// Configure the UART on the AVR. Defaults to 9600 baud on Arduino.
+func (uart UART) Configure(config UARTConfig) {
+	if config.BaudRate == 0 {
+		config.BaudRate = 9600
+	}
+
+	// Set baud rate based on prescale formula from
+	// https://www.microchip.com/webdoc/AVRLibcReferenceManual/FAQ_1faq_wrong_baud_rate.html
+	// ((F_CPU + UART_BAUD_RATE * 8L) / (UART_BAUD_RATE * 16L) - 1)
+	ps := ((CPU_FREQUENCY+config.BaudRate*8)/(config.BaudRate*16) - 1)
+	*avr.UBRR0H = avr.RegValue(ps >> 8)
+	*avr.UBRR0L = avr.RegValue(ps & 0xff)
+
+	// enable RX, TX and RX interrupt
+	*avr.UCSR0B = avr.UCSR0B_RXEN0 | avr.UCSR0B_TXEN0 | avr.UCSR0B_RXCIE0
+
+	// 8-bits data
+	*avr.UCSR0C = avr.UCSR0C_UCSZ01 | avr.UCSR0C_UCSZ00
+}
+
+// WriteByte writes a byte of data to the UART.
+func (uart UART) WriteByte(c byte) error {
+	// Wait until UART buffer is not busy.
+	for (*avr.UCSR0A & avr.UCSR0A_UDRE0) == 0 {
+	}
+	*avr.UDR0 = avr.RegValue(c) // send char
+	return nil
+}
+
+//go:interrupt USART_RX_vect
+func handleUSART_RX() {
+	// Read register to clear it.
+	data := *avr.UDR0
+
+	// Ensure no error.
+	if (*avr.UCSR0A & (avr.UCSR0A_FE0 | avr.UCSR0A_DOR0 | avr.UCSR0A_UPE0)) == 0 {
+		// Put data from UDR register into buffer.
+		bufferPut(byte(data))
+	}
+}
diff --git a/src/machine/machine_attiny.go b/src/machine/machine_attiny.go
new file mode 100644
index 000000000..eadcee2a5
--- /dev/null
+++ b/src/machine/machine_attiny.go
@@ -0,0 +1,48 @@
+// +build avr,attiny
+
+package machine
+
+import (
+	"device/avr"
+)
+
+// Configure sets the pin to input or output.
+func (p GPIO) Configure(config GPIOConfig) {
+	if config.Mode == GPIO_OUTPUT { // set output bit
+		*avr.DDRB |= 1 << p.Pin
+	} else { // configure input: clear output bit
+		*avr.DDRB &^= 1 << p.Pin
+	}
+}
+
+// Set changes the value of the GPIO pin. The pin must be configured as output.
+func (p GPIO) Set(value bool) {
+	if value { // set bits
+		*avr.PORTB |= 1 << p.Pin
+	} else { // clear bits
+		*avr.PORTB &^= 1 << p.Pin
+	}
+}
+
+// Get returns the current value of a GPIO pin.
+func (p GPIO) Get() bool {
+	val := *avr.PINB & (1 << p.Pin)
+	return (val > 0)
+}
+
+// Configure is a dummy implementation. UART has not been implemented for ATtiny
+// devices.
+func (uart UART) Configure(config UARTConfig) {
+}
+
+// WriteByte is a dummy implementation. UART has not been implemented for ATtiny
+// devices.
+func (uart UART) WriteByte(c byte) error {
+	return nil
+}
+
+// Tx is a dummy implementation. I2C has not been implemented for ATtiny
+// devices.
+func (i2c I2C) Tx(addr uint16, w, r []byte) error {
+	return nil
+}
diff --git a/src/machine/machine_avr.go b/src/machine/machine_avr.go
index dcced7574..5a6c4bab5 100644
--- a/src/machine/machine_avr.go
+++ b/src/machine/machine_avr.go
@@ -13,115 +13,6 @@ const (
 	GPIO_OUTPUT
 )
 
