Change RingBuffer to have buffer size of 128 and also set its members volatile since they are all accessed and modified in interrupt handlers.

hardware/arduino/sam/cores/arduino/RingBuffer.cpp

@@ -21,7 +21,7 @@
 
 RingBuffer::RingBuffer( void )
 {
-    memset( _aucBuffer, 0, SERIAL_BUFFER_SIZE ) ;
+    memset( (void *)_aucBuffer, 0, SERIAL_BUFFER_SIZE ) ;
     _iHead=0 ;
     _iTail=0 ;
 }

hardware/arduino/sam/cores/arduino/RingBuffer.h

@@ -25,14 +25,14 @@
 // using a ring buffer (I think), in which head is the index of the location
 // to which to write the next incoming character and tail is the index of the
 // location from which to read.
-#define SERIAL_BUFFER_SIZE 64
+#define SERIAL_BUFFER_SIZE 128
 
 class RingBuffer
 {
   public:
-    uint8_t _aucBuffer[SERIAL_BUFFER_SIZE] ;
-    int _iHead ;
-    int _iTail ;
+    volatile uint8_t _aucBuffer[SERIAL_BUFFER_SIZE] ;
+    volatile int _iHead ;
+    volatile int _iTail ;
 
   public:
     RingBuffer( void ) ;