8F328U在使用 delayMicroseconds()時會很不準

eddiewwm

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：

1. /* Delay for the given number of microseconds.  Assumes a 1, 8, 12, 16, 20 or 24 MHz clock. */
   
2. void delayMicroseconds(unsigned int us)
   
3. {
   
4.         // call = 4 cycles + 2 to 4 cycles to init us(2 for constant delay, 4 for variable)
   
5. 
   
6.         // calling avrlib's delay_us() function with low values (e.g. 1 or
   
7.         // 2 microseconds) gives delays longer than desired.
   
8.         //delay_us(us);
   
9. 
   
10. #if F_CPU >= 32000000L
    
11.         // for the 32 MHz clock for the aventurous ones, trying to overclock
    
12. 
    
13.         // zero delay fix
    
14. 
    
15.         if (us < 1) return; //  = 3 cycles, (4 when true)
    
16. 
    
17.         // the following loop takes a 1/6 of a microsecond (4 cycles)
    
18.         // per iteration, so execute it six times for each microsecond of
    
19.         // delay requested.
    
20.         us = (us << 3) + 2*us + us/2 + us/8; // ? cycles
    
21. 
    
22.         // account for the time taken in the preceeding commands.
    
23.         // we just burned 22 (24) cycles above, remove 5, (5*4=20)
    
24.         // us is at least 6 so we can substract 5
    
25. //        us -= 5; //=2 cycles
    
26. 
    
27. #elif F_CPU >= 24000000L
    
28.         // for the 24 MHz clock for the aventurous ones, trying to overclock
    
29. 
    
30.         // zero delay fix
    
31.         if (!us) return; //  = 3 cycles, (4 when true)
    
32. 
    
33.         // the following loop takes a 1/6 of a microsecond (4 cycles)
    
34.         // per iteration, so execute it six times for each microsecond of
    
35.         // delay requested.
    
36.         us *= 6; // x6 us, = 7 cycles
    
37. 
    
38.         // account for the time taken in the preceeding commands.
    
39.         // we just burned 22 (24) cycles above, remove 5, (5*4=20)
    
40.         // us is at least 6 so we can substract 5
    
41.         us -= 5; //=2 cycles
    
42. 
    
43. #elif F_CPU >= 20000000L
    
44.         // for the 20 MHz clock on rare Arduino boards
    
45. 
    
46.         // for a one-microsecond delay, simply return.  the overhead
    
47.         // of the function call takes 18 (20) cycles, which is 1us
    
48.         __asm__ __volatile__ (
    
49.                 "nop" "\n\t"
    
50.                 "nop" "\n\t"
    
51.                 "nop" "\n\t"
    
52.                 "nop"); //just waiting 4 cycles
    
53.         if (us <= 1) return; //  = 3 cycles, (4 when true)
    
54. 
    
55.         // the following loop takes a 1/5 of a microsecond (4 cycles)
    
56.         // per iteration, so execute it five times for each microsecond of
    
57.         // delay requested.
    
58.         us = (us << 2) + us; // x5 us, = 7 cycles
    
59. 
    
60.         // account for the time taken in the preceeding commands.
    
61.         // we just burned 26 (28) cycles above, remove 7, (7*4=28)
    
62.         // us is at least 10 so we can substract 7
    
63.         us -= 7; // 2 cycles
    
64. 
    
65. #elif F_CPU >= 16000000L
    
66.         // for the 16 MHz clock on most Arduino boards
    
67. 
    
68.         // for a one-microsecond delay, simply return.  the overhead
    
69.         // of the function call takes 14 (16) cycles, which is 1us
    
70. 
    
71.         if (us < 1) return; //  = 3 cycles, (4 when true)
    
72. 
    
73.         // the following loop takes 1/4 of a microsecond (4 cycles)
    
74.         // per iteration, so execute it four times for each microsecond of
    
75.         // delay requested.
    
76. 
    
77.         us = (us << 2) + us + us/4 + us/17  ; // ? cycles
    
78. 
    
79.         // account for the time taken in the preceeding commands.
    
80.         // we just burned 19 (21) cycles above, remove 5, (5*4=20)
    
81.         // us is at least 8 so we can substract 5
    
82. //        us -= 1; // = 2 cycles,
    
83. 
    
84. #elif F_CPU >= 12000000L
    
85.         // for the 12 MHz clock if somebody is working with USB
    
86. 
    
87.         // for a 1 microsecond delay, simply return.  the overhead
    
88.         // of the function call takes 14 (16) cycles, which is 1.5us
    
89.         if (us <= 1) return; //  = 3 cycles, (4 when true)
    
90. 
    
91.         // the following loop takes 1/3 of a microsecond (4 cycles)
    
92.         // per iteration, so execute it three times for each microsecond of
    
93.         // delay requested.
    
94.         us = (us << 1) + us; // x3 us, = 5 cycles
    
95. 
    
96.         // account for the time taken in the preceeding commands.
    
97.         // we just burned 20 (22) cycles above, remove 5, (5*4=20)
    
98.         // us is at least 6 so we can substract 5
    
99.         us -= 5; //2 cycles
    
100. 
     
101. #elif F_CPU >= 8000000L
     
102.         // for the 8 MHz internal clock
     
103. 
     
104.         // for a 1 and 2 microsecond delay, simply return.  the overhead
     
105.         // of the function call takes 14 (16) cycles, which is 2us
     
106.         if (us <= 2) return; //  = 3 cycles, (4 when true)
     
107. 
     
108.         // the following loop takes 1/2 of a microsecond (4 cycles)
     
109.         // per iteration, so execute it twice for each microsecond of
     
110.         // delay requested.
     
111.         us <<= 1; //x2 us, = 2 cycles
     
112. 
     
113.         // account for the time taken in the preceeding commands.
     
114.         // we just burned 17 (19) cycles above, remove 4, (4*4=16)
     
115.         // us is at least 6 so we can substract 4
     
116.         //us -= 4; // = 2 cycles
     
117. 
     
118. #else
     
119.         // for the 1 MHz internal clock (default settings for common Atmega microcontrollers)
     
120. 
     
121.         // the overhead of the function calls is 14 (16) cycles
     
122.         if (us <= 16) return; //= 3 cycles, (4 when true)
     
123.         if (us <= 25) return; //= 3 cycles, (4 when true), (must be at least 25 if we want to substract 22)
     
124. 
     
125.         // compensate for the time taken by the preceeding and next commands (about 22 cycles)
     
126.         us -= 22; // = 2 cycles
     
127.         // the following loop takes 4 microseconds (4 cycles)
     
128.         // per iteration, so execute it us/4 times
     
129.         // us is at least 4, divided by 4 gives us 1 (no zero delay bug)
     
130.         us >>= 2; // us div 4, = 4 cycles
     
131.         
     
132. 
     
133. #endif
     
134. 
     
135.         // busy wait
     
136.         __asm__ __volatile__ (
     
137.                 "1: sbiw %0,1" "\n\t" // 2 cycles
     
138.                 "brne 1b" : "=w" (us) : "0" (us) // 2 cycles
     
139.         );
     
140.         // return = 4 cycles
     
141. }
     
142. 
     
143. 
     

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。