nrf: implement UART interface

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

src/machine/board_pca10040.go

@@ -19,3 +19,9 @@ const (
 	BUTTON3 = 15
 	BUTTON4 = 16
 )
+
+// UART pins for NRF52840-DK
+const (
+	UART_TX_PIN = 6
+	UART_RX_PIN = 8
+)

src/machine/machine_avr.go

@@ -4,7 +4,6 @@ package machine
 
 import (
 	"device/avr"
-	"errors"
 )
 
 type GPIOMode uint8
@@ -274,14 +273,6 @@ func (i2c I2C) ReadByte() byte {
 }
 
 // UART
-
-type UARTConfig struct {
-	Baudrate uint32
-}
-
-type UART struct {
-}
-
 var (
 	// UART0 is the hardware serial port on the AVR.
 	UART0 = &UART{}
@@ -289,14 +280,14 @@ var (
 
 // 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
+	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)
+	ps := ((CPU_FREQUENCY+config.BaudRate*8)/(config.BaudRate*16) - 1)
 	*avr.UBRR0H = avr.RegValue(ps >> 8)
 	*avr.UBRR0L = avr.RegValue(ps & 0xff)
 
@@ -304,47 +295,6 @@ func (uart UART) Configure(config UARTConfig) {
 	*avr.UCSR0B |= avr.UCSR0B_RXCIE0
 }
 
-// Read from the RX buffer.
-func (uart UART) Read(data []byte) (n int, err error) {
-	// check if RX buffer is empty
-	size := uart.Buffered()
-	if size == 0 {
-		return 0, nil
-	}
-
-	// Make sure we do not read more from buffer than the data slice can hold.
-	if len(data) < size {
-		size = len(data)
-	}
-
-	// only read number of bytes used from buffer
-	for i := 0; i < size; i++ {
-		v, _ := uart.ReadByte()
-		data[i] = v
-	}
-
-	return size, nil
-}
-
-// Write data to the UART.
-func (uart UART) Write(data []byte) (n int, err error) {
-	for _, v := range data {
-		uart.WriteByte(v)
-	}
-	return len(data), nil
-}
-
-// ReadByte reads a single byte from the RX buffer.
-// If there is no data in the buffer, returns an error.
-func (uart UART) ReadByte() (byte, error) {
-	// check if RX buffer is empty
-	if uart.Buffered() == 0 {
-		return 0, errors.New("Buffer empty")
-	}
-
-	return bufferGet(), nil
-}
-
 // WriteByte writes a byte of data to the UART.
 func (uart UART) WriteByte(c byte) error {
 	// Wait until UART buffer is not busy.
@@ -354,38 +304,6 @@ func (uart UART) WriteByte(c byte) error {
 	return nil
 }
 
-// Buffered returns the number of bytes current stored in the RX buffer.
-func (uart UART) Buffered() int {
-	return int(bufferUsed())
-}
-
-const bufferSize = 64
-
-// Minimal ring buffer implementation inspired by post at
-// https://www.embeddedrelated.com/showthread/comp.arch.embedded/77084-1.php
-
-//go:volatile
-type volatileByte byte
-
-var rxbuffer [bufferSize]volatileByte
-var head volatileByte
-var tail volatileByte
-
-func bufferUsed() uint8 { return uint8(head - tail) }
-func bufferPut(val byte) {
-	if bufferUsed() != bufferSize {
-		head++
-		rxbuffer[head%bufferSize] = volatileByte(val)
-	}
-}
-func bufferGet() byte {
-	if bufferUsed() != 0 {
-		tail++
-		return byte(rxbuffer[tail%bufferSize])
-	}
-	return 0
-}
-
 //go:interrupt USART_RX_vect
 func handleUSART_RX() {
 	// Read register to clear it.

src/machine/machine_nrf.go

@@ -3,6 +3,7 @@
 package machine
 
 import (
+	"device/arm"
 	"device/nrf"
 )
 
@@ -35,3 +36,63 @@ func (p GPIO) Set(high bool) {
 func (p GPIO) Get() bool {
 	return (nrf.P0.IN>>p.Pin)&1 != 0
 }
+
+// UART
+var (
+	// UART0 is the hardware serial port on the NRF.
+	UART0 = &UART{}
+)
+
+// Configure the UART.
+func (uart UART) Configure(config UARTConfig) {
+	// Default baud rate to 115200.
+	if config.BaudRate == 0 {
+		config.BaudRate = 115200
+	}
+
+	uart.SetBaudRate(config.BaudRate)
+
+	// Set TX and RX pins from board.
+	nrf.UART0.PSELTXD = UART_TX_PIN
+	nrf.UART0.PSELRXD = UART_RX_PIN
+
+	nrf.UART0.ENABLE = nrf.UART_ENABLE_ENABLE_Enabled
+	nrf.UART0.TASKS_STARTTX = 1
+	nrf.UART0.TASKS_STARTRX = 1
+	nrf.UART0.INTENSET = nrf.UART_INTENSET_RXDRDY_Msk
+
+	// Enable RX IRQ.
+	arm.EnableIRQ(nrf.IRQ_UARTE0_UART0)
+}
+
+// SetBaudRate sets the communication speed for the UART.
+func (uart UART) SetBaudRate(br uint32) {
+	// Magic: calculate 'baudrate' register from the input number.
+	// Every value listed in the datasheet will be converted to the
+	// correct register value, except for 192600. I suspect the value
+	// listed in the nrf52 datasheet (0x0EBED000) is incorrectly rounded
+	// and should be 0x0EBEE000, as the nrf51 datasheet lists the
+	// nonrounded value 0x0EBEDFA4.
+	// Some background:
+	// https://devzone.nordicsemi.com/f/nordic-q-a/391/uart-baudrate-register-values/2046#2046
+	rate := uint32((uint64(br/400)*uint64(400*0xffffffff/16000000) + 0x800) & 0xffffff000)
+
+	nrf.UART0.BAUDRATE = nrf.RegValue(rate)
+}
+
+// WriteByte writes a byte of data to the UART.
+func (uart UART) WriteByte(c byte) error {
+	nrf.UART0.EVENTS_TXDRDY = 0
+	nrf.UART0.TXD = nrf.RegValue(c)
+	for nrf.UART0.EVENTS_TXDRDY == 0 {
+	}
+	return nil
+}
+
+//go:export UARTE0_UART0_IRQHandler
+func handleUART0() {
+	if nrf.UART0.EVENTS_RXDRDY != 0 {
+		bufferPut(byte(nrf.UART0.RXD))
+		nrf.UART0.EVENTS_RXDRDY = 0x0
+	}
+}

src/machine/uart.go

@@ -0,0 +1,85 @@
+// +build avr nrf
+
+package machine
+
+import "errors"
+
+type UARTConfig struct {
+	BaudRate uint32
+}
+
+type UART struct {
+}
+
+// Read from the RX buffer.
+func (uart UART) Read(data []byte) (n int, err error) {
+	// check if RX buffer is empty
+	size := uart.Buffered()
+	if size == 0 {
+		return 0, nil
+	}
+
+	// Make sure we do not read more from buffer than the data slice can hold.
+	if len(data) < size {
+		size = len(data)
+	}
+
+	// only read number of bytes used from buffer
+	for i := 0; i < size; i++ {
+		v, _ := uart.ReadByte()
+		data[i] = v
+	}
+
+	return size, nil
+}
+
+// Write data to the UART.
+func (uart UART) Write(data []byte) (n int, err error) {
+	for _, v := range data {
+		uart.WriteByte(v)
+	}
+	return len(data), nil
+}
+
+// ReadByte reads a single byte from the RX buffer.
+// If there is no data in the buffer, returns an error.
+func (uart UART) ReadByte() (byte, error) {
+	// check if RX buffer is empty
+	if uart.Buffered() == 0 {
+		return 0, errors.New("Buffer empty")
+	}
+
+	return bufferGet(), nil
+}
+
+// Buffered returns the number of bytes currently stored in the RX buffer.
+func (uart UART) Buffered() int {
+	return int(bufferUsed())
+}
+
+const bufferSize = 64
+
+// Minimal ring buffer implementation inspired by post at
+// https://www.embeddedrelated.com/showthread/comp.arch.embedded/77084-1.php
+
+//go:volatile
+type volatileByte byte
+
+var rxbuffer [bufferSize]volatileByte
+var head volatileByte
+var tail volatileByte
+
+func bufferUsed() uint8 { return uint8(head - tail) }
+func bufferPut(val byte) {
+	if bufferUsed() != bufferSize {
+		head++
+		rxbuffer[head%bufferSize] = volatileByte(val)
+	}
+}
+func bufferGet() byte {
+	if bufferUsed() != 0 {
+		tail++
+		return byte(rxbuffer[tail%bufferSize])
+	}
+	return 0
+}

src/runtime/runtime_nrf.go

@@ -31,6 +31,7 @@ func initUART() {
 	nrf.UART0.BAUDRATE = nrf.UART_BAUDRATE_BAUDRATE_Baud115200
 	nrf.UART0.TASKS_STARTTX = 1
 	nrf.UART0.PSELTXD = 6 // pin 6 for NRF52840-DK
+	nrf.UART0.PSELRXD = 8 // pin 8 for NRF52840-DK
 }
 
 func initLFCLK() {