nrf: implement UART interface

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

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
+	}
+}