Port delayMicroseconds from SAMD core

cores/nRF5/delay.h

@@ -25,6 +25,8 @@ extern "C" {
 
 #include <stdint.h>
 
+#include "variant.h"
+
 /**
  * \brief Returns the number of milliseconds since the Arduino board began running the current program.
  *
@@ -59,7 +61,42 @@ extern void delay( uint32_t dwMs ) ;
  *
  * \param dwUs the number of microseconds to pause (uint32_t)
  */
-extern void delayMicroseconds( uint32_t usec ) ;
+static __inline__ void delayMicroseconds( uint32_t ) __attribute__((always_inline, unused)) ;
+static __inline__ void delayMicroseconds( uint32_t usec )
+{
+  if ( usec == 0 )
+  {
+    return ;
+  }
+
+  /*
+   *  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
 }