diff --git a/cores/arduino/HardwareSerial.cpp b/cores/arduino/HardwareSerial.cpp
index 87794aa..284d8fc 100644
--- a/cores/arduino/HardwareSerial.cpp
+++ b/cores/arduino/HardwareSerial.cpp
@@ -5,7 +5,7 @@
 #include "HardwareSerial.h"
 #include <uart_lib.h> 
 #include "mik32_hal_irq.h"
-
+#include "wiring_LL.h" 
 
 // HardwareSerial class objects for use in Arduino IDE
 HardwareSerial Serial(0);
@@ -127,9 +127,9 @@ void HardwareSerial::rx_complete_irq(void)
   unsigned char c;
   // while there is something to receive, put the data into the buffer 
   // and edit the buffer index
-  while (!UART_IsRxFifoEmpty(uart))
+  while(!UART_IS_RX_FIFO_EMPTY(uart))
   {
-    c = UART_ReadByte(uart);
+    c = UART_READ_BYTE(uart);
     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 7ff9700..d7f60a1 100644
--- a/cores/arduino/HardwareSerial.h
+++ b/cores/arduino/HardwareSerial.h
@@ -100,7 +100,7 @@ class HardwareSerial : public Stream
     using Print::write; // pull in write(str)
     operator bool() { return isInited; }
 
-    void rx_complete_irq(void) __attribute__((optimize("O3")));
+    inline void rx_complete_irq(void) __attribute__((always_inline, optimize("O3")));
 };
 
 extern HardwareSerial Serial;