Add SPI support for Atmega based chips.

This is based on @Nerzal's #1398 PR, but is a bit of a refactor and
expansion to support all the Atmega based chips present in tinygo.

src/machine/machine_atmega.go

@@ -5,6 +5,8 @@ package machine
 import (
 	"device/avr"
 	"runtime/interrupt"
+	"runtime/volatile"
+	"unsafe"
 )
 
 // I2CConfig is used to store config info for I2C.
@@ -157,3 +159,108 @@ func (uart UART) WriteByte(c byte) error {
 	avr.UDR0.Set(c) // send char
 	return nil
 }
+
+// SPIConfig is used to store config info for SPI.
+type SPIConfig struct {
+	Frequency uint32
+	LSBFirst  bool
+	Mode      uint8
+}
+
+// SPI is for the Serial Peripheral Interface
+// Data is taken from http://ww1.microchip.com/downloads/en/DeviceDoc/ATmega48A-PA-88A-PA-168A-PA-328-P-DS-DS40002061A.pdf page 169 and following
+type SPI struct {
+	// The registers for the SPIx port set by the chip
+	spcr *volatile.Register8
+	spdr *volatile.Register8
+	spsr *volatile.Register8
+
+	// The io pins for the SPIx port set by the chip
+	sck Pin
+	sdi Pin
+	sdo Pin
+	cs  Pin
+}
+
+// Configure is intended to setup the SPI interface.
+func (s SPI) Configure(config SPIConfig) error {
+
+	// This is only here to help catch a bug with the configuration
+	// where a machine missed a value.
+	if s.spcr == (*volatile.Register8)(unsafe.Pointer(uintptr(0))) ||
+		s.spsr == (*volatile.Register8)(unsafe.Pointer(uintptr(0))) ||
+		s.spdr == (*volatile.Register8)(unsafe.Pointer(uintptr(0))) ||
+		s.sck == 0 || s.sdi == 0 || s.sdo == 0 || s.cs == 0 {
+		return errSPIInvalidMachineConfig
+	}
+
+	// Make the defaults meaningful
+	if config.Frequency == 0 {
+		config.Frequency = 4000000
+	}
+
+	// Default all port configuration bits to 0 for simplicity
+	s.spcr.Set(0)
+	s.spsr.Set(0)
+
+	// Setup pins output configuration
+	s.sck.Configure(PinConfig{Mode: PinOutput})
+	s.sdi.Configure(PinConfig{Mode: PinInput})
+	s.sdo.Configure(PinConfig{Mode: PinOutput})
+
+	// Prevent CS glitches if the pin is enabled Low (0, default)
+	s.cs.High()
+	// If the CS pin is not configured as output the SPI port operates in
+	// slave mode.
+	s.cs.Configure(PinConfig{Mode: PinOutput})
+
+	frequencyDivider := CPUFrequency() / config.Frequency
+
+	switch {
+	case frequencyDivider >= 128:
+		s.spcr.SetBits(avr.SPCR_SPR0 | avr.SPCR_SPR1)
+	case frequencyDivider >= 64:
+		s.spcr.SetBits(avr.SPCR_SPR1)
+	case frequencyDivider >= 32:
+		s.spcr.SetBits(avr.SPCR_SPR1)
+		s.spsr.SetBits(avr.SPSR_SPI2X)
+	case frequencyDivider >= 16:
+		s.spcr.SetBits(avr.SPCR_SPR0)
+	case frequencyDivider >= 8:
+		s.spcr.SetBits(avr.SPCR_SPR0)
+		s.spsr.SetBits(avr.SPSR_SPI2X)
+	case frequencyDivider >= 4:
+		// The clock is already set to all 0's.
+	default: // defaults to fastest which is /2
+		s.spsr.SetBits(avr.SPSR_SPI2X)
+	}
+
+	switch config.Mode {
+	case Mode1:
+		s.spcr.SetBits(avr.SPCR_CPHA)
+	case Mode2:
+		s.spcr.SetBits(avr.SPCR_CPOL)
+	case Mode3:
+		s.spcr.SetBits(avr.SPCR_CPHA | avr.SPCR_CPOL)
+	default: // default is mode 0
+	}
+
+	if config.LSBFirst {
+		s.spcr.SetBits(avr.SPCR_DORD)
+	}
+
+	// enable SPI, set controller, set clock rate
+	s.spcr.SetBits(avr.SPCR_SPE | avr.SPCR_MSTR)
+
+	return nil
+}
+
+// Transfer writes the byte into the register and returns the read content
+func (s SPI) Transfer(b byte) (byte, error) {
+	s.spdr.Set(uint8(b))
+
+	for !s.spsr.HasBits(avr.SPSR_SPIF) {
+	}
+
+	return byte(s.spdr.Get()), nil
+}

src/machine/machine_atmega1284p.go

@@ -69,3 +69,13 @@ func (p Pin) getPortMask() (*volatile.Register8, uint8) {
 		return avr.PORTD, 1 << uint8(p-portD)
 	}
 }
+
+// SPI configuration
+var SPI0 = SPI{
+	spcr: avr.SPCR,
+	spsr: avr.SPSR,
+	spdr: avr.SPDR,
+	sck:  PB7,
+	sdo:  PB5,
+	sdi:  PB6,
+	cs:   PB4}

src/machine/machine_atmega2560.go

@@ -126,3 +126,13 @@ func (p Pin) getPortMask() (*volatile.Register8, uint8) {
 		return avr.PORTA, 255
 	}
 }
+
+// SPI configuration
+var SPI0 = SPI{
+	spcr: avr.SPCR,
+	spdr: avr.SPDR,
+	spsr: avr.SPSR,
+	sck:  PB1,
+	sdo:  PB2,
+	sdi:  PB3,
+	cs:   PB0}

