refactor USB package with device class build tags

added basic HID keyboard support for SAMx51 (not fully-functional)

src/machine/usb/usb-cdc.go

@@ -0,0 +1,83 @@
+//go:build baremetal && usb.cdc
+// +build baremetal,usb.cdc
+
+package usb
+
+import (
+	"errors"
+)
+
+var (
+	ErrCDCInvalidPort = errors.New("invalid port")
+	ErrCDCEmptyBuffer = errors.New("buffer empty")
+)
+
+// CDC represents a virtual UART serial device emulation using the USB
+// CDC-ACM device class driver.
+type CDC struct {
+	// Port is the MCU's native USB core number. If in doubt, leave it
+	// uninitialized for default (0).
+	Port int
+	core *core
+}
+
+type CDCConfig struct {
+	BusSpeed Speed
+}
+
+func (cdc *CDC) Configure(config CDCConfig) error {
+
+	c := class{id: classDeviceCDC, config: 1}
+
+	// verify we have a free USB port and take ownership of it
+	var st status
+	cdc.core, st = initCore(cdc.Port, config.BusSpeed, c)
+	if !st.ok() {
+		return ErrCDCInvalidPort
+	}
+	return nil
+}
+
+func (cdc *CDC) Ready() bool {
+	return cdc.core.dc.cdcReady()
+}
+
+// Buffered returns the number of bytes currently stored in the Rx buffer.
+func (cdc *CDC) Buffered() int {
+	for !cdc.Ready() {
+	}
+	return cdc.core.dc.cdcAvailable()
+}
+
+// ReadByte reads a single byte from the Rx buffer.
+// If there is no data in the buffer, returns an error.
+func (cdc *CDC) ReadByte() (byte, error) {
+	for !cdc.Ready() {
+	}
+	n, ok := cdc.core.dc.cdcReadByte()
+	if !ok {
+		return 0, ErrCDCEmptyBuffer
+	}
+	return n, nil
+}
+
+// Read from the Rx buffer.
+func (cdc *CDC) Read(data []byte) (n int, err error) {
+	for !cdc.Ready() {
+	}
+	return cdc.core.dc.cdcRead(data)
+}
+
+// WriteByte writes a single byte of data to the virtual UART interface.
+func (cdc *CDC) WriteByte(c byte) error {
+	for !cdc.Ready() {
+	}
+	return cdc.core.dc.cdcWriteByte(c)
+}
+
+// Write data to the virtual UART.
+func (cdc *CDC) Write(data []byte) (n int, err error) {
+	for !cdc.Ready() {
+	}
+	return cdc.core.dc.cdcWrite(data)
+}