This commit contains 2 changes:

Added support for different size of TX and RX buffer sizes.
Added support for buffer sizes bigger than 256 bytes.

Added support for different size of TX and RX buffer sizes.
The default values remain the same. If you want to have different values
define SERIAL_TX_BUFFER_SIZE and SERIAL_RX_BUFFER_SIZE on the command
line

Added support for buffer sizes bigger than 256 bytes.
The type of the indexes is decided upon the size of the buffers. So
there is no increase in program/data size when the buffers are smaller
than 257

Author: jantje

Diff for: cores/arduino/HardwareSerial.cpp

@@ -83,7 +83,7 @@ void HardwareSerial::_tx_udr_empty_irq(void)
   // If interrupts are enabled, there must be more data in the output
   // buffer. Send the next byte
   unsigned char c = _tx_buffer[_tx_buffer_tail];
-  _tx_buffer_tail = (_tx_buffer_tail + 1) % SERIAL_BUFFER_SIZE;
+  _tx_buffer_tail = (_tx_buffer_tail + 1) % SERIAL_TX_BUFFER_SIZE;
 
   *_udr = c;
 
@@ -152,7 +152,7 @@ void HardwareSerial::end()
 
 int HardwareSerial::available(void)
 {
-  return (unsigned int)(SERIAL_BUFFER_SIZE + _rx_buffer_head - _rx_buffer_tail) % SERIAL_BUFFER_SIZE;
+  return (unsigned int)(SERIAL_RX_BUFFER_SIZE + _rx_buffer_head - _rx_buffer_tail) % SERIAL_RX_BUFFER_SIZE;
 }
 
 int HardwareSerial::peek(void)
@@ -171,7 +171,7 @@ int HardwareSerial::read(void)
     return -1;
   } else {
     unsigned char c = _rx_buffer[_rx_buffer_tail];
-    _rx_buffer_tail = (uint8_t)(_rx_buffer_tail + 1) % SERIAL_BUFFER_SIZE;
+    _rx_buffer_tail = (BUFPOINTER)(_rx_buffer_tail + 1) % SERIAL_RX_BUFFER_SIZE;
     return c;
   }
 }
@@ -207,7 +207,7 @@ size_t HardwareSerial::write(uint8_t c)
     sbi(*_ucsra, TXC0);
     return 1;
   }
-  uint8_t i = (_tx_buffer_head + 1) % SERIAL_BUFFER_SIZE;
+  BUFPOINTER i = (_tx_buffer_head + 1) % SERIAL_TX_BUFFER_SIZE;
 
   // If the output buffer is full, there's nothing for it other than to 
   // wait for the interrupt handler to empty it a bit

Diff for: cores/arduino/HardwareSerial.h

@@ -32,10 +32,19 @@
 // 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.
+#if !(defined(SERIAL_TX_BUFFER_SIZE)&&defined(SERIAL_RX_BUFFER_SIZE))
 #if (RAMEND < 1000)
-  #define SERIAL_BUFFER_SIZE 16
+#define SERIAL_TX_BUFFER_SIZE 16
+#define SERIAL_RX_BUFFER_SIZE 16
 #else
-  #define SERIAL_BUFFER_SIZE 64
+#define SERIAL_TX_BUFFER_SIZE 64
+#define SERIAL_RX_BUFFER_SIZE 64
+#endif
+#endif
+#if (SERIAL_TX_BUFFER_SIZE>255) || (SERIAL_RX_BUFFER_SIZE>255)
+#define BUFPOINTER uint16_t
+#else
+#define BUFPOINTER uint8_t
 #endif
 
 // Define config for Serial.begin(baud, config);
@@ -76,16 +85,16 @@ class HardwareSerial : public Stream
     // Has any byte been written to the UART since begin()
     bool _written;
 
-    volatile uint8_t _rx_buffer_head;
-    volatile uint8_t _rx_buffer_tail;
-    volatile uint8_t _tx_buffer_head;
-    volatile uint8_t _tx_buffer_tail;
+    volatile BUFPOINTER _rx_buffer_head;
+    volatile BUFPOINTER _rx_buffer_tail;
+    volatile BUFPOINTER _tx_buffer_head;
+    volatile BUFPOINTER _tx_buffer_tail;
 
     // Don't put any members after these buffers, since only the first
     // 32 bytes of this struct can be accessed quickly using the ldd
     // instruction.
-    unsigned char _rx_buffer[SERIAL_BUFFER_SIZE];
-    unsigned char _tx_buffer[SERIAL_BUFFER_SIZE];
+    unsigned char _rx_buffer[SERIAL_RX_BUFFER_SIZE];
+    unsigned char _tx_buffer[SERIAL_TX_BUFFER_SIZE];
 
   public:
     inline HardwareSerial(