Minor updates.

- Updated makefile to be more universally compatible by not requiring
grep or ruby.

- Edited XON/XOFF flow control usage, noting that FTDI-based Arduinos
are known to work, but not Atmega8U2-based Arduino. Still officially
not supported, but added for advanced users.

- Minor edits.

Makefile

@@ -40,7 +40,7 @@ FUSES      = -U hfuse:w:0xd2:m -U lfuse:w:0xff:m
 # Tune the lines below only if you know what you are doing:
 
 AVRDUDE = avrdude $(PROGRAMMER) -p $(DEVICE) -B 10 -F 
-COMPILE = avr-gcc -Wall -Os -DF_CPU=$(CLOCK) -mmcu=$(DEVICE) -I. -ffunction-sections
+COMPILE = avr-gcc -Wall -Os -DF_CPU=$(CLOCK) -mmcu=$(DEVICE) -I. -ffunction-sections 
 
 # symbolic targets:
 all:	grbl.hex
@@ -81,8 +81,7 @@ main.elf: $(OBJECTS)
 grbl.hex: main.elf
 	rm -f grbl.hex
 	avr-objcopy -j .text -j .data -O ihex main.elf grbl.hex
-	avr-objdump -h main.elf | grep .bss | ruby -e 'puts "\n\n--- Requires %s bytes of SRAM" % STDIN.read.match(/0[0-9a-f]+\s/)[0].to_i(16)'
-	avr-size *.hex *.elf *.o
+	avr-size -C --mcu=$(DEVICE) main.elf
 # If you have an EEPROM section, you must also create a hex file for the
 # EEPROM and add it to the "flash" target.
 

config.h

@@ -115,12 +115,15 @@
 #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 USB-to-serial chip latency (Atmega8U2/FTDI)
-// when connecting to an Arduino through the USB port. This problem has to do with having no control
-// of the USB packets and causing standard terminal programs not being able to honor the XON/XOFF 
-// control characters on time. However, with specially programmed UI's or avoiding the USB interface
-// completely, XON/XOFF flow control should work. In any case, please report any successes to grbl
-// administrators!
+// Officially not supported due to problems involving the Atmega8U2 USB-to-serial chips on current
+// future Arduinos boards. 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!
 #define ENABLE_XONXOFF 0 // Boolean. Default disabled.
 
 // -----------------------------------------------

gcode.c

@@ -569,7 +569,7 @@ static int next_statement(char *letter, double *double_ptr, char *line, uint8_t
   - Evaluation of expressions
   - Variables
   - Multiple home locations
-  - Multiple coordinate systems (May be added in the future)
+  - Multiple coordinate systems (Additional ones maybe added in config.h)
   - Probing
   - Override control
   - Tool changes

limits.c

@@ -1,5 +1,5 @@
 /*
-  limits.h - code pertaining to limit-switches and performing the homing cycle
+  limits.c - code pertaining to limit-switches and performing the homing cycle
   Part of Grbl
 
   Copyright (c) 2009-2011 Simen Svale Skogsrud

serial.c

@@ -111,24 +111,24 @@ ISR(USART_UDRE_vect)
   uint8_t tail = tx_buffer_tail;
   
   #if ENABLE_XONXOFF
-    switch (flow_ctrl) {
-      case SEND_XOFF: UDR0 = XOFF_CHAR; flow_ctrl = XOFF_SENT; break;
-      case SEND_XON: UDR0 = XON_CHAR; flow_ctrl = XON_SENT; break;
-      default:  
+    if (flow_ctrl == SEND_XOFF) { 
+      UDR0 = XOFF_CHAR; 
+      flow_ctrl = XOFF_SENT; 
+    } else if (flow_ctrl == SEND_XON) { 
+      UDR0 = XON_CHAR; 
+      flow_ctrl = XON_SENT; 
+    } else
   #endif
+  { 
+    // Send a byte from the buffer	
+    UDR0 = tx_buffer[tail];
   
-  // Send a byte from the buffer	
-  UDR0 = tx_buffer[tail];
-
-  // Update tail position
-  tail++;
-  if (tail == TX_BUFFER_SIZE) { tail = 0; }
-
-  tx_buffer_tail = tail;
-
-  #if ENABLE_XONXOFF
-    }
-  #endif
+    // Update tail position
+    tail++;
+    if (tail == TX_BUFFER_SIZE) { tail = 0; }
+  
+    tx_buffer_tail = tail;
+  }
   
   // Turn off Data Register Empty Interrupt to stop tx-streaming if this concludes the transfer
   if (tail == tx_buffer_head) { UCSR0B &= ~(1 << UDRIE0); }
@@ -144,7 +144,7 @@ uint8_t serial_read()
     if (rx_buffer_tail == RX_BUFFER_SIZE) { rx_buffer_tail = 0; }
 
     #if ENABLE_XONXOFF
-      if ((get_rx_buffer_count() < RX_BUFFER_LOW) && flow_ctrl != XON_SENT) { 
+      if ((get_rx_buffer_count() < RX_BUFFER_LOW) && flow_ctrl == XOFF_SENT) { 
         flow_ctrl = SEND_XON;
         UCSR0B |=  (1 << UDRIE0); // Force TX
       }
@@ -183,7 +183,7 @@ ISR(USART_RX_vect)
         rx_buffer_head = next_head;    
         
         #if ENABLE_XONXOFF
-          if ((get_rx_buffer_count() >= RX_BUFFER_FULL) && flow_ctrl != XOFF_SENT) {
+          if ((get_rx_buffer_count() >= RX_BUFFER_FULL) && flow_ctrl == XON_SENT) {
             flow_ctrl = SEND_XOFF;
             UCSR0B |=  (1 << UDRIE0); // Force TX
           }