上次制作无屏幕版本的万用表在测量交流信号时，没有频率和占空比测量功能，这次使用ESP32采集交流信号，计算频率和占空比。

 

频率为周期的倒数，占空比高电平时间T1与总周期时间T的比值，范围0%-100%

下面介绍3种实现方法，也是我逐一调试程序验证的过程。

1、先试一下用MCPWM捕获功能实现频率测量，参照官方例程配置比较简单

void M_pwm_cap_start()
{
    M_pwm_init();
    //7. Capture configuration
    //comment if you don't want to use capture submodule, also u can comment the capture gpio signals
    //configure CAP0, CAP1 and CAP2 signal to start capture counter on rising edge
    //we generate a gpio_test_signal of 20ms on GPIO 12 and connect it to one of the capture signal, the disp_captured_function displays the time between rising edge
    //In general practice you can connect Capture  to external signal, measure time between rising edge or falling edge and take action accordingly
    mcpwm_capture_enable(MCPWM_UNIT_0, MCPWM_SELECT_CAP0, MCPWM_POS_EDGE, 0);  //capture signal on rising edge, prescale = 0 i.e. 800,000,000 counts is equal to one second
    //enable interrupt, so each this a rising edge occurs interrupt is triggered
   
   //create queue
    cap_queue = xQueueCreate(1, sizeof(capture)); //comment if you don't want to use capture module
    MCPWM[MCPWM_UNIT_0]->int_ena.val = CAP0_INT_EN;  //Enable interrupt on  CAP0, CAP1 and CAP2 signal
    mcpwm_isr_register(MCPWM_UNIT_0, isr_handler, NULL, ESP_INTR_FLAG_IRAM, NULL);  //Set ISR Handler
    xTaskCreate(disp_captured_signal, "mcpwm_config", 4096, NULL, 5, NULL);  //comment if you don't want to use capture module
}

static void IRAM_ATTR isr_handler()
{
    uint32_t mcpwm_intr_status;
    capture evt;
    mcpwm_intr_status = MCPWM[MCPWM_UNIT_0]->int_st.val; //Read interrupt status
    if (mcpwm_intr_status & CAP0_INT_EN) { //Check for interrupt on rising edge on CAP0 signal
        evt.capture_signal = mcpwm_capture_signal_get_value(MCPWM_UNIT_0, MCPWM_SELECT_CAP0); //get capture signal counter value
        evt.sel_cap_signal = MCPWM_SELECT_CAP0;
        xQueueSendFromISR(cap_queue, &evt, NULL);
    }

    MCPWM[MCPWM_UNIT_0]->int_clr.val = mcpwm_intr_status;
}

static void disp_captured_signal(void *arg)
{
	int add=0;
    float frequency;
    uint32_t *current_cap_value = (uint32_t *)malloc(2*sizeof(uint32_t));
    uint32_t *previous_cap_value = (uint32_t *)malloc(2*sizeof(uint32_t));
    capture evt;
    while (1) {
        xQueueReceive(cap_queue, &evt, portMAX_DELAY);
        if (evt.sel_cap_signal == MCPWM_SELECT_CAP0) {
            current_cap_value[0] = evt.capture_signal - previous_cap_value[0];
            previous_cap_value[0] = evt.capture_signal;
            current_cap_value[0] = (current_cap_value[0] / 10000) * (10000000000 / rtc_clk_apb_freq_get());
            
			if(++add>50)
			{
				add=0;
                frequency=1000000/current_cap_value[0];
				printf("CAP0 : %d us\n", current_cap_value[0]);//print high time
				printf("frequency : %.2f Hz\n", frequency);
			}
        }

        vTaskDelay(10/portTICK_RATE_MS);
    }
}

用信号发生器产生PWM信号，测试试一下测量频率

 

