Fixed delayMicrosecond for non-constant case.

Author: cmaglie

cores/arduino/delay.h

@@ -61,40 +61,40 @@ extern void delay( uint32_t dwMs ) ;
  * \param dwUs the number of microseconds to pause (uint32_t)
  */
 static __inline__ void delayMicroseconds( uint32_t ) __attribute__((always_inline, unused)) ;
-static __inline__ void delayMicroseconds( uint32_t ul_usec )
+static __inline__ void delayMicroseconds( uint32_t usec )
 {
-  if ( ul_usec == 0 )
+  if ( usec == 0 )
   {
     return ;
   }
 
-  uint32_t ul = ul_usec * (VARIANT_MCK / 3000000); // = ul_usec * 16 with VARIANT_MCK @ 48MHz
-
-/* following 'for' loop will generate this:
- * loop:
- * subs r3, #1 // 1 Core cycle
- * bne.n loop // 1 or 2 Core cycles, depends on pipeline break (50/50?)
-
-  for ( ; ul != 0 ; ul-- )
-  {
-    __asm__ volatile("");
-  }
-*/
-
-/*
- * __asm__ [__volatile__] ( AssemblerTemplate : [OutputOperands] [ : [InputOperands] [ : [Clobbers] ] ] )
- *
- * __asm__ documentation for GCC: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html
- * __volatile__ documentation for GCC: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile
- *
- */
-  __asm__ __volatile__( "1:      \n"
-                        "sub %0, #1              \n" /* substract 1 from %0 (ul) */
-                        "bne 1b   \n"
-                        : /* no outputs */
-                        : "r" (ul) /* 'ul' variable is '%0', '+' means R/W constraint on this variable/register, 'r' means 'register' (versus 'm' for 'memory' */
-                        : /* no clobber, ie nothing should be damaged by this asm code */
-                      ) ;
+  /*
+   *  The following loop:
+   *
+   *    for (; ul; ul--) {
+   *      __asm__ volatile("");
+   *    }
+   *
+   *  produce the following assembly code:
+   *
+   *    loop:
+   *      subs r3, #1        // 1 Core cycle
+   *      bne.n loop         // 1 Core cycle + 1 if branch is taken
+   */
+
+  // VARIANT_MCK / 1000000 == cycles needed to delay 1uS
+  //                     3 == cycles used in a loop
+  uint32_t n = usec * (VARIANT_MCK / 1000000) / 3;
+  __asm__ __volatile__(
+    "1:              \n"
+    "   sub %0, #1   \n" // substract 1 from %0 (n)
+    "   bne 1b       \n" // if result is not 0 jump to 1
+    : "+r" (n)           // '%0' is n variable with RW constraints
+    :                    // no input
+    :                    // no clobber
+  );
+  // https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html
+  // https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile
 }
 
 #ifdef __cplusplus