示例:在 8 位寄存器中使用相位累加器进行 5 位抖动,其中占空比 = 1 到 31 [% = 占空比 / (1 << 位)]。
// Easier to do in assembly, where there is access to the carry flag
unsigned bits = 5; // states = 1 << bits
unsigned carry = 7; // keep carry bit within an 8 bit register, limits bits
unsigned frq = ((1 << carry) * duty) / (1 << bits); // More than 8 bit intermediate value
unsigned phs = 0;
for (i = 0; i < (1 << bits); i++) {
phs += frq; // Carry is high bit
output((phs >> carry) & 1); // Output carry
phs &= (1 << carry) - 1; // Discard carry
}
抖动模式如下所示:
00: 00000000000000000000000000000000
01: 00000000000000000000000000000001
02: 00000000000000010000000000000001
03: 00000000001000000000010000000001
04: 00000001000000010000000100000001
05: 00000010000010000001000001000001
06: 00000100001000010000010000100001
07: 00001000010001000010001000010001
08: 00010001000100010001000100010001
09: 00010001001000100100010010001001
10: 00010010010010010001001001001001
11: 00100100100100100100100100100101
12: 00100101001001010010010100100101
13: 00101001010010100101001010010101
14: 00101010010101010010101001010101
15: 00101010101010100101010101010101
16: 01010101010101010101010101010101
17: 01010101010101011010101010101011
18: 01010101101010110101010110101011
19: 01010110101101011010110101101011
20: 01011011010110110101101101011011
21: 01011011011011011011011011011011
22: 01101101101101110110110110110111
23: 01101110110111011011101101110111
24: 01110111011101110111011101110111
25: 01110111101110111101110111101111
26: 01111011110111110111101111011111
27: 01111101111101111110111110111111
28: 01111111011111110111111101111111
29: 01111111110111111111101111111111
30: 01111111111111110111111111111111
31: 01111111111111111111111111111111
如果您没有足够宽的整数(或者在没有乘法或除法的情况下在汇编程序中),则可能需要一次一位地在循环中计算 frq 。
可选地,可以预先计算抖动模式并将其编码为查找表中的常数。
只有 2 的幂的模式没有噪音;除非您进行音频或射频合成,否则这无关紧要。否则其他模式有小鸟。在输出一次模式后打乱模式位的顺序会增加噪音,但会消除小鸟。可以使用具有长重复周期且不添加或删除位(1 和 0 的数量保持不变,只是它们的顺序发生变化)的 LFSR 函数来执行此操作。
请注意,要以 60 Hz 帧速率输出完整模式,需要 60 Hz * (1 << bits) = 1.92 kHz 的抖动频率。可能可以在没有闪烁的情况下为 LED 提供低得多的抖动频率,例如 (1 << bits) = 32 Hz。实验!