src/machine/machine_atmega328p.go

@@ -88,3 +88,13 @@ func (pwm PWM) Set(value uint16) {
 		panic("Invalid PWM pin")
 	}
 }
+
+// SPI configuration
+var SPI0 = SPI{
+	spcr: avr.SPCR,
+	spdr: avr.SPDR,
+	spsr: avr.SPSR,
+	sck:  PB5,
+	sdo:  PB3,
+	sdi:  PB4,
+	cs:   PB2}

src/machine/machine_atmega328pb.go

@@ -0,0 +1,109 @@
+// +build avr,atmega328pb
+
+package machine
+
+import (
+	"device/avr"
+	"runtime/volatile"
+)
+
+const irq_USART0_RX = avr.IRQ_USART0_RX
+
+// getPortMask returns the PORTx register and mask for the pin.
+func (p Pin) getPortMask() (*volatile.Register8, uint8) {
+	switch {
+	case p >= PB0 && p <= PB7: // port B
+		return avr.PORTB, 1 << uint8(p-portB)
+	case p >= PC0 && p <= PC7: // port C
+		return avr.PORTC, 1 << uint8(p-portC)
+	default: // port D
+		return avr.PORTD, 1 << uint8(p-portD)
+	}
+}
+
+// InitPWM initializes the registers needed for PWM.
+func InitPWM() {
+	// use waveform generation
+	avr.TCCR0A.SetBits(avr.TCCR0A_WGM00)
+
+	// set timer 0 prescale factor to 64
+	avr.TCCR0B.SetBits(avr.TCCR0B_CS01 | avr.TCCR0B_CS00)
+
+	// set timer 1 prescale factor to 64
+	avr.TCCR1B.SetBits(avr.TCCR1B_CS11)
+
+	// put timer 1 in 8-bit phase correct pwm mode
+	avr.TCCR1A.SetBits(avr.TCCR1A_WGM10)
+
+	// set timer 2 prescale factor to 64
+	avr.TCCR2B.SetBits(avr.TCCR2B_CS22)
+
+	// configure timer 2 for phase correct pwm (8-bit)
+	avr.TCCR2A.SetBits(avr.TCCR2A_WGM20)
+}
+
+// Configure configures a PWM pin for output.
+func (pwm PWM) Configure() error {
+	switch pwm.Pin / 8 {
+	case 0: // port B
+		avr.DDRB.SetBits(1 << uint8(pwm.Pin))
+	case 2: // port D
+		avr.DDRD.SetBits(1 << uint8(pwm.Pin-16))
+	}
+	return nil
+}
+
+// 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 := uint8(value >> 8)
+	switch pwm.Pin {
+	case PD3:
+		// connect pwm to pin on timer 2, channel B
+		avr.TCCR2A.SetBits(avr.TCCR2A_COM2B1)
+		avr.OCR2B.Set(value8) // set pwm duty
+	case PD5:
+		// connect pwm to pin on timer 0, channel B
+		avr.TCCR0A.SetBits(avr.TCCR0A_COM0B1)
+		avr.OCR0B.Set(value8) // set pwm duty
+	case PD6:
+		// connect pwm to pin on timer 0, channel A
+		avr.TCCR0A.SetBits(avr.TCCR0A_COM0A1)
+		avr.OCR0A.Set(value8) // set pwm duty
+	case PB1:
+		// connect pwm to pin on timer 1, channel A
+		avr.TCCR1A.SetBits(avr.TCCR1A_COM1A1)
+		// this is a 16-bit value, but we only currently allow the low order bits to be set
+		avr.OCR1AL.Set(value8) // set pwm duty
+	case PB2:
+		// connect pwm to pin on timer 1, channel B
+		avr.TCCR1A.SetBits(avr.TCCR1A_COM1B1)
+		// this is a 16-bit value, but we only currently allow the low order bits to be set
+		avr.OCR1BL.Set(value8) // set pwm duty
+	case PB3:
+		// connect pwm to pin on timer 2, channel A
+		avr.TCCR2A.SetBits(avr.TCCR2A_COM2A1)
+		avr.OCR2A.Set(value8) // set pwm duty
+	default:
+		panic("Invalid PWM pin")
+	}
+}
+
+// SPI configuration
+var SPI0 = SPI{
+	spcr: avr.SPCR0,
+	spdr: avr.SPDR0,
+	spsr: avr.SPSR0,
+	sck:  PB5,
+	sdo:  PB3,
+	sdi:  PB4,
+	cs:   PB2}
+
+var SPI1 = SPI{
+	spcr: avr.SPCR1,
+	spdr: avr.SPDR1,
+	spsr: avr.SPSR1,
+	sck:  PC1,
+	sdo:  PE3,
+	sdi:  PC0,
+	cs:   PE2}

src/machine/spi.go

@@ -1,4 +1,4 @@
-// +build !baremetal sam stm32,!stm32f7x2 fe310 k210
+// +build !baremetal sam stm32,!stm32f7x2 fe310 k210 atmega
 
 package machine
 
@@ -13,7 +13,8 @@ const (
 )
 
 var (
-	ErrTxInvalidSliceSize = errors.New("SPI write and read slices must be same size")
+	ErrTxInvalidSliceSize      = errors.New("SPI write and read slices must be same size")
+	errSPIInvalidMachineConfig = errors.New("SPI port was not configured properly by the machine")
 )
 
 // Tx handles read/write operation for SPI interface. Since SPI is a syncronous write/read