Updating delayMicroseconds() to work on an 8 MHz clock as well as a 16 MHz one.

targets/arduino/wiring.c

@@ -65,6 +65,9 @@ void delayMicroseconds(unsigned int us)
 	// 2 microseconds) gives delays longer than desired.
 	//delay_us(us);
 
+#if F_CPU >= 16000000L
+    // for the 16 MHz clock on most Arduino boards
+
 	// for a one-microsecond delay, simply return.  the overhead
 	// of the function call yields a delay of approximately 1 1/8 us.
 	if (--us == 0)
@@ -77,6 +80,26 @@ void delayMicroseconds(unsigned int us)
 
 	// account for the time taken in the preceeding commands.
 	us -= 2;
+#else
+    // for the 8 MHz internal clock on the ATmega168
+
+    // for a one- or two-microsecond delay, simply return.  the overhead of
+    // the function calls takes more than two microseconds.  can't just
+    // subtract two, since us is unsigned; we'd overflow.
+	if (--us == 0)
+		return;
+	if (--us == 0)
+		return;
+
+	// the following loop takes half of a microsecond (4 cycles)
+	// per iteration, so execute it twice for each microsecond of
+	// delay requested.
+	us <<= 1;
+    
+    // partially compensate for the time taken by the preceeding commands.
+    // we can't subtract any more than this or we'd overflow w/ small delays.
+    us--;
+#endif
 
 	// disable interrupts, otherwise the timer 0 overflow interrupt that
 	// tracks milliseconds will make us delay longer than we want.