Send ZLP (zero length packet) on flush, if last write filled buffer

Also, wait for write access before sending ZLP

Author: sandeepmistry

hardware/arduino/avr/cores/arduino/USBCore.cpp

@@ -191,6 +191,8 @@ static inline u8 FrameNumber()
 //==================================================================
 //==================================================================
 
+bool _sendZlp[USB_ENDPOINTS];
+
 u8 USBGetConfiguration(void)
 {
 	return _usbConfiguration;
@@ -300,6 +302,7 @@ int USB_Send(u8 ep, const void* d, int len)
 				while (n--)
 					Send8(*data++);
 			}
+			_sendZlp[ep & USB_ENDPOINTS_MASK] = !ReadWriteAllowed() && (len == 0);
 			if (!ReadWriteAllowed() || ((len == 0) && (ep & TRANSFER_RELEASE)))	// Release full buffer
 				ReleaseTX();
 		}
@@ -625,8 +628,15 @@ ISR(USB_COM_vect)
 void USB_Flush(u8 ep)
 {
 	SetEP(ep);
-	if (FifoByteCount())
+
+	// wait for write access if a ZLP is needed
+	if (_sendZlp[ep])
+		while(!ReadWriteAllowed());
+
+	if (FifoByteCount() || _sendZlp[ep])
 		ReleaseTX();
+
+	_sendZlp[ep] = false;
 }
 
 static inline void USB_ClockDisable()