-func (p GPIO) Configure(config GPIOConfig) {
-	if config.Mode == GPIO_OUTPUT { // set output bit
-		if p.Pin < 8 {
-			*avr.DDRD |= 1 << p.Pin
-		} else {
-			*avr.DDRB |= 1 << (p.Pin - 8)
-		}
-	} else { // configure input: clear output bit
-		if p.Pin < 8 {
-			*avr.DDRD &^= 1 << p.Pin
-		} else {
-			*avr.DDRB &^= 1 << (p.Pin - 8)
-		}
-	}
-}
-
-func (p GPIO) Set(value bool) {
-	if value { // set bits
-		if p.Pin < 8 {
-			*avr.PORTD |= 1 << p.Pin
-		} else {
-			*avr.PORTB |= 1 << (p.Pin - 8)
-		}
-	} else { // clear bits
-		if p.Pin < 8 {
-			*avr.PORTD &^= 1 << p.Pin
-		} else {
-			*avr.PORTB &^= 1 << (p.Pin - 8)
-		}
-	}
-}
-
-// Get returns the current value of a GPIO pin.
-func (p GPIO) Get() bool {
-	if p.Pin < 8 {
-		val := *avr.PIND & (1 << p.Pin)
-		return (val > 0)
-	} else {
-		val := *avr.PINB & (1 << (p.Pin - 8))
-		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")
-	}
-}
-
 // InitADC initializes the registers needed for ADC.
 func InitADC() {
 	// set a2d prescaler so we are inside the desired 50-200 KHz range at 16MHz.
@@ -158,159 +49,15 @@ func (a ADC) Get() uint16 {
 	return uint16(low) | uint16(high<<8)
 }
 
-// I2C on the Arduino.
+// I2C on AVR.
 type I2C struct {
 }
 
-// I2C0 is the only I2C interface on the Arduino.
+// I2C0 is the only I2C interface on most AVRs.
 var I2C0 = I2C{}
 
