diff --git a/cores/arduino/HardwareSerial.cpp b/cores/arduino/HardwareSerial.cpp
index a0a6642..6eb0bf9 100644
--- a/cores/arduino/HardwareSerial.cpp
+++ b/cores/arduino/HardwareSerial.cpp
@@ -121,18 +121,16 @@ int HardwareSerial::availableForWrite(void)
 void HardwareSerial::rx_complete_irq(void)
 {
   // which UART to use
-  UART_TypeDef* uart = UART_0;
-  if (uartNum == 1)
-    uart = UART_1;
-
-  // find next index in buffer with upper limit
-  uint8_t i = (uint8_t)(_rx_buffer_head + 1)%SERIAL_RX_BUFFER_SIZE;
+  UART_TypeDef* uart = (uartNum == 1) ? UART_1 : UART_0;
+  rx_buffer_index_t i;
   unsigned char c;
   // while there is something to receive, put the data into the buffer 
   // and edit the buffer index
   while(!UART_IS_RX_FIFO_EMPTY(uart))
   {
     c = UART_READ_BYTE(uart);
+    // find next index in buffer with upper limit
+    i = (unsigned int)(_rx_buffer_head + 1)%SERIAL_RX_BUFFER_SIZE;
     if (i != _rx_buffer_tail) 
     {
       // write if there is space in the buffer
diff --git a/cores/arduino/HardwareSerial.h b/cores/arduino/HardwareSerial.h
index 792bf13..5791740 100644
--- a/cores/arduino/HardwareSerial.h
+++ b/cores/arduino/HardwareSerial.h
@@ -41,7 +41,15 @@
 // often work, but occasionally a race condition can occur that makes
 // Serial behave erratically. See https://github.com/arduino/Arduino/issues/2405
 
+#if !defined(SERIAL_RX_BUFFER_SIZE)
 #define SERIAL_RX_BUFFER_SIZE 64
+#endif
+
+#if  (SERIAL_RX_BUFFER_SIZE>256)
+typedef uint16_t rx_buffer_index_t;
+#else
+typedef uint8_t rx_buffer_index_t;
+#endif
 
 // Define config for Serial.begin(baud, config);
 // parity:      7...4 bits = 0 (no), 2 (even), 3 (odd)
@@ -68,8 +76,8 @@
 class HardwareSerial : public Stream
 {
   protected:
-    volatile uint8_t _rx_buffer_head;
-    volatile uint8_t _rx_buffer_tail;
+    volatile rx_buffer_index_t _rx_buffer_head;
+    volatile rx_buffer_index_t _rx_buffer_tail;
     unsigned char _rx_buffer[SERIAL_RX_BUFFER_SIZE];
   
   private: