From bbec45d3df8bb593a2466ded232f5691e470a04f Mon Sep 17 00:00:00 2001
From: "David A. Mellis" <d.mellis@arduino.cc>
Date: Sat, 3 Dec 2005 17:03:26 +0000
Subject: [PATCH] Improved accuracy of delayMicroseconds().

---
 readme.txt               |  1 +
 targets/arduino/wiring.c | 37 ++++++++++++++++++++++++++++++++++---
 targets/arduino/wiring.h |  4 ++--
 3 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/readme.txt b/readme.txt
index aa5232545..8ed76c2b1 100644
--- a/readme.txt
+++ b/readme.txt
@@ -25,6 +25,7 @@ order to read the same physical pin.
 Now flushing the serial port before uploading (should fix some errors).
 Added a printNewline() function.
 Updated icon of the Windows executable.
+Improved accuracy of the delayMicroseconds() function.
 
 0002 - 2005.10.05
 New build process no longer uses makefiles; now controlled by preferences.txt.
diff --git a/targets/arduino/wiring.c b/targets/arduino/wiring.c
index 8c657f456..4f2356342 100755
--- a/targets/arduino/wiring.c
+++ b/targets/arduino/wiring.c
@@ -19,7 +19,7 @@
   Free Software Foundation, Inc., 59 Temple Place, Suite 330,
   Boston, MA  02111-1307  USA
 
-  $Id: wiring.c,v 1.7 2005/05/28 21:04:15 mellis Exp $
+  $Id$
 */
 
 #include <avr/io.h>
@@ -292,9 +292,40 @@ void delay(unsigned long ms)
 	timerPause(ms);
 }
 
-void delayMicroseconds(unsigned long us)
+/* Delay for the given number of microseconds.  Assumes a 16 MHz clock. 
+ * Disables interrupts, which will disrupt the millis() function if used
+ * too frequently. */
+void delayMicroseconds(unsigned int us)
 {
-	delay_us(us);
+	// calling avrlib's delay_us() function with low values (e.g. 1 or
+	// 2 microseconds) gives delays longer than desired.
+	//delay_us(us);
+
+	// 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)
+		return;
+
+	// the following loop takes a quarter of a microsecond (4 cycles)
+	// per iteration, so execute it four times for each microsecond of
+	// delay requested.
+	us <<= 2;
+
+	// account for the time taken in the preceeding commands.
+	us -= 2;
+
+	// disable interrupts, otherwise the timer 0 overflow interrupt that
+	// tracks milliseconds will make us delay longer than we want.
+	cli();
+
+	// busy wait
+	__asm__ __volatile__ (
+		"1: sbiw %0,1" "\n\t" // 2 cycles
+		"brne 1b" : "=w" (us) : "0" (us) // 2 cycles
+	);
+
+	// reenable interrupts.
+	sei();
 }
 
 int main(void)
diff --git a/targets/arduino/wiring.h b/targets/arduino/wiring.h
index 1b57ffe3c..87ccdd2ce 100755
--- a/targets/arduino/wiring.h
+++ b/targets/arduino/wiring.h
@@ -19,7 +19,7 @@
   Free Software Foundation, Inc., 59 Temple Place, Suite 330,
   Boston, MA  02111-1307  USA
 
-  $Id: wiring.h,v 1.4 2005/05/24 17:47:41 mellis Exp $
+  $Id$
 */
 
 #ifndef Wiring_h
@@ -57,7 +57,7 @@ typedef uint8_t byte;
 
 void delay(unsigned long);
 void delay_ms(unsigned short ms);
-void delayMicroseconds(unsigned long us);
+void delayMicroseconds(unsigned int us);
 void pinMode(int, int);
 void digitalWrite(int, int);
 int digitalRead(int);