Fix bug with premature step end. Refactored _delay_ms() and square() for better portability.

- Fixed a premature step end bug dating back to Simen's 0.7b edge
version is fixed, from which this code is forked from. Caused by Timer2
constantly overflowing calling the Step Reset Interrupt every 128usec.
Now Timer2 is always disabled after a step end and should free up some
cycles for the main program. Could be more than one way to fix this
problem. I'm open to suggestions.

- _delay_ms() refactored to accept only constants to comply with
current compilers. square() removed since not available with some
compilers.

stepper.c

@@ -130,30 +130,25 @@ static uint8_t iterate_trapezoid_cycle_counter()
 // config_step_timer. It pops blocks from the block_buffer and executes them by pulsing the stepper pins appropriately. 
 // It is supported by The Stepper Port Reset Interrupt which it uses to reset the stepper port after each pulse. 
 // The bresenham line tracer algorithm controls all three stepper outputs simultaneously with these two interrupts.
-// NOTE: ISR_NOBLOCK allows SIG_OVERFLOW2 to trigger on-time regardless of time in this handler.
-
-// TODO: ISR_NOBLOCK is the same as the old SIGNAL with sei() method, but is optimizable by the compiler. On
-// an oscilloscope there is a weird hitch in the step pulse during high load operation. Very infrequent, but
-// when this does happen most of the time the pulse falling edge is randomly delayed by 20%-50% of the total 
-// intended pulse time, but sometimes it pulses less than 3usec. The former likely caused by the serial 
-// interrupt doing its thing, not that big of a deal, but the latter cause is unknown and worrisome. Need
-// to track down what is causing this problem. Functionally, this shouldn't cause any noticeable issues
-// as long as stepper drivers have a pulse minimum of 1usec or so (Pololu and any Allegro IC are ok).
-// ** This seems to be an inherent issue that dates all the way back to Simen's v0.6b or earlier. **
-
-ISR(TIMER1_COMPA_vect,ISR_NOBLOCK)
+ISR(TIMER1_COMPA_vect)
 {        
   if (busy) { return; } // The busy-flag is used to avoid reentering this interrupt
-  busy = true;
   
   // Set the direction pins a couple of nanoseconds before we step the steppers
   STEPPING_PORT = (STEPPING_PORT & ~DIRECTION_MASK) | (out_bits & DIRECTION_MASK);
   // Then pulse the stepping pins
   STEPPING_PORT = (STEPPING_PORT & ~STEP_MASK) | out_bits;
-  // Reset step pulse reset timer so that The Stepper Port Reset Interrupt can reset the signal after
-  // exactly settings.pulse_microseconds microseconds.
-  TCNT2 = step_pulse_time;
-    
+  // Enable step pulse reset timer so that The Stepper Port Reset Interrupt can reset the signal after
+  // exactly settings.pulse_microseconds microseconds, independent of the main Timer1 prescaler.
+  TCNT2 = step_pulse_time; // Reload timer counter
+  TCCR2B = (1<<CS21); // Begin timer2. Full speed, 1/8 prescaler
+
+  busy = true;
+  // Re-enable interrupts to allow ISR_TIMER2_OVERFLOW to trigger on-time and allow serial communications
+  // regardless of time in this handler. The following code prepares the stepper driver for the next
+  // step interrupt compare and will always finish before returning to the main program.
+  sei();
+  
   // If there is no current block, attempt to pop one from the buffer
   if (current_block == NULL) {
     // Anything in the buffer? If so, initialize next motion.
@@ -296,12 +291,15 @@ ISR(TIMER1_COMPA_vect,ISR_NOBLOCK)
   busy = false;
 }
 
-// This interrupt is set up by SIG_OUTPUT_COMPARE1A when it sets the motor port bits. It resets
+// This interrupt is set up by ISR_TIMER1_COMPAREA when it sets the motor port bits. It resets
 // the motor port after a short period (settings.pulse_microseconds) completing one step cycle.
+// TODO: It is possible for the serial interrupts to delay this interrupt by a few microseconds, if
+// they execute right before this interrupt. Not a big deal, but could use some TLC at some point.
 ISR(TIMER2_OVF_vect)
 {
   // Reset stepping pins (leave the direction pins)
   STEPPING_PORT = (STEPPING_PORT & ~STEP_MASK) | (settings.invert_mask & STEP_MASK); 
+  TCCR2B = 0; // Disable Timer2 to prevent re-entering this interrupt when it's not needed. 
 }
 
 // Reset and clear stepper subsystem variables
@@ -332,8 +330,8 @@ void st_init()
   TCCR1A &= ~(3<<COM1B0); 
 	
   // Configure Timer 2
-  TCCR2A = 0;         // Normal operation
-  TCCR2B = (1<<CS21); // Full speed, 1/8 prescaler
+  TCCR2A = 0; // Normal operation
+  TCCR2B = 0; // Disable timer until needed.
   TIMSK2 |= (1<<TOIE2);      
 
   // Start in the idle state