zero delay fix, ADC prescaler fix

Author: Cano

hardware/arduino/avr/cores/arduino/wiring.c

@@ -123,118 +123,114 @@ void delay(unsigned long ms)
 void delayMicroseconds(unsigned int us)
 {
   // call = 4 cycles + 2 to 4 cycles to init us(2 for constant delay, 4 for variable)
+  
 	// 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 >= 24000000L
 	// for the 24 MHz clock for the aventurous ones, trying to overclock
 
-	// for a one-microsecond delay, simply wait 6 cycles and return. The overhead
-	// of the function call yields a delay of exactly one microsecond.
-	__asm__ __volatile__ (
-		"nop" "\n\t"
-		"nop" "\n\t"
-		"nop" "\n\t"
-		"nop" "\n\t"
-		"nop" "\n\t"
-		"nop"); //just waiting 6 cycles
-	if (--us == 0)
-		return;
+	// zero delay fix
+  if (!us) return; //  = 3 cycles, (4 when true)
 
 	// the following loop takes a 1/6 of a microsecond (4 cycles)
 	// per iteration, so execute it six times for each microsecond of
 	// delay requested.
-	us *= 6; // x6 us
+	us *= 6; // x6 us, = 7 cycles
 
 	// account for the time taken in the preceeding commands.
-	us -= 2;
+	// we just burned 22 (24) cycles above, remove 5, (5*4=20)
+  // us is at least 6 so we can substract 5
+	us -= 5; //=2 cycles
 
 #elif F_CPU >= 20000000L
 	// for the 20 MHz clock on rare Arduino boards
 
-	// for a one-microsecond delay, simply wait 2 cycles and return. The overhead
-	// of the function call yields a delay of exactly one microsecond.
+	// for a one-microsecond delay, simply return.  the overhead
+	// of the function call takes 18 (20) cycles, which is 1us
 	__asm__ __volatile__ (
 		"nop" "\n\t"
-		"nop"); //just waiting 2 cycle
-	if (--us == 0)
-		return;
+		"nop" "\n\t"
+		"nop" "\n\t"
+		"nop"); //just waiting 4 cycles
+  if (us <= 1) return; //  = 3 cycles, (4 when true)
 
 	// 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
+	us = (us << 2) + us; // x5 us, = 7 cycles
 
 	// account for the time taken in the preceeding commands.
-	us -= 2;
+	// we just burned 26 (28) cycles above, remove 7, (7*4=28)
+  // us is at least 10 so we can substract 7
+	us -= 7; // 2 cycles
 
 #elif 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)
-		return;
+	// of the function call takes 14 (16) cycles, which is 1us
+	if (us <= 1) return; //  = 3 cycles, (4 when true)
 
 	// the following loop takes 1/4 of a microsecond (4 cycles)
 	// per iteration, so execute it four times for each microsecond of
 	// delay requested.
-	us <<= 2; // x4 us
+	us <<= 2; // x4 us, = 4 cycles
 
 	// account for the time taken in the preceeding commands.
-	us -= 2;
+	// we just burned 19 (21) cycles above, remove 5, (5*4=20)
+  // us is at least 8 so we can substract 5
+	us -= 5; // = 2 cycles, 
 
 #elif F_CPU >= 12000000L
 	// for the 12 MHz clock if somebody is working with USB
 
-	// for a one-microsecond delay, simply return.  the overhead
-	// of the function call yields a delay of approximately 1.5 us.
-	if (--us == 0)
-		return;
+	// for a 1 microsecond delay, simply return.  the overhead
+	// of the function call takes 14 (16) cycles, which is 1.5us
+	if (us <= 1) return; //  = 3 cycles, (4 when true)
 
 	// the following loop takes 1/3 of a microsecond (4 cycles)
 	// per iteration, so execute it three times for each microsecond of
 	// delay requested.
-	us = (us << 1) + us; // x3 us
+	us = (us << 1) + us; // x3 us, = 5 cycles
 
 	// account for the time taken in the preceeding commands.
-	us -= 2;
+	// we just burned 20 (22) cycles above, remove 5, (5*4=20)
+  // us is at least 6 so we can substract 5
+	us -= 5; //2 cycles
+
 #elif F_CPU >= 8000000L
 	// for the 8 MHz internal clock
 
