8000000Hz / 510 =
15686Hz。每16次輸出構成一個周期正弦波,正弦波的頻率為980.4Hz。PWM由OC0(PB4)引腳輸出。參考程序如下(ICCAVR)。
//ICC-AVR application builder : 2004-08
// Target : M128
// Crystal: 8.0000Mhz
#include
#include
#pragma data:code
// 128點正弦波樣本表
const unsigned char auc_SinParam[128] = {
64,67,70,73,76,79,82,85,88,91,94,96,99,102,104,106,109,111,113,115,117,118,120,121,
123,124,125,126,126,127,127,127,127,127,127,127,126,126,125,124,123,121,120,118,
117,115,113,111,109,106,104,102,99,96,94,91,88,85,82,79,76,73,70,67,64,60,57,54,51,48,
45,42,39,36,33,31,28,25,23,21,18,16,14,12,10,9,7,6,4,3,2,1,1,0,0,0,0,0,0,0,1,1,2,3,4,6,
7,9,10,12,14,16,18,21,23,25,28,31,33,36,39,42,45,48,51,54,57,60};
#pragma data:data
unsigned char x_SW = 8,X_LUT = 0;
#pragma interrupt_handler timer0_ovf_isr:17
void timer0_ovf_isr(void)
{
X_LUT += x_SW; // 新樣點指針
if (X_LUT > 127) X_LUT -= 128; // 樣點指針調整
OCR0 = auc_SinParam[X_LUT]; // 取樣點指針到比較匹配寄存器
}
void main(void)
{
DDRB |= 0x10; // PB4(OC0)輸出
TCCR0 = 0x71; // 相位調整PWM模式,分頻系數=1,正向控制OC0
TIMSK = 0x01; // T/C0溢出中斷允許
SEI(); // 使能全局中斷
while(1)
{……};
}
每次計數器溢出中斷的服務中取出一個正弦波的樣點值到比較匹配寄存器中,用于調整下一個PWM的脈沖寬度,這樣在PB4引腳上輸出了按正弦波調制的PWM方波。當PB4的輸出通過一個低通濾波器后,便得到一個980.4Hz的正弦波了。如要得到更精確的1KHz的正弦波,可使用定時/計數器T/C1,選擇工作模式10,設置ICR1=250為計數器的上限值。
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