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# 8F328U在使用 delayMicroseconds()時會很不準 发表于 2019-4-18 14:58:27 | 显示全部楼层 |阅读模式
 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 } 复制代码 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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