Limit pin internal pull-resistors enabled. Re-wrote read_double() function. Correctly changed all 'double's to 'float's.

- Limit pin internal pull-resistors now enabled. Normal high operation.
This will be the standard going forward.

- Updated all of the 'double' variable types to 'float' to reflect what
happens when compiled for the Arduino. Also done for compatibility
reasons to @jgeisler0303 's Grbl simulator code.

- G-code parser will now ignore 'E' exponent values, since they are
reserved g-code characters for some machines. Thanks @csdexter!

- The read_double() function was re-written and optimized for use in
Grbl. The strtod() avr lib was removed.

nuts_bolts.c

@@ -24,20 +24,92 @@
 #include <stdlib.h>
 #include <util/delay.h>
 
-int read_double(char *line, uint8_t *char_counter, double *double_ptr)                  
-{
-  char *start = line + *char_counter;
-  char *end;
-  
-  *double_ptr = strtod(start, &end);
-  if(end == start) { 
-    return(false); 
-  };
+#define MAX_INT_DIGITS 8 // Maximum number of digits in int32 (and float)
+extern float __floatunsisf (unsigned long);
 
-  *char_counter = end - line;
+// Extracts a floating point value from a string. The following code is based loosely on
+// the avr-libc strtod() function by Michael Stumpf and Dmitry Xmelkov and many freely
+// available conversion method examples, but has been highly optimized for Grbl. For known
+// CNC applications, the typical decimal value is expected to be in the range of E0 to E-4.
+// Scientific notation is officially not supported by g-code, and the 'E' character may
+// be a g-code word on some CNC systems. So, 'E' notation will not be recognized. 
+// NOTE: Thanks to Radu-Eosif Mihailescu for identifying the issues with using strtod().
+int read_float(char *line, uint8_t *char_counter, float *float_ptr)                  
+{
+  char *ptr = line + *char_counter;
+  unsigned char c;
+    
+  // Grab first character and increment pointer. No spaces assumed in line.
+  c = *ptr++;
+  
+  // Capture initial positive/minus character
+  bool isnegative = false;
+  if (c == '-') {
+    isnegative = true;
+    c = *ptr++;
+  } else if (c == '+') {
+    c = *ptr++;
+  }
+  
+  // Extract number into fast integer. Track decimal in terms of exponent value.
+  uint32_t intval = 0;
+  int8_t exp = 0;
+  uint8_t ndigit = 0;
+  bool isdecimal = false;
+  while(1) {
+    c -= '0';
+    if (c <= 9) {
+      ndigit++;
+      if (ndigit <= MAX_INT_DIGITS) {
+        if (isdecimal) { exp--; }
+        intval = (((intval << 2) + intval) << 1) + c; // intval*10 + c
+      } else {
+        if (!(isdecimal)) { exp++; }  // Drop overflow digits
+      }
+    } else if (c == (('.'-'0') & 0xff)  &&  !(isdecimal)) {
+      isdecimal = true;
+    } else {
+      break;
+    }
+    c = *ptr++;
+  }
+  
+  // Return if no digits have been read.
+  if (!ndigit) { return(false); };
+  
+  // Convert integer into floating point.
+  float fval;
+  fval = __floatunsisf(intval);
+  
+  // Apply decimal. Should perform no more than two floating point multiplications for the
+  // expected range of E0 to E-4.
+  if (fval != 0) {
+    while (exp <= -2) {
+      fval *= 0.01; 
+      exp += 2;
+    }
+    if (exp < 0) { 
+      fval *= 0.1; 
+    } else if (exp > 0) {
+      do {
+        fval *= 10.0;
+      } while (--exp > 0);
+    } 
+  }
+
+  // Assign floating point value with correct sign.    
+  if (isnegative) {
+    *float_ptr = -fval;
+  } else {
+    *float_ptr = fval;
+  }
+
+  *char_counter = ptr - line - 1; // Set char_counter to next statement
+  
   return(true);
 }
 
+
 // Delays variable defined milliseconds. Compiler compatibility fix for _delay_ms(),
 // which only accepts constants in future compiler releases.
 void delay_ms(uint16_t ms) 
@@ -45,6 +117,7 @@ void delay_ms(uint16_t ms)
   while ( ms-- ) { _delay_ms(1); }
 }
 
+
 // Delays variable defined microseconds. Compiler compatibility fix for _delay_us(),
 // which only accepts constants in future compiler releases. Written to perform more 
 // efficiently with larger delays, as the counter adds parasitic time in each iteration.