Fixing delayMicroseconds() timing for 20 MHz clocks. (Erdem U. Altinyurt)

http://code.google.com/p/arduino/issues/detail?id=306

cores/arduino/wiring.c

@@ -124,8 +124,26 @@ void delayMicroseconds(unsigned int us)
 	// calling avrlib's delay_us() function with low values (e.g. 1 or
 	// 2 microseconds) gives delays longer than desired.
 	//delay_us(us);
+#if F_CPU >= 20000000L
+	// for the 20 MHz clock on rare Arduino boards
 
-#if F_CPU >= 16000000L
+	// for a one-microsecond delay, simply wait 2 cycle and return. The overhead
+	// of the function call yields a delay of exactly a one microsecond.
+	__asm__ __volatile__ (
+		"nop" "\n\t"
+		"nop"); //just waiting 2 cycle
+	if (--us == 0)
+		return;
+
+	// the following loop takes a 1/5 of a microsecond (4 cycles)
+	// per iteration, so execute it five times for each microsecond of
+	// delay requested.
+	us = (us<<2) + us; // x5 us
+
+	// account for the time taken in the preceeding commands.
+	us -= 2;
+
+#elif F_CPU >= 16000000L
 	// for the 16 MHz clock on most Arduino boards
 
 	// for a one-microsecond delay, simply return.  the overhead