8F328U在使用 delayMicroseconds()時會很不準
本帖最后由 eddiewwm 于 2020-3-2 22:26 编辑實測 8F328U在使用 delayMicroseconds()時會很不準，會快很多，delay 1000us時，得出的約是 765us。
可參考網上的文章 https://www.arduino.cn/thread-12468-1-1.html 作出改進。
LGT 原來在 wiring.c 中的設定更改為：
/* 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 >= 32000000L
// for the 32 MHz clock for the aventurous ones, trying to overclock
// zero delay fix
if (us < 1) 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 = (us << 3) + 2*us + us/2 + us/8; // ? cycles
// account for the time taken in the preceeding commands.
// 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 >= 24000000L
// for the 24 MHz clock for the aventurous ones, trying to overclock
// 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, = 7 cycles
// account for the time taken in the preceeding commands.
// 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 return.the overhead
// of the function call takes 18 (20) cycles, which is 1us
__asm__ __volatile__ (
"nop" "\n\t"
"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, = 7 cycles
// account for the time taken in the preceeding commands.
// 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 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 = (us << 2) + us + us/4 + us/17; // ? cycles
// account for the time taken in the preceeding commands.
// we just burned 19 (21) cycles above, remove 5, (5*4=20)
// us is at least 8 so we can substract 5
// us -= 1; // = 2 cycles,
#elif F_CPU >= 12000000L
// for the 12 MHz clock if somebody is working with USB
// 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, = 5 cycles
// account for the time taken in the preceeding commands.
// 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 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, = 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 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 is at least 4, divided by 4 gives us 1 (no zero delay bug)
us >>= 2; // us div 4, = 4 cycles
#endif
// busy wait
__asm__ __volatile__ (
"1: sbiw %0,1" "\n\t" // 2 cycles
"brne 1b" : "=w" (us) : "0" (us) // 2 cycles
);
// return = 4 cycles
}
針對 8F328U 板用的是 16MHz主頻 作出了以下的更改
#elif F_CPU >= 16000000L
.......
us = (us << 2) ; // x5 us, = 7 cycles
delay 1000us 時得到的是 ：
us = (us << 2); // x==> 765us
us = (us << 2) + us; // x==> 954us
us *= 6; // x==> 1140us
在不作任何改動下，就祇能乘以系數 1.31 作補償了。
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