machine/atsamd51: use only the necessary UART interrupts

A small footnote in the datasheet says that interrupt source numbers
correspond to the bit position in INTFLAG. We only need the RXC
interrupt for UART. In other words, ony the _2 interrupts (RXC is in the
2nd bit position) needs to be used for UART to work correctly.

In the future, more interrupts may be needed. They can then be added as
necessary.

src/machine/machine_atsamd51.go

@@ -628,6 +628,7 @@ type UART struct {
 	Buffer *RingBuffer
 	Bus    *sam.SERCOM_USART_INT_Type
 	SERCOM uint8
+	IRQVal uint32 // RXC interrupt
 }
 
 var (
@@ -639,6 +640,7 @@ var (
 		Buffer: NewRingBuffer(),
 		Bus:    sam.SERCOM3_USART_INT,
 		SERCOM: 3,
+		IRQVal: sam.IRQ_SERCOM3_2, // RXC interrupt
 	}
 
 	// The second hardware serial port on the SAMD51. Uses the SERCOM0 interface.
@@ -646,6 +648,7 @@ var (
 		Buffer: NewRingBuffer(),
 		Bus:    sam.SERCOM0_USART_INT,
 		SERCOM: 0,
+		IRQVal: sam.IRQ_SERCOM0_2, // RXC interrupt
 	}
 )
 
@@ -697,7 +700,7 @@ func (uart UART) Configure(config UARTConfig) error {
 	config.TX.Configure(PinConfig{Mode: txPinMode})
 	config.RX.Configure(PinConfig{Mode: rxPinMode})
 
-	// reset SERCOM0
+	// reset SERCOM
 	uart.Bus.CTRLA.SetBits(sam.SERCOM_USART_INT_CTRLA_SWRST)
 	for uart.Bus.CTRLA.HasBits(sam.SERCOM_USART_INT_CTRLA_SWRST) ||
 		uart.Bus.SYNCBUSY.HasBits(sam.SERCOM_USART_INT_SYNCBUSY_SWRST) {
@@ -747,19 +750,12 @@ func (uart UART) Configure(config UARTConfig) error {
 	uart.Bus.INTENSET.Set(sam.SERCOM_USART_INT_INTENSET_RXC)
 
 	// Enable RX IRQ.
-	switch uart.SERCOM {
-	case 0:
-		arm.EnableIRQ(sam.IRQ_SERCOM0_0)
-		arm.EnableIRQ(sam.IRQ_SERCOM0_1)
-		arm.EnableIRQ(sam.IRQ_SERCOM0_2)
-		arm.EnableIRQ(sam.IRQ_SERCOM0_OTHER)
-	default:
-		// Currently assumes SERCOM3
-		arm.EnableIRQ(sam.IRQ_SERCOM3_0)
-		arm.EnableIRQ(sam.IRQ_SERCOM3_1)
-		arm.EnableIRQ(sam.IRQ_SERCOM3_2)
-		arm.EnableIRQ(sam.IRQ_SERCOM3_OTHER)
-	}
+	// This is a small note at the bottom of the NVIC section of the datasheet:
+	// > The integer number specified in the source refers to the respective bit
+	// > position in the INTFLAG register of respective peripheral.
+	// Therefore, if we only need to listen to the RXC interrupt source (in bit
+	// position 2), we only need interrupt source 2 for this SERCOM device.
+	arm.EnableIRQ(uart.IRQVal)
 
 	return nil
 }
@@ -787,53 +783,15 @@ func (uart UART) WriteByte(c byte) error {
 	return nil
 }
 
-//go:export SERCOM3_0_IRQHandler
-func handleSERCOM3_0() {
-	handleUART1()
-}
-
-//go:export SERCOM3_1_IRQHandler
-func handleSERCOM3_1() {
-	handleUART1()
-}
-
 //go:export SERCOM3_2_IRQHandler
 func handleSERCOM3_2() {
-	handleUART1()
-}
-
-//go:export SERCOM3_OTHER_IRQHandler
-func handleSERCOM3_OTHER() {
-	handleUART1()
-}
-
-func handleUART1() {
 	// should reset IRQ
 	UART1.Receive(byte((UART1.Bus.DATA.Get() & 0xFF)))
 	UART1.Bus.INTFLAG.SetBits(sam.SERCOM_USART_INT_INTFLAG_RXC)
 }
 
-//go:export SERCOM0_0_IRQHandler
-func handleSERCOM0_0() {
-	handleUART2()
-}
-
-//go:export SERCOM0_1_IRQHandler
-func handleSERCOM0_1() {
-	handleUART2()
-}
-
 //go:export SERCOM0_2_IRQHandler
 func handleSERCOM0_2() {
-	handleUART2()
-}
-
-//go:export SERCOM0_OTHER_IRQHandler
-func handleSERCOM0_OTHER() {
-	handleUART2()
-}
-
-func handleUART2() {
 	// should reset IRQ
 	UART2.Receive(byte((UART2.Bus.DATA.Get() & 0xFF)))
 	UART2.Bus.INTFLAG.SetBits(sam.SERCOM_USART_INT_INTFLAG_RXC)