sam: Optimized delayMicroseconds() (Rob Tillaart)

See #1121

Author: cmaglie

build/shared/revisions.txt

@@ -11,6 +11,7 @@ ARDUINO 1.5.6 BETA
 [core]
 * sam: Fixed wrap-around bug in delay() (Mark Tillotson)
 * sam: Fixed regression in analogRead() (fails to read multiple channels) (Mark Tillotson)
+* sam: Optimized delayMicroseconds() (Rob Tillaart) #1121
 * Optimized Print::print(String&) method, now uses internal string buffer to perform block write
 
 ARDUINO 1.5.5 BETA 2013.11.28

hardware/arduino/sam/cores/arduino/wiring.h

@@ -64,11 +64,12 @@ extern void delay( uint32_t dwMs ) ;
  */
 static inline void delayMicroseconds(uint32_t) __attribute__((always_inline, unused));
 static inline void delayMicroseconds(uint32_t usec){
+    if (usec == 0) return;
     uint32_t n = usec * (VARIANT_MCK / 3000000);
     asm volatile(
         "L_%=_delayMicroseconds:"       "\n\t"
         "subs   %0, #1"                 "\n\t"
-        "bge    L_%=_delayMicroseconds" "\n"
+        "bne    L_%=_delayMicroseconds" "\n"
         : "+r" (n) :
     );
 }