fix for USB deactivation/restore (power-saving)

Author: ppescher

Diff for: sam/1.0.4/cores/arduino/USB/CDC.cpp

@@ -165,6 +165,8 @@ void Serial_::end(void)
 
 void Serial_::accept(void)
 {
+	if (!Is_otg_enabled()) return;
+	
 	static uint32_t guard = 0;
 
 	// synchronized access to guard
@@ -202,12 +204,16 @@ void Serial_::accept(void)
 
 int Serial_::available(void)
 {
+	if (!Is_otg_enabled()) return 0;
+	
 	ring_buffer *buffer = &cdc_rx_buffer;
 	return (unsigned int)(CDC_SERIAL_BUFFER_SIZE + buffer->head - buffer->tail) % CDC_SERIAL_BUFFER_SIZE;
 }
 
 int Serial_::peek(void)
 {
+	if (!Is_otg_enabled()) return -1;
+	
 	ring_buffer *buffer = &cdc_rx_buffer;
 
 	if (buffer->head == buffer->tail)
@@ -222,6 +228,8 @@ int Serial_::peek(void)
 
 int Serial_::read(void)
 {
+	if (!Is_otg_enabled()) return -1;
+	
 	ring_buffer *buffer = &cdc_rx_buffer;
 
 	// if the head isn't ahead of the tail, we don't have any characters
@@ -241,11 +249,15 @@ int Serial_::read(void)
 
 void Serial_::flush(void)
 {
+	if (!Is_otg_enabled()) return;
+	
 	USBD_Flush(CDC_TX);
 }
 
 size_t Serial_::write(const uint8_t *buffer, size_t size)
 {
+	if (!Is_otg_enabled()) return 0;
+	
 	/* only try to send bytes if the high-level CDC connection itself
 	 is open (not just the pipe) - the OS should set lineState when the port
 	 is opened and clear lineState when the port is closed.