machine/samd21: switch UART to use new pin configuration

This allows all possible UART pin configurations to be used and avoids
some tricky configuration.

src/machine/board_arduino_nano33.go

@@ -68,10 +68,10 @@ const (
 
 // UART1 on the Arduino Nano 33 connects to the onboard NINA-W102 WiFi chip.
 var (
-	UART1 = UART{Bus: sam.SERCOM5_USART,
+	UART1 = UART{
 		Buffer: NewRingBuffer(),
-		Mode:   PinSERCOMAlt,
+		Bus:    sam.SERCOM5_USART,
-		IRQVal: sam.IRQ_SERCOM5,
+		SERCOM: 5,
 	}
 )
 
@@ -88,10 +88,10 @@ func handleUART1() {
 
 // UART2 on the Arduino Nano 33 connects to the normal TX/RX pins.
 var (
-	UART2 = UART{Bus: sam.SERCOM3_USART,
+	UART2 = UART{
 		Buffer: NewRingBuffer(),
-		Mode:   PinSERCOMAlt,
+		Bus:    sam.SERCOM3_USART,
-		IRQVal: sam.IRQ_SERCOM3,
+		SERCOM: 3,
 	}
 )
 

src/machine/board_circuitplay_express.go

@@ -70,10 +70,10 @@ const (
 
 // UART1 on the Circuit Playground Express.
 var (
-	UART1 = UART{Bus: sam.SERCOM1_USART,
+	UART1 = UART{
 		Buffer: NewRingBuffer(),
-		Mode:   PinSERCOM,
+		Bus:    sam.SERCOM4_USART,
-		IRQVal: sam.IRQ_SERCOM1,
+		SERCOM: 4,
 	}
 )
 

src/machine/board_feather-m0.go

@@ -53,10 +53,10 @@ const (
 
 // UART1 on the Feather M0.
 var (
-	UART1 = UART{Bus: sam.SERCOM1_USART,
+	UART1 = UART{
 		Buffer: NewRingBuffer(),
-		Mode:   PinSERCOM,
+		Bus:    sam.SERCOM1_USART,
-		IRQVal: sam.IRQ_SERCOM1,
+		SERCOM: 1,
 	}
 )
 

src/machine/board_itsybitsy-m0.go

@@ -55,10 +55,10 @@ const (
 
 // UART1 on the ItsyBitsy M0.
 var (
-	UART1 = UART{Bus: sam.SERCOM1_USART,
+	UART1 = UART{
 		Buffer: NewRingBuffer(),
-		Mode:   PinSERCOM,
+		Bus:    sam.SERCOM1_USART,
-		IRQVal: sam.IRQ_SERCOM1,
+		SERCOM: 1,
 	}
 )
 

src/machine/board_trinket.go

@@ -44,10 +44,10 @@ const (
 
 // UART1 on the Trinket M0.
 var (
-	UART1 = UART{Bus: sam.SERCOM1_USART,
+	UART1 = UART{
 		Buffer: NewRingBuffer(),
-		Mode:   PinSERCOM,
+		Bus:    sam.SERCOM0_USART,
-		IRQVal: sam.IRQ_SERCOM1,
+		SERCOM: 0,
 	}
 )
 

src/machine/machine_atsamd21.go

@@ -364,8 +364,7 @@ func waitADCSync() {
 type UART struct {
 	Buffer *RingBuffer
 	Bus    *sam.SERCOM_USART_Type
-	Mode   PinMode
+	SERCOM uint8
-	IRQVal uint32
 }
 
 var (
@@ -374,66 +373,54 @@ var (
 )
 
 const (
-	sampleRate16X  = 16
+	sampleRate16X = 16
-	lsbFirst       = 1
+	lsbFirst      = 1
-	sercomRXPad0   = 0
-	sercomRXPad1   = 1
-	sercomRXPad2   = 2
-	sercomRXPad3   = 3
-	sercomTXPad0   = 0 // Only for UART
-	sercomTXPad2   = 1 // Only for UART
-	sercomTXPad023 = 2 // Only for UART with TX on PAD0, RTS on PAD2 and CTS on PAD3
 )
 
