dda.c: allow protected re-entry for time based stepper calculations.

This becomes more important when acceleration is calculated
there, too.

dda.c

@@ -717,6 +717,7 @@ void dda_step(DDA *dda) {
   Updating speed 500 times a second is easily enough for smooth acceleration!
 */
 void dda_clock() {
+  static volatile uint8_t busy = 0;
   DDA *dda;
   static DDA *last_dda = NULL;
   static uint8_t endstop_stop = 0; ///< Stop due to endstop trigger
@@ -732,6 +733,13 @@ void dda_clock() {
   if (dda == NULL)
     return;
 
+  // Lengthy calculations ahead!
+  // Make sure we didn't re-enter, then allow nested interrupts.
+  if (busy)
+    return;
+  busy = 1;
+  sei();
+
   // Caution: we mangle step counters here without locking interrupts. This
   //          means, we trust dda isn't changed behind our back, which could
   //          in principle (but rarely) happen if endstops are checked not as
@@ -808,6 +816,9 @@ void dda_clock() {
   } /* if (endstop_stop == 0) */
 
   last_dda = dda;
+
+  cli(); // Compensate sei() above.
+  busy = 0;
 }
 
 /// update global current_position struct

timer.c

@@ -57,8 +57,6 @@ ISR(TIMER1_COMPB_vect) {
 	/*
 	clock stuff
 	*/
-  dda_clock();
-
 	clock_counter_10ms += TICK_TIME_MS;
 	if (clock_counter_10ms >= 10) {
 		clock_counter_10ms -= 10;
@@ -77,6 +75,8 @@ ISR(TIMER1_COMPB_vect) {
 		}
 	}
 
+  dda_clock();
+
 	// restore status register
 	MEMORY_BARRIER();
 	SREG = sreg_save;