atsamd2x: avoid infinite loop when USBCDC is disconnected

src/machine/machine_atsamd21.go

@@ -1468,6 +1468,7 @@ type USBCDC struct {
 	Buffer            *RingBuffer
 	TxIdx             volatile.Register8
 	waitTxc           bool
+	waitTxcRetryCount uint8
 	sent              bool
 }
 
@@ -1476,6 +1477,7 @@ const (
 	usbcdcTxBankMask          uint8 = ^usbcdcTxSizeMask
 	usbcdcTxBank1st           uint8 = 0x00
 	usbcdcTxBank2nd           uint8 = usbcdcTxSizeMask + 1
+	usbcdcTxMaxRetriesAllowed uint8 = 5
 )
 
 // Flush flushes buffered data.
@@ -1491,6 +1493,7 @@ func (usbcdc *USBCDC) Flush() error {
 				return nil
 			}
 			usbcdc.waitTxc = true
+			usbcdc.waitTxcRetryCount = 0
 
 			// set the data
 			usbEndpointDescriptors[usb_CDC_ENDPOINT_IN].DeviceDescBank[1].ADDR.Set(uint32(uintptr(unsafe.Pointer(&udd_ep_in_cache_buffer[usb_CDC_ENDPOINT_IN][bk]))))
@@ -1537,6 +1540,14 @@ func (usbcdc *USBCDC) WriteByte(c byte) error {
 
 			if ok {
 				break
+			} else if usbcdcTxMaxRetriesAllowed < UART0.waitTxcRetryCount {
+				mask := interrupt.Disable()
+				UART0.waitTxc = false
+				UART0.waitTxcRetryCount = 0
+				usbcdc.TxIdx.Set(0)
+				usbLineInfo.lineState = 0
+				interrupt.Restore(mask)
+				break
 			} else {
 				mask := interrupt.Disable()
 				if UART0.sent {
@@ -1764,6 +1775,10 @@ func handleUSB(intr interrupt.Interrupt) {
 				}
 			}
 		}
+
+		if i == usb_CDC_ENDPOINT_IN && UART0.waitTxc {
+			UART0.waitTxcRetryCount++
+		}
 	}
 
 	UART0.Flush()