-// I2CConfig is used to store config info for I2C.
-type I2CConfig struct {
-	Frequency uint32
-}
-
-// Configure is intended to setup the I2C interface.
-func (i2c I2C) Configure(config I2CConfig) {
-	// Default I2C bus speed is 100 kHz.
-	if config.Frequency == 0 {
-		config.Frequency = TWI_FREQ_100KHZ
-	}
-
-	// Activate internal pullups for twi.
-	*avr.PORTC |= (avr.DIDR0_ADC4D | avr.DIDR0_ADC5D)
-
-	// Initialize twi prescaler and bit rate.
-	*avr.TWSR |= (avr.TWSR_TWPS0 | avr.TWSR_TWPS1)
-
-	// twi bit rate formula from atmega128 manual pg. 204:
-	// SCL Frequency = CPU Clock Frequency / (16 + (2 * TWBR))
-	// NOTE: TWBR should be 10 or higher for master mode.
-	// It is 72 for a 16mhz board with 100kHz TWI
-	*avr.TWBR = avr.RegValue(((CPU_FREQUENCY / config.Frequency) - 16) / 2)
-
-	// Enable twi module.
-	*avr.TWCR = avr.TWCR_TWEN
-}
-
-// Tx does a single I2C transaction at the specified address.
-// It clocks out the given address, writes the bytes in w, reads back len(r)
-// bytes and stores them in r, and generates a stop condition on the bus.
-func (i2c I2C) Tx(addr uint16, w, r []byte) error {
-	if len(w) != 0 {
-		i2c.start(uint8(addr), true) // start transmission for writing
-		for _, b := range w {
-			i2c.writeByte(b)
-		}
-	}
-	if len(r) != 0 {
-		i2c.start(uint8(addr), false) // re-start transmission for reading
-		for i := range r {            // read each char
-			r[i] = i2c.readByte()
-		}
-	}
-	if len(w) != 0 || len(r) != 0 {
-		// Stop the transmission after it has been started.
-		i2c.stop()
-	}
-	return nil
-}
-
-// start starts an I2C communication session.
-func (i2c I2C) start(address uint8, write bool) {
-	// Clear TWI interrupt flag, put start condition on SDA, and enable TWI.
-	*avr.TWCR = (avr.TWCR_TWINT | avr.TWCR_TWSTA | avr.TWCR_TWEN)
-
-	// Wait till start condition is transmitted.
-	for (*avr.TWCR & avr.TWCR_TWINT) == 0 {
-	}
-
-	// Write 7-bit shifted peripheral address.
-	address <<= 1
-	if !write {
-		address |= 1 // set read flag
-	}
-	i2c.writeByte(address)
-}
-
-// stop ends an I2C communication session.
-func (i2c I2C) stop() {
-	// Send stop condition.
-	*avr.TWCR = (avr.TWCR_TWEN | avr.TWCR_TWINT | avr.TWCR_TWSTO)
-
-	// Wait for stop condition to be executed on bus.
-	for (*avr.TWCR & avr.TWCR_TWSTO) == 0 {
-	}
-}
-
-// writeByte writes a single byte to the I2C bus.
-func (i2c I2C) writeByte(data byte) {
-	// Write data to register.
-	*avr.TWDR = avr.RegValue(data)
-
-	// Clear TWI interrupt flag and enable TWI.
-	*avr.TWCR = (avr.TWCR_TWEN | avr.TWCR_TWINT)
-
-	// Wait till data is transmitted.
-	for (*avr.TWCR & avr.TWCR_TWINT) == 0 {
-	}
-}
-
-// readByte reads a single byte from the I2C bus.
-func (i2c I2C) readByte() byte {
-	// Clear TWI interrupt flag and enable TWI.
-	*avr.TWCR = (avr.TWCR_TWEN | avr.TWCR_TWINT | avr.TWCR_TWEA)
-
-	// Wait till read request is transmitted.
-	for (*avr.TWCR & avr.TWCR_TWINT) == 0 {
-	}
-
-	return byte(*avr.TWDR)
-}
-
 // UART
 var (
 	// UART0 is the hardware serial port on the AVR.
 	UART0 = &UART{}
 )
-
-// Configure the UART on the AVR. Defaults to 9600 baud on Arduino.
-func (uart UART) Configure(config UARTConfig) {
-	if config.BaudRate == 0 {
-		config.BaudRate = 9600
-	}
-
-	// Set baud rate based on prescale formula from
-	// https://www.microchip.com/webdoc/AVRLibcReferenceManual/FAQ_1faq_wrong_baud_rate.html
-	// ((F_CPU + UART_BAUD_RATE * 8L) / (UART_BAUD_RATE * 16L) - 1)
-	ps := ((CPU_FREQUENCY+config.BaudRate*8)/(config.BaudRate*16) - 1)
-	*avr.UBRR0H = avr.RegValue(ps >> 8)
-	*avr.UBRR0L = avr.RegValue(ps & 0xff)
-
-	// enable RX, TX and RX interrupt
-	*avr.UCSR0B = avr.UCSR0B_RXEN0 | avr.UCSR0B_TXEN0 | avr.UCSR0B_RXCIE0
-
-	// 8-bits data
-	*avr.UCSR0C = avr.UCSR0C_UCSZ01 | avr.UCSR0C_UCSZ00
-}
-
-// WriteByte writes a byte of data to the UART.
-func (uart UART) WriteByte(c byte) error {
-	// Wait until UART buffer is not busy.
-	for (*avr.UCSR0A & avr.UCSR0A_UDRE0) == 0 {
-	}
-	*avr.UDR0 = avr.RegValue(c) // send char
-	return nil
-}
-
-//go:interrupt USART_RX_vect
-func handleUSART_RX() {
-	// Read register to clear it.
-	data := *avr.UDR0
-
-	// Ensure no error.
-	if (*avr.UCSR0A & (avr.UCSR0A_FE0 | avr.UCSR0A_DOR0 | avr.UCSR0A_UPE0)) == 0 {
-		// Put data from UDR register into buffer.
-		bufferPut(byte(data))
-	}
-}
diff --git a/src/runtime/runtime_atmega.go b/src/runtime/runtime_atmega.go
new file mode 100644
index 000000000..474f34c40
--- /dev/null
+++ b/src/runtime/runtime_atmega.go
@@ -0,0 +1,35 @@
+// +build avr,atmega
+
+package runtime
+
+import (
+	"device/avr"
+)
+
+// Sleep for a given period. The period is defined by the WDT peripheral, and is
+// on most chips (at least) 3 bits wide, in powers of two from 16ms to 2s
+// (0=16ms, 1=32ms, 2=64ms...). Note that the WDT is not very accurate: it can
+// be off by a large margin depending on temperature and supply voltage.
+//
+// TODO: disable more peripherals etc. to reduce sleep current.
+func sleepWDT(period uint8) {
+	// Configure WDT
+	avr.Asm("cli")
+	avr.Asm("wdr")
+	// Start timed sequence.
+	*avr.WDTCSR |= avr.WDTCSR_WDCE | avr.WDTCSR_WDE
+	// Enable WDT and set new timeout
+	*avr.WDTCSR = avr.WDTCSR_WDIE | avr.RegValue(period)
+	avr.Asm("sei")
+
+	// Set sleep mode to idle and enable sleep mode.
+	// Note: when using something other than idle, the UART won't work
+	// correctly. This needs to be fixed, though, so we can truly sleep.
+	*avr.SMCR = (0 << 1) | avr.SMCR_SE
+
+	// go to sleep
+	avr.Asm("sleep")
+
+	// disable sleep
+	*avr.SMCR = 0
+}
diff --git a/src/runtime/runtime_attiny.go b/src/runtime/runtime_attiny.go
new file mode 100644
index 000000000..e0f6cc2eb
--- /dev/null
+++ b/src/runtime/runtime_attiny.go
@@ -0,0 +1,16 @@
+// +build avr,attiny
+
+package runtime
+
+import (
+	"device/avr"
+)
+
+func sleepWDT(period uint8) {
+	// TODO: use the watchdog timer instead of a busy loop.
+	for i := 0x45; i != 0; i-- {
+		for i := 0xff; i != 0; i-- {
+			avr.Asm("nop")
+		}
+	}
+}
diff --git a/src/runtime/runtime_avr.go b/src/runtime/runtime_avr.go
index e01d2dce1..deeaa4ed1 100644
--- a/src/runtime/runtime_avr.go
+++ b/src/runtime/runtime_avr.go
@@ -83,34 +83,6 @@ func sleepTicks(d timeUnit) {
 	}
 }
 
