Don't disable the stepper timer interrupt while stepping.

After lots of try and error the conclusion was, disabling this
interrupt makes the timer vulnerable to be messed up by
characters incoming over the serial line. So, now the
interrupt is enabled as a move starts and not disabled before
the move, and all subsequent moves are done.

dda.c

@@ -330,6 +330,7 @@ void dda_start(DDA *dda) {
 
 		// set timeout for first step
 		setTimer(dda->c >> 8);
+		enableTimerInterrupt();
 	}
 }
 

dda_queue.c

@@ -26,10 +26,7 @@ uint8_t queue_empty() {
 // It calls a few other functions, though.
 // -------------------------------------------------------
 void queue_step() {
-	disableTimerInterrupt();
-
 	// do our next step
-	// NOTE: dda_step makes this interrupt interruptible after steps have been sent but before new speed is calculated.
 	if (movebuffer[mb_tail].live) {
 		if (movebuffer[mb_tail].waitfor_temp) {
 			if (temp_achieved()) {
@@ -42,6 +39,7 @@ void queue_step() {
 			#endif
 		}
 		else {
+			// NOTE: dda_step makes this interrupt interruptible after steps have been sent but before new speed is calculated.
 			dda_step(&(movebuffer[mb_tail]));
 		}
 	}
@@ -50,13 +48,6 @@ void queue_step() {
 	// the dda dies not directly after its last step, but when the timer fires and there's no steps to do
 	if (movebuffer[mb_tail].live == 0)
 		next_move();
-
-	#if STEP_INTERRUPT_INTERRUPTIBLE
-		cli();
-	#endif
-	// check queue, if empty we don't need to interrupt again until re-enabled in dda_create
-	if (queue_empty() == 0)
-		enableTimerInterrupt();
 }
 
 void enqueue(TARGET *t) {
@@ -93,9 +84,11 @@ void enqueue(TARGET *t) {
 	#endif
 
 	// fire up in case we're not running yet
-	enableTimerInterrupt();
+	if (TIMSK1 == 0)
+		next_move();
 }
 
+// sometimes called from normal program execution, sometimes from interrupt context
 void next_move() {
 	if (queue_empty() == 0) {
 		// next item
@@ -104,6 +97,8 @@ void next_move() {
 		dda_start(&movebuffer[t]);
 		mb_tail = t;
 	}
+	else
+		disableTimerInterrupt();
 
 	#ifdef	XONXOFF
 	// restart transmission if the move buffer is only half full

timer.c

@@ -21,9 +21,6 @@ void setupTimerInterrupt()
 	TCCR1B = MASK(WGM12);
 	// no interrupts yet
 	TIMSK1 = 0;
-
-	//start off with a slow frequency.
-	setTimer(F_CPU / 100);
 }
 
 // the following are all from reprap project 5D firmware with some modification to reduce redundancy