开源ESP32彩屏WIFI/BLE智能万用表制过程(3.频率占空比测量)

1万 · 2021-8-17 16:56

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
}

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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;
}

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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);
}
}

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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);
}

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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);
}

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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。

开源ESP32彩屏WIFI/BLE智能万用表制过程(3.频率占空比测量)

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