-// Sleep for a given period. The period is defined by the WDT peripheral, and is
-// on most chips (at least) 3 bits wide, in powers of two from 16ms to 2s
-// (0=16ms, 1=32ms, 2=64ms...). Note that the WDT is not very accurate: it can
-// be off by a large margin depending on temperature and supply voltage.
-//
-// TODO: disable more peripherals etc. to reduce sleep current.
-func sleepWDT(period uint8) {
-	// Configure WDT
-	avr.Asm("cli")
-	avr.Asm("wdr")
-	// Start timed sequence.
-	*avr.WDTCSR |= avr.WDTCSR_WDCE | avr.WDTCSR_WDE
-	// Enable WDT and set new timeout (0.5s)
-	*avr.WDTCSR = avr.WDTCSR_WDIE | avr.RegValue(period)
-	avr.Asm("sei")
-
-	// Set sleep mode to idle and enable sleep mode.
-	// Note: when using something other than idle, the UART won't work
-	// correctly. This needs to be fixed, though, so we can truly sleep.
-	*avr.SMCR = (0 << 1) | avr.SMCR_SE
-
-	// go to sleep
-	avr.Asm("sleep")
-
-	// disable sleep
-	*avr.SMCR = 0
-}
-
 func ticks() timeUnit {
 	return currentTime
 }
diff --git a/targets/arduino.json b/targets/arduino.json
index be7c52601..795a99564 100644
--- a/targets/arduino.json
+++ b/targets/arduino.json
@@ -1,7 +1,7 @@
 {
+	"inherits": ["avr"],
 	"llvm-target": "avr-atmel-none",
-	"build-tags": ["arduino", "atmega328p", "atmega", "avr5", "avr", "js", "wasm"],
-	"linker": "avr-gcc",
+	"build-tags": ["arduino", "atmega328p", "atmega", "avr5"],
 	"pre-link-args": [
 		"-nostartfiles",
 		"-mmcu=avr5",
@@ -13,6 +13,5 @@
 		"targets/avr.S",
 		"src/device/avr/atmega328p.s"
 	],
-	"objcopy": "avr-objcopy",
 	"flash": "avrdude -c arduino -p atmega328p -P {port} -U flash:w:{hex}"
 }
diff --git a/targets/avr.json b/targets/avr.json
new file mode 100644
index 000000000..debb54786
--- /dev/null
+++ b/targets/avr.json
@@ -0,0 +1,5 @@
+{
+	"build-tags": ["avr", "js", "wasm"],
+	"linker": "avr-gcc",
+	"objcopy": "avr-objcopy"
+}
diff --git a/targets/digispark.json b/targets/digispark.json
new file mode 100644
index 000000000..02d9b87e4
--- /dev/null
+++ b/targets/digispark.json
@@ -0,0 +1,17 @@
+{
+	"inherits": ["avr"],
+	"llvm-target": "avr-atmel-none",
+	"build-tags": ["digispark", "attiny85", "attiny", "avr2", "avr25"],
+	"pre-link-args": [
+		"-nostartfiles",
+		"-mmcu=attiny85",
+		"-Wl,--defsym=_bootloader_size=2180",
+		"-Wl,--defsym=_stack_size=128",
+		"-T", "src/device/avr/attiny85.ld",
+		"-T", "targets/avr.ld",
+		"-Wl,--gc-sections",
+		"targets/avr.S",
+		"src/device/avr/attiny85.s"
+	],
+	"flash": "micronucleus --run {hex}"
+}
diff --git a/tools/gen-device-avr.py b/tools/gen-device-avr.py
index 1b7efe3fd..00cf7ba1f 100755
--- a/tools/gen-device-avr.py
+++ b/tools/gen-device-avr.py
@@ -93,10 +93,16 @@ def readATDF(path):
                     continue
 
