STM32在嵌入式过程中得到了广泛的应用，在很多的驱动设备的场景中，需要正弦波的输出，因此使用STM32生成正弦波具有比较重要的实际意义。
FreeRTOS是一种广泛使用的嵌入式实时操作系统，因此使用FreeRTOS实现正弦波的输出。
STM32的DAC可以自动生成三角波，但是无法自动生成正弦波，因此，本文采用查表法的方式生成正弦波。查表法即将一个正弦波周期进行采样，然后将采样得到的点存在一个数组里，然后将数组里值以此使用DAC进行输出即可。STM32的DAC是12bit的，因此DAC的赋值范围为[0, 4095]，正弦波的采样值的数组如下：
const u16 Sine12bit2[500] = {
2048,2073,2099,2125,2150,2176,2202,2227,2253,2279,2304,2330,2355,2380,
2406,2431,2456,2482,2507,2532,2557,2582,2606,2631,2656,2680,2705,2729,2753,
2777,2801,2825,2849,2872,2896,2919,2942,2966,2988,3011,3034,3056,3079,3101,
3123,3145,3166,3188,3209,3230,3251,3272,3292,3313,3333,3353,3372,3392,3411,
3430,3449,3468,3486,3504,3522,3540,3558,3575,3592,3609,3625,3641,3657,3673,
3689,3704,3719,3734,3748,3762,3776,3790,3803,3816,3829,3842,3854,3866,3878,
3889,3900,3911,3921,3932,3942,3951,3961,3970,3978,3987,3995,4003,4010,4017,
4024,4031,4037,4043,4048,4054,4059,4063,4068,4072,4075,4079,4082,4085,4087,
4089,4091,4092,4094,4094,4095,4095,4095,4094,4094,4092,4091,4089,4087,4085,
4082,4079,4075,4072,4068,4063,4059,4054,4048,4043,4037,4031,4024,4017,4010,
4003,3995,3987,3978,3970,3961,3951,3942,3932,3921,3911,3900,3889,3878,3866,
3854,3842,3829,3816,3803,3790,3776,3762,3748,3734,3719,3704,3689,3673,3657,
3641,3625,3609,3592,3575,3558,3540,3522,3504,3486,3468,3449,3430,3411,3392,
3372,3353,3333,3313,3292,3272,3251,3230,3209,3188,3166,3145,3123,3101,3079,
3056,3034,3011,2988,2966,2943,2919,2896,2872,2849,2825,2801,2777,2753,2729,
2705,2680,2656,2631,2606,2582,2557,2532,2507,2482,2456,2431,2406,2381,2355,
2330,2304,2279,2253,2227,2202,2176,2150,2125,2099,2073,2048,2022,1996,1970,
1945,1919,1893,1868,1842,1816,1791,1765,1740,1715,1689,1664,1639,1613,1588,
1563,1538,1513,1489,1464,1439,1415,1390,1366,1342,1318,1294,1270,1246,1223,
1199,1176,1153,1129,1107,1084,1061,1039,1016,994,972,950,929,907,886,865,
844,823,803,782,762,742,723,703,684,665,646,627,609,591,573,555,537,520,
503,486,470,454,438,422,406,391,376,361,347,333,319,305,292,279,266,253,
241,229,217,206,195,184,174,163,153,144,134,125,117,108,100,92,85,78,71,
64,58,52,47,41,36,32,27,23,20,16,13,10,8,6,4,3,1,1,0,0,0,1,1,3,4,6,8,10,
13,16,20,23,27,32,36,41,47,52,58,64,71,78,85,92,100,108,117,125,134,144,
153,163,174,184,195,206,217,229,241,253,266,279,292,305,319,333,347,361,
376,391,406,422,438,454,470,486,503,520,537,555,573,591,609,627,646,665,
684,703,723,742,762,782,803,823,844,865,886,907,929,950,972,994,1016,1039,
1061,1084,1107,1129,1153,1176,1199,1223,1246,1270,1294,1318,1342,1366,1390,
1415,1439,1464,1489,1513,1538,1563,1588,1613,1639,1664,1689,1715,1740,1765,
1791,1816,1842,1868,1893,1919,1945,1970,1996,2022};
上表是将幅值为3.3V的正弦波通过12bit的ADC进行500次采样后得到的数值，我们只需将上述数组中的数值依次赋值给DAC，我们即可通过DAC得到正弦波的输出。
为了保证正弦波频率的准确性和稳定性，采用在定时器中断内控制DAC的输出，代码如下：
void HAL_TIM_PeriodElapsedCallback(TIM_HandleTypeDef *htim)
{
float *buffer;
BaseType_t xTaskWokenByReceive=pdFALSE;
BaseType_t err;
if(htim==(&TIM3_Handler))
{
if(DAC_index >= 499)
DAC_index = 0;
else
DAC_index++;
buffer=mymalloc(SRAMIN,sizeof(float));
if(DAC_amp_Queue!=NULL)
{
memset(buffer,0,sizeof(float));
err=xQueueReceiveFromISR(DAC_amp_Queue,buffer,&xTaskWokenByReceive);//
if(err==pdTRUE) //½ÓÊÕµ½ÏûÏ¢
{
DAC_amp = *buffer;
}
}
myfree(SRAMIN,buffer); //ÊÍ·ÅÄÚ´æ
LED1=!LED1;
}
HAL_DAC_SetValue(&DAC1_Handler,DAC_CHANNEL_1,DAC_ALIGN_12B_R,Sine12bit2[DAC_index]*DAC_amp);
portYIELD_FROM_ISR(xTaskWokenByReceive);
}从上面程序中可以看到，采用的消息队列的方式传递sin波 的幅值，幅值大小是通过按键进行改变的，如下：
//ÖжϲâÊÔÈÎÎñº¯Êý
void key_task(void *pvParameters)
{
u8 key;
BaseType_t err;
while(1)
{
key=KEY_Scan(0); //ɨÃè°´¼ü
switch(key)
{
case WKUP_PRES: //KEY_UP¿ØÖÆLED1
DAC_amp += 0.1f;
if(DAC_amp > 1)
DAC_amp = 1;
break;
case KEY1_PRES: //KEY1¿ØÖÆ·äÃùÆ÷
DAC_amp -= 0.1f;
if(DAC_amp < 0)
DAC_amp = 0;
break;
}
err=xQueueSend(DAC_amp_Queue,&DAC_amp,10);
if(err==errQUEUE_FULL) //·¢ËÍ°´¼üÖµ
{
printf("¶ÓÁÐKey_QueueÒÑÂú£¬Êý¾Ý·¢ËÍʧ°Ü!\r\n");
}
vTaskDelay(100);
}
}由此，可以通过FreeRTOS获得正弦波输出。
需要原工程文件的可以点击这里。
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