Added step pulse delay after direction set (Compile-time option only). Updated read me.

Added a compile-time only experimental feature that creates a
user-specified time delay between a step pulse and a direction pin set
(in config.h). This is for users with hardware-specific issues
(opto-couplers) that need more than a few microseconds between events,
which can lead to slowly progressing step drift after many many
direction changes. We suggest to try the hack/fix posted in the Wiki
before using this, as this experimental feature may cause Grbl to take
a performance hit at high step rates and about complex curves.

config.h

@@ -75,7 +75,7 @@
 // entering g-code into grbl, i.e. locating part zero or simple manual machining. If the axes drift,
 // grbl has no way to know this has happened, since stepper motors are open-loop control. Depending
 // on the machine, this parameter may need to be larger or smaller than the default time.
-// NOTE: If commented out, the delay will not be compiled.
+// NOTE: If set to zero, the delay will not be compiled.
 #define STEPPER_IDLE_LOCK_TIME 25 // (milliseconds) - Integer > 0
 
 // The temporal resolution of the acceleration management subsystem. Higher number give smoother
@@ -114,19 +114,36 @@
 // time step. Also, keep in mind that the Arduino delay timer is not very accurate for long delays.
 #define DWELL_TIME_STEP 50 // Integer (1-255) (milliseconds)
 
-// FOR ADVANCED USERS ONLY: Toggles XON/XOFF software flow control for serial communications. 
-// Officially not supported due to problems involving the Atmega8U2 USB-to-serial chips on all 
-// new and future Arduinos. The firmware on these chips do not support XON/XOFF flow control 
-// characters and the intermediate buffer in the chips cause latency and overflow problems with
-// standard terminal programs. However, using specifically-programmed UI's to manage this latency 
-// problem has been confirmed to work, as well as, using older FTDI FT232RL-based Arduinos
-// (Duemilanove) since their firmaware correctly manage the XON/XOFF characters. Other unconfirmed
-// methods include using an FTDI board/cable or directly communicate on the RX/TX pins on the 
-// Arduino, both of which circumvent the Atmega8U2 chip altogether. In any case, please report any
-// successes to grbl administrators!
+// ---------------------------------------------------------------------------------------
+// FOR ADVANCED USERS ONLY: 
+
+// Toggles XON/XOFF software flow control for serial communications. Not officially supported
+// due to problems involving the Atmega8U2 USB-to-serial chips on current Arduinos. The firmware
+// on these chips do not support XON/XOFF flow control characters and the intermediate buffer 
+// in the chips cause latency and overflow problems with standard terminal programs. However, 
+// using specifically-programmed UI's to manage this latency problem has been confirmed to work.
+// As well as, older FTDI FT232RL-based Arduinos(Duemilanove) are known to work with standard
+// terminal programs since their firmware correctly manage these XON/XOFF characters. In any
+// case, please report any successes to grbl administrators!
 #define ENABLE_XONXOFF 0 // Boolean. Default disabled.
 
-// -----------------------------------------------
+// Creates a delay between the direction pin setting and corresponding step pulse by creating
+// another interrupt (Timer2 compare) to manage it. The main Grbl interrupt (Timer1 compare) 
+// sets the direction pins, and does not immediately set the stepper pins, as it would in 
+// normal operation. The Timer2 compare fires next to set the stepper pins after the step 
+// pulse delay time, and Timer2 overflow will complete the step pulse, except now delayed 
+// by the step pulse time plus the step pulse delay. (Thanks langwadt for the idea!)
+//   This is an experimental feature that should only be used if your setup requires a longer
+// delay between direction and step pin settings (some opto coupler based drivers), as it may
+// adversely effect Grbl's high-end performance (>10kHz). Please notify Grbl administrators 
+// of your successes or difficulties, as we will monitor this and possibly integrate this as a 
+// standard feature for future releases. However, we suggest to first try our direction delay
+// hack/solution posted in the Wiki involving inverting the stepper pin mask.
+// NOTE: If set greater than zero, step pulse delay will be compiled and enabled. Also, the 
+// total delay added with the Grbl settings pulse microseconds must not exceed 127 ms.
+#define STEP_PULSE_DELAY 0 // Step pulse delay in microseconds. Default disabled.
+
+// ---------------------------------------------------------------------------------------
 
 // TODO: The following options are set as compile-time options for now, until the next EEPROM 
 // settings version has solidified. 

