diff --git a/CMakeLists.txt b/CMakeLists.txt
index 23a91e6751..609c575e81 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -252,7 +252,7 @@ set(FW_SOURCES
     SdFatUtil.cpp
     SdFile.cpp
     SdVolume.cpp
-    sm4.c
+    sm4.cpp
     sound.cpp
     speed_lookuptable.cpp
     spi.c
diff --git a/Firmware/sm4.c b/Firmware/sm4.cpp
similarity index 97%
rename from Firmware/sm4.c
rename to Firmware/sm4.cpp
index 45ecb46130..009875f951 100644
--- a/Firmware/sm4.c
+++ b/Firmware/sm4.cpp
@@ -1,5 +1,4 @@
-//sm4.c - simple 4-axis stepper control
-
+//sm4.cpp - simple 4-axis stepper control
 #include "sm4.h"
 #include <avr/io.h>
 #include <avr/pgmspace.h>
@@ -70,7 +69,7 @@ void sm4_set_dir(uint8_t axis, uint8_t dir)
 
 void sm4_set_dir_bits(uint8_t dir_bits)
 {
-    register uint8_t portL = PORTL;
+	uint8_t portL = PORTL;
 	portL &= 0xb8; //set direction bits to zero
 	//TODO -optimize in asm
 #if ((MOTHERBOARD == BOARD_RAMBO_MINI_1_0) || (MOTHERBOARD == BOARD_RAMBO_MINI_1_3))
@@ -96,7 +95,7 @@ void sm4_do_step(uint8_t axes_mask)
 #ifdef TMC2130_DEDGE_STEPPING
 	PINC = (axes_mask & 0x0f); // toggle step signals by mask
 #else
-    register uint8_t portC = PORTC & 0xf0;
+	uint8_t portC = PORTC & 0xf0;
 	PORTC = portC | (axes_mask & 0x0f); //set step signals by mask
 	asm("nop");
 	PORTC = portC; //set step signals to zero
diff --git a/Firmware/sm4.h b/Firmware/sm4.h
index a1897bca26..da834f73de 100644
--- a/Firmware/sm4.h
+++ b/Firmware/sm4.h
@@ -5,12 +5,6 @@
 #include <inttypes.h>
 #include "config.h"
 
-
-#if defined(__cplusplus)
-extern "C" {
-#endif //defined(__cplusplus)
-
-
 // callback prototype for stop condition (return 0 - continue, return 1 - stop)
 typedef uint8_t (*sm4_stop_cb_t)();