allow the Low Power library to not break the Serial connection

Author: polygamma

cores/arduino/USB/CDC.cpp

@@ -204,6 +204,10 @@ void Serial_::flush(void)
 	usb.flush(CDC_ENDPOINT_IN);
 }
 
+void Serial_::clear(void) {
+	usb.clear(CDC_ENDPOINT_IN);
+}
+
 size_t Serial_::write(const uint8_t *buffer, size_t size)
 {
 	uint32_t r = usb.send(CDC_ENDPOINT_IN, buffer, size);

cores/arduino/USB/USBAPI.h

@@ -94,6 +94,7 @@ class USBDeviceClass {
 	int recv(uint32_t ep);
 	uint32_t available(uint32_t ep);
 	void flush(uint32_t ep);
+	void clear(uint32_t ep);
 	void stall(uint32_t ep);
 
 	// private?
@@ -125,6 +126,7 @@ class Serial_ : public Stream
 	virtual int peek(void);
 	virtual int read(void);
 	virtual void flush(void);
+	virtual void clear(void);
 	virtual size_t write(uint8_t);
 	virtual size_t write(const uint8_t *buffer, size_t size);
 	using Print::write; // pull in write(str) from Print

cores/arduino/USB/USBCore.cpp

@@ -508,6 +508,17 @@ void USBDeviceClass::flush(uint32_t ep)
 	}
 }
 
+void USBDeviceClass::clear(uint32_t ep) {
+	usbd.epBank1SetAddress(ep, &udd_ep_in_cache_buffer[ep]);
+	usbd.epBank1SetByteCount(ep, 0);
+
+	// Clear the transfer complete flag
+	usbd.epBank1AckTransferComplete(ep);
+
+	// RAM buffer is full, we can send data (IN)
+	usbd.epBank1SetReady(ep);
+}
+
 void USBDeviceClass::stall(uint32_t ep)
 {
 	// TODO: test