Add core support for multiple UARTs (#152)

* machine/uart: add core support for multiple UARTs by allowing for multiple RingBuffers
* machine/uart: complete core support for multiple UARTs
* machine/uart: no need to store pointer to UART, better to treat like I2C and SPI
* machine/uart: increase ring buffer size to 128 bytes
* machine/uart: improve godocs comments and use comma-ok idiom for buffer Put/Get methods

src/machine/buffer.go

@@ -0,0 +1,49 @@
+package machine
+
+const bufferSize = 128
+
+//go:volatile
+type volatileByte byte
+
+// RingBuffer is ring buffer implementation inspired by post at
+// https://www.embeddedrelated.com/showthread/comp.arch.embedded/77084-1.php
+//
+// It has some limitations currently due to how "volatile" variables that are
+// members of a struct are not compiled correctly by TinyGo.
+// See https://github.com/aykevl/tinygo/issues/151 for details.
+type RingBuffer struct {
+	rxbuffer [bufferSize]volatileByte
+	head     volatileByte
+	tail     volatileByte
+}
+
+// NewRingBuffer returns a new ring buffer.
+func NewRingBuffer() *RingBuffer {
+	return &RingBuffer{}
+}
+
+// Used returns how many bytes in buffer have been used.
+func (rb *RingBuffer) Used() uint8 {
+	return uint8(rb.head - rb.tail)
+}
+
+// Put stores a byte in the buffer. If the buffer is already
+// full, the method will return false.
+func (rb *RingBuffer) Put(val byte) bool {
+	if rb.Used() != bufferSize {
+		rb.head++
+		rb.rxbuffer[rb.head%bufferSize] = volatileByte(val)
+		return true
+	}
+	return false
+}
+
+// Get returns a byte from the buffer. If the buffer is empty,
+// the method will return a false as the second value.
+func (rb *RingBuffer) Get() (byte, bool) {
+	if rb.Used() != 0 {
+		rb.tail++
+		return byte(rb.rxbuffer[rb.tail%bufferSize]), true
+	}
+	return 0, false
+}