Port AVR Serial_::readBreak() API to SAM core

hardware/arduino/sam/cores/arduino/USB/CDC.cpp

@@ -55,6 +55,8 @@ static volatile LineInfo _usbLineInfo = {
     0x00   // lineState
 };
 
+static volatile int32_t breakValue = -1;
+
 _Pragma("pack(1)")
 static const CDCDescriptor _cdcInterface =
 {
@@ -144,6 +146,7 @@ bool WEAK CDC_Setup(USBSetup& setup)
 
 		if (CDC_SEND_BREAK == r)
 		{
+			breakValue = ((uint16_t)setup.wValueH << 8) | setup.wValueL;
 			return true;
 		}
 	}
@@ -305,6 +308,28 @@ Serial_::operator bool()
 	return result;
 }
 
+int32_t Serial_::readBreak() {
+	uint8_t enableInterrupts = ((__get_PRIMASK() & 0x1) == 0 && 
+								(__get_FAULTMASK() & 0x1) == 0);
+
+	// disable interrupts,
+	// to avoid clearing a breakValue that might occur 
+	// while processing the current break value
+	__disable_irq();
+
+	int ret = breakValue;
+
+	breakValue = -1;
+
+	if (enableInterrupts)
+	{
+		// re-enable the interrupts
+		__enable_irq();
+	}
+
+	return ret;
+}
+
 unsigned long Serial_::baud() {
 	return _usbLineInfo.dwDTERate;
 }

hardware/arduino/sam/cores/arduino/USB/USBAPI.h

@@ -63,6 +63,23 @@ public:
 	using Print::write; // pull in write(str) from Print
 	operator bool();
 
+	// This method allows processing "SEND_BREAK" requests sent by
+	// the USB host. Those requests indicate that the host wants to
+	// send a BREAK signal and are accompanied by a single uint16_t
+	// value, specifying the duration of the break. The value 0
+	// means to end any current break, while the value 0xffff means
+	// to start an indefinite break.
+	// readBreak() will return the value of the most recent break
+	// request, but will return it at most once, returning -1 when
+	// readBreak() is called again (until another break request is
+	// received, which is again returned once).
+	// This also mean that if two break requests are received
+	// without readBreak() being called in between, the value of the
+	// first request is lost.
+	// Note that the value returned is a long, so it can return
+	// 0-0xffff as well as -1.
+	int32_t readBreak();
+
 	// 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.