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