delay.c: delay_us() and delay_ms() are now calibrated.

See comments in the code for the result.

This also fixes a bug where corner cases like delay(0) or
delay(13107) whould result in an extra long pause.
Note: 65535 / (F_CPU / 4000000) = 13107 on 20 MHz.

This costs 28 bytes binary size.

delay.c

@@ -12,21 +12,53 @@
 #error Delay functions only work with F_CPU >= 4000000UL 
 #endif
 
-/// delay microseconds
-/// \param delay time to wait in microseconds
+/** Delay in microseconds
+  \param delay time to wait in microseconds
+
+  Calibrated in SimulAVR.
+
+  Accuracy on 20 MHz CPU clock: -1/+3 clock cycles over the whole range(!).
+  Accuracy on 16 MHz CPU clock: delay is about 0.8% too short.
+
+  Exceptions are delays of 0..2 on 20 MHz, which are all 0.75 us and delays
+  of 0..3 on 16 MHz, which are all 0.93us.
+*/
 void delay_us(uint16_t delay) {
 	wd_reset();
+
+  // Compensate call overhead, as close as possible.
+  #define OVERHEAD_CALL_CLOCKS 39 // clock cycles
+  #define OVERHEAD_CALL_DIV ((OVERHEAD_CALL_CLOCKS / (F_CPU / 1000000)) + 1)
+  #define OVERHEAD_CALL_REM ((OVERHEAD_CALL_DIV * (F_CPU / 1000000)) - \
+                             OVERHEAD_CALL_CLOCKS)
+
+  if (delay > OVERHEAD_CALL_DIV) {
+    delay -= OVERHEAD_CALL_DIV;
+    if (OVERHEAD_CALL_REM >= 2)
+      _delay_loop_2((OVERHEAD_CALL_REM + 2) / 4);
+  }
+  else {
+    return;
+  }
+
 	while (delay > (65536L / (F_CPU / 4000000L))) {
-		_delay_loop_2(65534); // we use 65534 here to compensate for the time that the surrounding loop takes. TODO: exact figure needs tuning
+    #define OVERHEAD_LOOP_CLOCKS 13
+
+    _delay_loop_2(65536 - (OVERHEAD_LOOP_CLOCKS + 2) / 4);
 		delay -= (65536L / (F_CPU / 4000000L));
 		wd_reset();
 	}
-	_delay_loop_2(delay * (F_CPU / 4000000L));
+  if (delay)
+    _delay_loop_2(delay * (F_CPU / 4000000L));
 	wd_reset();
 }
 
-/// delay milliseconds
-/// \param delay time to wait in milliseconds
+/** Delay in microseconds
+  \param delay time to wait in milliseconds
+
+  Accuracy on 20 MHz: delay < 0.04% too long over the whole range.
+  Accuracy on 16 MHz: delay < 0.8% too short over the whole range.
+*/
 void delay_ms(uint32_t delay) {
 	wd_reset();
 	while (delay > 65) {
@@ -34,6 +66,6 @@ void delay_ms(uint32_t delay) {
 		delay -= 65;
 		wd_reset();
 	}
-	delay_us(delay * 1000);
+  delay_us(delay * 1000 - 2);
 	wd_reset();
 }