 // Configure the UART.
-func (uart UART) Configure(config UARTConfig) {
+func (uart UART) Configure(config UARTConfig) error {
 	// Default baud rate to 115200.
 	if config.BaudRate == 0 {
 		config.BaudRate = 115200
 	}
 
-	// determine pins
+	// Use default pins if pins are not set.
-	if config.TX == 0 {
+	if config.TX == 0 && config.RX == 0 {
 		// use default pins
 		config.TX = UART_TX_PIN
 		config.RX = UART_RX_PIN
 	}
 
-	// determine pads
+	// Determine transmit pinout.
-	var txpad, rxpad int
+	txPinMode, txPad, ok := findPinPadMapping(uart.SERCOM, config.TX)
-	switch config.TX {
+	if !ok {
-	case PA10:
+		return ErrInvalidOutputPin
-		txpad = sercomTXPad2
+	}
-	case PA18:
+	var txPinOut uint32
-		txpad = sercomTXPad2
+	// See table 25-9 of the datasheet (page 459) for how pads are mapped to
-	case PA16:
+	// pinout values.
-		txpad = sercomTXPad0
+	switch txPad {
-	case PA22:
+	case 0:
-		txpad = sercomTXPad0
+		txPinOut = 0
+	case 2:
+		txPinOut = 1
 	default:
-		panic("Invalid TX pin for UART")
+		// TODO: flow control (RTS/CTS)
+		return ErrInvalidOutputPin
 	}
 
-	switch config.RX {
+	// Determine receive pinout.
-	case PA11:
+	rxPinMode, rxPad, ok := findPinPadMapping(uart.SERCOM, config.RX)
-		rxpad = sercomRXPad3
+	if !ok {
-	case PA18:
+		return ErrInvalidInputPin
-		rxpad = sercomRXPad2
-	case PA16:
-		rxpad = sercomRXPad0
-	case PA19:
-		rxpad = sercomRXPad3
-	case PA17:
-		rxpad = sercomRXPad1
-	case PA23:
-		rxpad = sercomRXPad1
-	default:
-		panic("Invalid RX pin for UART")
 	}
+	// As you can see in table 25-8 on page 459 of the datasheet, input pins
+	// are mapped directly.
+	rxPinOut := rxPad
 
 	// configure pins
-	config.TX.Configure(PinConfig{Mode: uart.Mode})
+	config.TX.Configure(PinConfig{Mode: txPinMode})
-	config.RX.Configure(PinConfig{Mode: uart.Mode})
+	config.RX.Configure(PinConfig{Mode: rxPinMode})
 
 	// reset SERCOM0
 	uart.Bus.CTRLA.SetBits(sam.SERCOM_USART_CTRLA_SWRST)
@@ -467,8 +454,8 @@ func (uart UART) Configure(config UARTConfig) {
 	// set UART pads. This is not same as pins...
 	//  SERCOM_USART_CTRLA_TXPO(txPad) |
 	//   SERCOM_USART_CTRLA_RXPO(rxPad);
-	uart.Bus.CTRLA.SetBits(uint32((txpad << sam.SERCOM_USART_CTRLA_TXPO_Pos) |
+	uart.Bus.CTRLA.SetBits((txPinOut << sam.SERCOM_USART_CTRLA_TXPO_Pos) |
-		(rxpad << sam.SERCOM_USART_CTRLA_RXPO_Pos)))
+		(rxPinOut << sam.SERCOM_USART_CTRLA_RXPO_Pos))
 
 	// Enable Transceiver and Receiver
 	//sercom->USART.CTRLB.reg |= SERCOM_USART_CTRLB_TXEN | SERCOM_USART_CTRLB_RXEN ;
@@ -484,7 +471,12 @@ func (uart UART) Configure(config UARTConfig) {
 	uart.Bus.INTENSET.Set(sam.SERCOM_USART_INTENSET_RXC)
 
 	// Enable RX IRQ.
-	arm.EnableIRQ(uart.IRQVal)
+	// IRQ lines are in the same order as SERCOM instance numbers on SAMD21
+	// chips, so the IRQ number can be trivially determined from the SERCOM
+	// number.
+	arm.EnableIRQ(sam.IRQ_SERCOM0 + uint32(uart.SERCOM))
+
+	return nil
 }
 
 // SetBaudRate sets the communication speed for the UART.