diff options
| -rw-r--r-- | cores/arduino/wiring.c | 78 |
1 files changed, 68 insertions, 10 deletions
diff --git a/cores/arduino/wiring.c b/cores/arduino/wiring.c index 5cbe241..6efda6e 100644 --- a/cores/arduino/wiring.c +++ b/cores/arduino/wiring.c @@ -119,17 +119,41 @@ void delay(unsigned long ms) } } -/* Delay for the given number of microseconds. Assumes a 8 or 16 MHz clock. */ +/* Delay for the given number of microseconds. Assumes a 1, 8, 12, 16, 20 or 24 MHz clock. */ 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 >= 20000000L +#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; + + // 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 + + // account for the time taken in the preceeding commands. + us -= 2; + +#elif F_CPU >= 20000000L // for the 20 MHz clock on rare Arduino boards - // for a one-microsecond delay, simply wait 2 cycle and return. The overhead - // of the function call yields a delay of exactly a one microsecond. + // for a one-microsecond delay, simply wait 2 cycles and return. The overhead + // of the function call yields a delay of exactly one microsecond. __asm__ __volatile__ ( "nop" "\n\t" "nop"); //just waiting 2 cycle @@ -152,15 +176,31 @@ void delayMicroseconds(unsigned int us) if (--us == 0) return; - // the following loop takes a quarter of a microsecond (4 cycles) + // 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; + us <<= 2; // x4 us // account for the time taken in the preceeding commands. us -= 2; -#else - // for the 8 MHz internal clock on the ATmega168 + +#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; + + // 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 + + // account for the time taken in the preceeding commands. + us -= 2; +#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 @@ -170,14 +210,31 @@ void delayMicroseconds(unsigned int us) if (--us == 0) return; - // the following loop takes half of a microsecond (4 cycles) + // 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; + 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--; + +#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 following loop takes 4 microseconds (4 cycles) + // per iteration, so execute it us/4 times + us >>= 2; // us div 4 #endif // busy wait @@ -185,6 +242,7 @@ void delayMicroseconds(unsigned int us) "1: sbiw %0,1" "\n\t" // 2 cycles "brne 1b" : "=w" (us) : "0" (us) // 2 cycles ); + // return = 4 cycles } void init() |