对于几百Hz以内测量还是很准确的，但是没找到测量占空比的方法，测试其他方法。

2、利用ESP32的pcnt脉冲计数器，先初始化

static void pcnt_example_init(void)
{
    /* Prepare configuration for the PCNT unit */
    pcnt_config_t pcnt_config = {
        // Set PCNT input signal and control GPIOs
        .pulse_gpio_num = PCNT_INPUT_SIG_IO,
        .ctrl_gpio_num = PCNT_INPUT_CTRL_IO,
        .channel = PCNT_CHANNEL_0,
        .unit = PCNT_TEST_UNIT,
        // What to do on the positive / negative edge of pulse input?
        .pos_mode = PCNT_COUNT_INC,   // Count up on the positive edge
        .neg_mode = PCNT_COUNT_DIS,   // Keep the counter value on the negative edge
        // What to do when control input is low or high?
        .lctrl_mode = PCNT_MODE_KEEP, // Reverse counting direction if low
        .hctrl_mode = PCNT_MODE_KEEP,    // Keep the primary counter mode if high
        // Set the maximum and minimum limit values to watch
        .counter_h_lim = PCNT_H_LIM_VAL,
        .counter_l_lim = PCNT_L_LIM_VAL,
    };
    /* Initialize PCNT unit */
    pcnt_unit_config(&pcnt_config);

    /* Configure and enable the input filter */
    pcnt_set_filter_value(PCNT_TEST_UNIT, 100);
    pcnt_filter_enable(PCNT_TEST_UNIT);

    /* Set threshold 0 and 1 values and enable events to watch */
    pcnt_set_event_value(PCNT_TEST_UNIT, PCNT_EVT_THRES_1, PCNT_THRESH1_VAL);
    pcnt_event_enable(PCNT_TEST_UNIT, PCNT_EVT_THRES_1);
    pcnt_set_event_value(PCNT_TEST_UNIT, PCNT_EVT_THRES_0, PCNT_THRESH0_VAL);
    pcnt_event_enable(PCNT_TEST_UNIT, PCNT_EVT_THRES_0);
    /* Enable events on zero, maximum and minimum limit values */
    pcnt_event_enable(PCNT_TEST_UNIT, PCNT_EVT_ZERO);
    pcnt_event_enable(PCNT_TEST_UNIT, PCNT_EVT_H_LIM);
    pcnt_event_enable(PCNT_TEST_UNIT, PCNT_EVT_L_LIM);

    /* Initialize PCNT's counter */
    pcnt_counter_pause(PCNT_TEST_UNIT);
    pcnt_counter_clear(PCNT_TEST_UNIT);

    /* Register ISR handler and enable interrupts for PCNT unit */
    pcnt_isr_register(pcnt_example_intr_handler, NULL, 0, &user_isr_handle);
    pcnt_intr_enable(PCNT_TEST_UNIT);

    /* Everything is set up, now go to counting */
    pcnt_counter_resume(PCNT_TEST_UNIT);
}

然后用定时器，每秒钟获取一次计数值，这样就能算出信号频率，每秒钟的脉冲个数就是频率。

void test_timer_periodic_cb(void *arg) {
    int16_t count = 0;
    pcnt_get_counter_value(PCNT_TEST_UNIT, &count);
    printf("frequency :%d Hz\n ", count);
    pcnt_counter_clear(PCNT_TEST_UNIT);
}

这个方法可以测量比较高的频率，下图是测量4513Hz频率信号，但是和第一种方法一样，测量不了占空比，继续测量其他方法。

 3、利用ESP32外部中断引脚+定时器计数器，在上升沿中断时使能计数器，下降沿中断时获取高电平计数值n1，继续计数，直到又一个上升沿中断获取计数值n2，根据计数器的周期，可以得到计数1次的周期，从而得到高电平时间和整个周期的时间，然后就能算出占空比和频率了。

 

这种方式可以精确的测量出占空比和频率，但存在一个问题，当测量频率比较高时，要频繁的进入中断程序，大大降低了CPU的效率，继续改进。

4、由于我们不需要在每个信号周期都计算一次占空比和频率，所以可以再加入一个定时器，每隔1秒中断一次，在中断程序中开启外部中断，在开启外部中断测量了一次完整的信号后，关闭外部中断，这样测量速率就由这个定时器决定，不会频繁进入外部中断，经过几个小时的程序调试，最终完成占空比和频率的测量程序，下图是测量120Hz，20%的PWM信号。