readme.textile

@@ -19,9 +19,11 @@ Grbl includes full acceleration management with look ahead. That means the contr
   - Program stop(M0,M1*,M2,M30) initial support. Optional stop to do.
   - System reset re-initializes grbl without resetting the Arduino and retains machine/home position and work coordinates.
   - Restructured planner and stepper modules to become independent and ready for future features.
-  - Planned features: Jog mode, status reporting, runtime settings such as toggling block delete, XON/XOFF flow control.
-  - Reduce serial read buffer to 128 characters and increased write buffer to 64 characters.
+  - Reduced serial read buffer to 128 characters and increased write buffer to 64 characters.
   - Misc bug fixes and removed deprecated acceleration enabled code.  
+  - Planned features: Jog mode, full-featured status reporting, runtime settings such as toggling block delete.
+  - Advanced compile-time options: Up to 6 work coordinate systems(G54-G59), XON/XOFF flow control (limited support), direction and step pulse time delay.
+  
 
 *Important note for Atmega 168 users:* Going forward, support for Atmega 168 will be dropped due to its limited memory and speed. However, legacy Grbl v0.51 "in the branch called 'v0_51' is still available for use.
 

stepper.c

@@ -62,6 +62,10 @@ static uint8_t step_pulse_time; // Step pulse reset time after step rise
 static uint8_t out_bits;        // The next stepping-bits to be output
 static volatile uint8_t busy;   // True when SIG_OUTPUT_COMPARE1A is being serviced. Used to avoid retriggering that handler.
 
+#if STEP_PULSE_DELAY > 0
+  static uint8_t step_bits;  // Stores out_bits output to complete the step pulse delay
+#endif
+
 //         __________________________
 //        /|                        |\     _________________         ^
 //       / |                        | \   /|               |\        |
@@ -86,8 +90,16 @@ static void st_wake_up()
 {
   // Initialize stepper output bits
   out_bits = (0) ^ (settings.invert_mask); 
-  // Set step pulse time. Ad hoc computation from oscilloscope.
+  // Initialize step pulse timing from settings. Here to ensure updating after re-writing.
+  #if STEP_PULSE_DELAY > 0
+    // Set total step pulse time after direction pin set. Ad hoc computation from oscilloscope.
+    step_pulse_time = -(((settings.pulse_microseconds+STEP_PULSE_DELAY-2)*TICKS_PER_MICROSECOND) >> 3);
+    // Set delay between direction pin write and step command.
+    OCR2A = -(((settings.pulse_microseconds)*TICKS_PER_MICROSECOND) >> 3);
+  #else // Normal operation
+    // Set step pulse time. Ad hoc computation from oscilloscope. Uses two's complement.
     step_pulse_time = -(((settings.pulse_microseconds-2)*TICKS_PER_MICROSECOND) >> 3);
+  #endif
   // Enable steppers by resetting the stepper disable port
   STEPPERS_DISABLE_PORT &= ~(1<<STEPPERS_DISABLE_BIT);
   // Enable stepper driver interrupt
@@ -101,7 +113,7 @@ void st_go_idle()
   TIMSK1 &= ~(1<<OCIE1A); 
   // Force stepper dwell to lock axes for a defined amount of time to ensure the axes come to a complete
   // stop and not drift from residual inertial forces at the end of the last movement.
-  #ifdef STEPPER_IDLE_LOCK_TIME
+  #if STEPPER_IDLE_LOCK_TIME > 0
     _delay_ms(STEPPER_IDLE_LOCK_TIME);   
   #endif
   // Disable steppers by setting stepper disable
@@ -133,7 +145,11 @@ ISR(TIMER1_COMPA_vect)
   // 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
+  #if STEP_PULSE_DELAY > 0
+    step_bits = (STEPPING_PORT & ~STEP_MASK) | out_bits; // Store out_bits to prevent overwriting.
+  #else  // Normal operation
     STEPPING_PORT = (STEPPING_PORT & ~STEP_MASK) | out_bits;
+  #endif
   // 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
@@ -298,6 +314,18 @@ ISR(TIMER2_OVF_vect)
   TCCR2B = 0; // Disable Timer2 to prevent re-entering this interrupt when it's not needed. 
 }
 
+#if STEP_PULSE_DELAY > 0
+  // This interrupt is used only when STEP_PULSE_DELAY is enabled. Here, the step pulse is
+  // initiated after the STEP_PULSE_DELAY time period has elapsed. The ISR TIMER2_OVF interrupt
+  // will then trigger after the appropriate settings.pulse_microseconds, as in normal operation.
+  // The new timing between direction, step pulse, and step complete events are setup in the
+  // st_wake_up() routine.
+  ISR(TIMER2_COMPA_vect) 
+  { 
+    STEPPING_PORT = step_bits; // Begin step pulse.
+  }
+#endif
+
 // Reset and clear stepper subsystem variables
 void st_reset()
 {
@@ -330,6 +358,10 @@ void st_init()
   TCCR2B = 0; // Disable timer until needed.
   TIMSK2 |= (1<<TOIE2);      
   
+  #if STEP_PULSE_DELAY > 0
+    TIMSK2 |= (1<<OCIE2A); // Enable Timer2 Compare Match A interrupt
+  #endif
+  
   // Start in the idle state
   st_go_idle();
 }