                 for bitfieldEl in regEl.getElementsByTagName('bitfield'):
+                    mask = bitfieldEl.getAttribute('mask')
+                    if len(mask) == 2:
+                        # Two devices (ATtiny102 and ATtiny104) appear to have
+                        # an error in the bitfields, leaving out the '0x'
+                        # prefix.
+                        mask = '0x' + mask
                     reg['bitfields'].append({
                         'name':        regName + '_' + bitfieldEl.getAttribute('name'),
                         'description': bitfieldEl.getAttribute('caption'),
-                        'value':       int(bitfieldEl.getAttribute('mask'), 0),
+                        'value':       int(mask, 0),
                     })
 
                 if regName in allRegisters:
@@ -112,6 +118,11 @@ def readATDF(path):
 
                 peripheral['registers'].append(reg)
 
+    ramSize = 0 # for devices with no RAM
+    for ramSegmentName in ['IRAM', 'INTERNAL_SRAM', 'SRAM']:
+        if ramSegmentName in memorySizes['data']['segments']:
+            ramSize = memorySizes['data']['segments'][ramSegmentName]
+
     device.metadata = {
         'file':             os.path.basename(path),
         'descriptorSource': 'http://packs.download.atmel.com/',
@@ -121,7 +132,7 @@ def readATDF(path):
         'arch':             arch,
         'family':           family,
         'flashSize':        memorySizes['prog']['size'],
-        'ramSize':          memorySizes['data']['segments'].get('IRAM', memorySizes['data']['segments'].get('INTERNAL_SRAM')),
+        'ramSize':          ramSize,
         'numInterrupts':    len(device.interrupts),
     }
 
@@ -228,15 +239,21 @@ __vector_default:
 '''.format(**device.metadata))
     num = 0
     for intr in device.interrupts:
+        jmp = 'jmp'
+        if device.metadata['flashSize'] <= 8 * 1024:
+            # When a device has 8kB or less flash, rjmp (2 bytes) must be used
+            # instead of jmp (4 bytes).
+            # https://www.avrfreaks.net/forum/rjmp-versus-jmp
+            jmp = 'rjmp'
         if intr['index'] < num:
             # Some devices have duplicate interrupts, probably for historical
             # reasons.
             continue
         while intr['index'] > num:
-            out.write('    jmp __vector_default\n')
+            out.write('    {jmp} __vector_default\n'.format(jmp=jmp))
             num += 1
         num += 1
-        out.write('    jmp __vector_{name}\n'.format(**intr))
+        out.write('    {jmp} __vector_{name}\n'.format(jmp=jmp, **intr))
 
     out.write('''
     ; Define default implementations for interrupts, redirecting to