Expose serial settings from CDC virtual serial port

This allows a sketch to find out the settings chosen by the USB host
(computer) and act accordingly.

Other than reading the DTR flag and checking if the baudrate is 1200,
the regular CDC code doesn't actually use any of these settings.

By exposing these settings to the sketch, it can for example copy them
to the hardware UART, turning the Leonardo into a proper USB-to-serial
device. This can be useful to let the computer directly talk to whatever
device is connected to the hardware serial port (like an XBee module).

The Teensy core already supported these methods. This code was
independently developed, but the method names were chosen to match the
Teensy code, for compatibility (except that `dtr()` and `rtr()` return
`bool`, while the Teensy version return a `uint8_t`).

This change is applied to both the avr and sam cores, which have a very
similar CDC implementation.

Author: matthijskooijman

cores/arduino/USB/CDC.cpp

@@ -299,6 +299,30 @@ Serial_::operator bool()
 	return result;
 }
 
+unsigned long Serial_::baud() {
+	return _usbLineInfo.dwDTERate;
+}
+
+uint8_t Serial_::stopbits() {
+	return _usbLineInfo.bCharFormat;
+}
+
+uint8_t Serial_::paritytype() {
+	return _usbLineInfo.bParityType;
+}
+
+uint8_t Serial_::numbits() {
+	return _usbLineInfo.bDataBits;
+}
+
+bool Serial_::dtr() {
+	return _usbLineInfo.lineState & 0x1;
+}
+
+bool Serial_::rts() {
+	return _usbLineInfo.lineState & 0x2;
+}
+
 Serial_ SerialUSB;
 
 #endif

cores/arduino/USB/USBAPI.h

@@ -61,6 +61,28 @@ class Serial_ : public Stream
 	virtual size_t write(const uint8_t *buffer, size_t size);
 	using Print::write; // pull in write(str) from Print
 	operator bool();
+
+	// These return the settings specified by the USB host for the
+	// serial port. These aren't really used, but are offered here
+	// in case a sketch wants to act on these settings.
+	uint32_t baud();
+	uint8_t stopbits();
+	uint8_t paritytype();
+	uint8_t numbits();
+	bool dtr();
+	bool rts();
+	enum {
+		ONE_STOP_BIT = 0,
+		ONE_AND_HALF_STOP_BIT = 1,
+		TWO_STOP_BITS = 2,
+	};
+	enum {
+		NO_PARITY = 0,
+		ODD_PARITY = 1,
+		EVEN_PARITY = 2,
+		MARK_PARITY = 3,
+		SPACE_PARITY = 4,
+	};
 };
 extern Serial_ SerialUSB;