-	// 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;
+	// for a 1 and 2 microsecond delay, simply return.  the overhead
+	// of the function call takes 14 (16) cycles, which is 2us
+	if (us <= 2) return; //  = 3 cycles, (4 when true)
 
 	// the following loop takes 1/2 of a microsecond (4 cycles)
 	// per iteration, so execute it twice for each microsecond of
 	// delay requested.
-	us <<= 1; //x2 us
-    
-	// 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--;
+	us <<= 1; //x2 us, = 2 cycles
+
+	// account for the time taken in the preceeding commands.
+	// we just burned 17 (19) cycles above, remove 4, (4*4=16)
+  // us is at least 6 so we can substract 4
+	us -= 4; // = 2 cycles
 
 #else
 	// for the 1 MHz internal clock (default settings for common Atmega microcontrollers)
 
-	// the overhead of the function calls takes about 16 microseconds.
-	if (us <= 16) //4 cycles spent here
-		return;
-	if (us <= 22) { //4 cycles spent here
-    return;
-	}
-	
-	// compensate for the time taken by the preceeding and next commands.
-	us -= 22;
+	// the overhead of the function calls is 14 (16) cycles
+	if (us <= 16) return; //= 3 cycles, (4 when true)
+	if (us <= 25) return; //= 3 cycles, (4 when true), (must be at least 25 if we want to substract 22)
 
+	// compensate for the time taken by the preceeding and next commands (about 22 cycles)
+	us -= 22; // = 2 cycles
 	// the following loop takes 4 microseconds (4 cycles)
 	// per iteration, so execute it us/4 times
-	us >>= 2; // us div 4
+  // us is at least 4, divided by 4 gives us 1 (no zero delay bug)
+	us >>= 2; // us div 4, = 4 cycles
+	
+
 #endif
 
 	// busy wait
@@ -360,14 +356,32 @@ void init()
 #endif
 
 #if defined(ADCSRA)
-	// set a2d prescale factor to 128
-	// 16 MHz / 128 = 125 KHz, inside the desired 50-200 KHz range.
-	// XXX: this will not work properly for other clock speeds, and
-	// this code should use F_CPU to determine the prescale factor.
-	sbi(ADCSRA, ADPS2);
-	sbi(ADCSRA, ADPS1);
-	sbi(ADCSRA, ADPS0);
-
+	// set a2d prescaler so we are inside the desired 50-200 KHz range.
+  #if F_CPU >= 16000000 // 16 MHz / 128 = 125 KHz
+    sbi(ADCSRA, ADPS2);
+    sbi(ADCSRA, ADPS1);
+    sbi(ADCSRA, ADPS0);
+  #elif F_CPU >= 8000000 // 8 MHz / 64 = 125 KHz
+    sbi(ADCSRA, ADPS2);
+    sbi(ADCSRA, ADPS1);
+    cbi(ADCSRA, ADPS0);
+  #elif F_CPU >= 4000000 // 4 MHz / 32 = 125 KHz
+    sbi(ADCSRA, ADPS2);
+    cbi(ADCSRA, ADPS1);
+    sbi(ADCSRA, ADPS0);
+  #elif F_CPU >= 2000000 // 2 MHz / 16 = 125 KHz
+    sbi(ADCSRA, ADPS2);
+    cbi(ADCSRA, ADPS1);
+    cbi(ADCSRA, ADPS0);
+  #elif F_CPU >= 1000000 // 1 MHz / 8 = 125 KHz
+    cbi(ADCSRA, ADPS2);
+    sbi(ADCSRA, ADPS1);
+    sbi(ADCSRA, ADPS0);
+  #else // 128 kHz / 2 = 64 KHz -> This is the closest you can get, the prescaler is 2
+    cbi(ADCSRA, ADPS2);
+    cbi(ADCSRA, ADPS1);
+    sbi(ADCSRA, ADPS0);
+  #endif
 	// enable a2d conversions
 	sbi(ADCSRA, ADEN);
 #endif