diff --git a/gcode.c b/gcode.c
index becac9d..7efd8fc 100644
--- a/gcode.c
+++ b/gcode.c
@@ -169,8 +169,10 @@ uint8_t gc_execute_line(char *line) {
       switch(int_value) {
         case 0: gc.motion_mode = MOTION_MODE_SEEK; break;
         case 1: gc.motion_mode = MOTION_MODE_LINEAR; break;
+#ifdef __AVR_ATmega328P__        
         case 2: gc.motion_mode = MOTION_MODE_CW_ARC; break;
         case 3: gc.motion_mode = MOTION_MODE_CCW_ARC; break;
+#endif        
         case 4: next_action = NEXT_ACTION_DWELL; break;
         case 17: select_plane(X_AXIS, Y_AXIS, Z_AXIS); break;
         case 18: select_plane(X_AXIS, Z_AXIS, Y_AXIS); break;
@@ -264,6 +266,7 @@ uint8_t gc_execute_line(char *line) {
       mc_line(target[X_AXIS], target[Y_AXIS], target[Z_AXIS], 
         (gc.inverse_feed_rate_mode) ? inverse_feed_rate : gc.feed_rate, gc.inverse_feed_rate_mode);
       break;
+#ifdef __AVR_ATmega328P__
       case MOTION_MODE_CW_ARC: case MOTION_MODE_CCW_ARC:
       if (radius_mode) {
         /* 
@@ -394,6 +397,7 @@ uint8_t gc_execute_line(char *line) {
       mc_line(target[X_AXIS], target[Y_AXIS], target[Z_AXIS], 
         (gc.inverse_feed_rate_mode) ? inverse_feed_rate : gc.feed_rate, gc.inverse_feed_rate_mode);
       break;
+#endif      
     }    
   }
   
diff --git a/main.c b/main.c
index 0b9a354..6c33037 100644
--- a/main.c
+++ b/main.c
@@ -31,9 +31,9 @@
 #include "settings.h"
 #include "wiring_serial.h"
 
-#ifndef __AVR_ATmega328P__
-#  error "As of version 0.6 Grbl only supports atmega328p. If you want to run Grbl on an 168 check out 0.51 ('git co v0_51')"
-#endif
+// #ifndef __AVR_ATmega328P__
+// #  error "As of version 0.6 Grbl only supports atmega328p. If you want to run Grbl on an 168 check out 0.51 ('git co v0_51')"
+// #endif
 
 int main(void)
 {
diff --git a/motion_control.c b/motion_control.c
index f1f1b45..11c7dec 100644
--- a/motion_control.c
+++ b/motion_control.c
@@ -42,6 +42,7 @@ void mc_dwell(uint32_t milliseconds)
 // axis in axis_l which will be the axis for linear travel if you are tracing a helical motion.
 // position is a pointer to a vector representing the current position in millimeters.
 
+#ifdef __AVR_ATmega328P__
 // The arc is approximated by generating a huge number of tiny, linear segments. The length of each 
 // segment is configured in settings.mm_per_arc_segment.  
 void mc_arc(double theta, double angular_travel, double radius, double linear_travel, int axis_1, int axis_2, 
@@ -77,6 +78,7 @@ void mc_arc(double theta, double angular_travel, double radius, double linear_tr
   }
   plan_set_acceleration_manager_enabled(acceleration_manager_was_enabled);
 }
+#endif
 
 void mc_go_home()
 {
diff --git a/motion_control.h b/motion_control.h
index d2f409b..90a05da 100644
--- a/motion_control.h
+++ b/motion_control.h
@@ -31,12 +31,14 @@
 // NOTE: Although this function structurally belongs in this module, there is nothing to do but
 // to forward the request to the planner. For efficiency the function is implemented with a macro.
 
+#ifdef __AVR_ATmega328P__
 // Execute an arc. theta == start angle, angular_travel == number of radians to go along the arc,
 // positive angular_travel means clockwise, negative means counterclockwise. Radius == the radius of the
 // circle in millimeters. axis_1 and axis_2 selects the circle plane in tool space. Stick the remaining
 // axis in axis_l which will be the axis for linear travel if you are tracing a helical motion.
 void mc_arc(double theta, double angular_travel, double radius, double linear_travel, int axis_1, int axis_2, 
   int axis_linear, double feed_rate, int invert_feed_rate, double *position);
+#endif
   
 // Dwell for a couple of time units
 void mc_dwell(uint32_t milliseconds);
diff --git a/planner.c b/planner.c
index 7684632..9dbe363 100644
--- a/planner.c
+++ b/planner.c
@@ -64,7 +64,11 @@
 #include "wiring_serial.h"
 
 // The number of linear motions that can be in the plan at any give time
-#define BLOCK_BUFFER_SIZE 20  
+#ifdef __AVR_ATmega328P__
+#define BLOCK_BUFFER_SIZE 20
+#else
+#define BLOCK_BUFFER_SIZE 5
+#endif
 
 static block_t block_buffer[BLOCK_BUFFER_SIZE];  // A ring buffer for motion instructions
 static volatile int block_buffer_head;           // Index of the next block to be pushed
diff --git a/wiring_serial.c b/wiring_serial.c
index 20f8756..62c4be4 100644
--- a/wiring_serial.c
+++ b/wiring_serial.c
@@ -31,7 +31,11 @@
 // using a ring buffer (I think), in which rx_buffer_head is the index of the
 // location to which to write the next incoming character and rx_buffer_tail
 // is the index of the location from which to read.
-#define RX_BUFFER_SIZE 200
+#ifdef __AVR_ATmega328P__
+#define RX_BUFFER_SIZE 256
+#else
+#define RX_BUFFER_SIZE 64
+#endif
 
 unsigned char rx_buffer[RX_BUFFER_SIZE];