PWM Ep. 1 : Square Wave Frequency and Duty Cycle Measurement -- Raspberry Pi Pico and STM32F103C8T6

本文章探討利用Raspberry Pi Pico來量測輸入PWM方波的頻率與佔空比。以STM32F103C8T6來產生PWM方波讓Pico來量測。

一、RP2040 PWM：

(source: RP2040 datasheet)

RP2040 共有8 PWM slices，每個slice有channel A與B，與GPIO對應表如自如所示。每個slice channel A and B均能輸出PWM方波。但只有channel B能作為輸入PWM用。

RP2040輸出方波的波型主要由下列三個主要參數決定。

1. 8.4除瀕(8 bits整數，4 bits小數) ：
   0: max(256)。整數：1~255，小數:0~(15/16)。
   pwm_set_clkdiv(slice_num, div);
2. 16 bit 計數值(TOP register):
   pwm_set_wrap(slice_num, top_wap);
   top_wap: 0~65535
3. 16 bit 計數比較值(CC register):
   pwm_set_chan_level(slice_num, PWM_CHAN_A, cc);
   cc: 0~65536

例如：頻率為500k佔空比為40%的方波

系統clock為125M Hz，

pwm_set_clkdiv(slice_num 12.5) --> 125M/12.5=10M

pwm_set_warp(slice_num, 19) --> 10M/(19+1) = 500k

pwm_set_chan_level(slice_num, PWM_CHAN_A, 8) --> 8/(19+1) = 40%

(Source RP2040 datasheet)

• PWM輸入：

僅有每個slice的Channel B能接受輸入。

(source RP2040 datasheet)

將Channel B設為輸入有三種模式，分別為偵測High Leve, rising edge與falling edge。

pwm_config_set_clkdiv_mode(&cfg, mode);

pwm_set_clkdiv_mode(slice_num, mode);

mode:PWM_DIV_B_HIGH, PWM_DIV_B_RISING, PWM_DIV_B_FALLING

1. 設定PWM_DIV_B_HIGH時可用來偵測Duty Cycle:
   當channel偵測到input PWM pulse level為high時，CC count 加 1，register為16 bits，因此最答值為65535。例如：
   slice_num : 3, click div: 100, 偵測時間為50ms。則channel B最大clock數為：
   125M Hz/100*(50/1000) =  62500 Hz。
   pwm_set_clkdiv_mode(slice_num, PWM_DIV_B_HIGH);    pwm_set_clkdiv(slice_num, 100);
   pwm_set_enabled(slice_num, true);
   sleep_ms(50);
   pwm_set_enabled(slice_num, false);
   上述為channel B 50ms總clock數。
   count=pwm_get_counter(slice_num);
   當channel B偵測到input PWM pulse為high時, count+1，因此
   count / 625000 * 100%即為Duty cycle。
   
2. 設定PWM_DIV_B_RISING or PWM_DIV_B_FALLING時可用來偵測Frequency:
   當channel B偵測到input PWM pulse為RISING or FALLING時count+1。pwm_config cfg = pwm_get_default_config();    pwm_config_set_clkdiv_mode(&cfg, PWM_DIV_B_RISING);    pwm_config_set_clkdiv(&cfg, 1);   
   pwm_init(slice_num, &cfg, false);    gpio_set_function(gpio, GPIO_FUNC_PWM);
   以125M Hz偵測。
   pwm_set_enabled(slice_num, true);       
   sleep_ms(delay_ms);       
   pwm_set_enabled(slice_num, false);       
   rising_count = pwm_get_counter(slice_num);
   經過delay_ms後，偵測到rising_count個RISING EDGE，因此
   Freq = rising_count / delay_ms*1000 Hz
   但是rising_count最多為65535(16 bit register)，因此必須根據input pulse frequence調整delay_ms數。

其他詳細程式碼附於文末。

二、STM32 PWM:

STM32 PWM由timer產生。

如上圖timer 1 clock為72 M Hz。

clock source設為internal clock(72M Hz)，prescaler, counter period(ARR)分別為1, 299，因此PWM的Frequency為72M Hz/(1+1)/(299+1) = 120K Hz。

Duty cycle=150/(299+1)*100% = 50% 

htim1.Instance->ARR = fs_presc;

htim1.Instance->CCR1 = fs_presc/duty;

改變ARR與CCR可以改變frequency 與duty cycle

三、展示影片：

四、程式碼：

 
#include <stdio.h>
 
#include "pico/stdlib.h"
#include "hardware/pwm.h"
#include "hardware/clocks.h"
#include "string.h"
#include "pico_tft/pico_tft.h"
#include "pico_tft/tft_string/tft_string.h"
#include "pico_tft/fonts/font_ubuntu_mono_24.h"
 
const uint MEASURE_PIN_1 = 3;
const uint MEASURE_PIN_2 = 5;
bool pwm_slice_warp_1=false;
bool pwm_slice_warp_2=false;
 
void on_pwm_wrap() {
 
   uint slice_num_1 = pwm_gpio_to_slice_num(MEASURE_PIN_1);
    if (pwm_get_irq_status_mask() & (1 << slice_num_1)) {
        pwm_clear_irq(slice_num_1);
        pwm_slice_warp_1=true;
        
    }
 
    uint slice_num_2 = pwm_gpio_to_slice_num(MEASURE_PIN_2);
    if (pwm_get_irq_status_mask() & (1 << slice_num_2)) {
        pwm_clear_irq(slice_num_2);
        pwm_slice_warp_2=true;
        
    }
}
 
 
float measure_frequency(uint gpio) {
    uint16_t measure_ms[] = {1,5,10,100,1000};
    bool *pwm_slice_warp=false;
 
    // Only the PWM B pins can be used as inputs.
    assert(pwm_gpio_to_channel(gpio) == PWM_CHAN_B);
    uint slice_num = pwm_gpio_to_slice_num(gpio);
 
    if (gpio == MEASURE_PIN_1) pwm_slice_warp = &pwm_slice_warp_1;
    if (gpio == MEASURE_PIN_2) pwm_slice_warp = &pwm_slice_warp_2;
 
    // Count once for every 1 cycles the PWM B input is high
    pwm_config cfg = pwm_get_default_config();
    pwm_config_set_clkdiv_mode(&cfg, PWM_DIV_B_RISING);
    pwm_config_set_clkdiv(&cfg, 1);
    pwm_init(slice_num, &cfg, false);
    gpio_set_function(gpio, GPIO_FUNC_PWM);
    
    pwm_set_irq_enabled(slice_num, true);
    irq_set_exclusive_handler(PWM_IRQ_WRAP, on_pwm_wrap);
    irq_set_enabled(PWM_IRQ_WRAP, true);
 
    uint32_t ms=0;
    uint16_t count=0;
    uint8_t meauser_count = sizeof(measure_ms)/sizeof(measure_ms[0]);
    *pwm_slice_warp=false;
    for (int i=0; i < meauser_count;i++) {
        ms = measure_ms[i];
        pwm_set_enabled(slice_num, true);
        sleep_ms(ms);
        pwm_set_enabled(slice_num, false);
        count = pwm_get_counter(slice_num);
        pwm_set_counter(slice_num,0);
 
        if (*pwm_slice_warp) {
            *pwm_slice_warp=false;
            if (i > 0) {
                 ms = measure_ms[i-1];
                pwm_set_enabled(slice_num, true);
                sleep_ms(ms);
                pwm_set_enabled(slice_num, false);
                count = pwm_get_counter(slice_num);
                pwm_set_counter(slice_num,0);
            }
            break;
            
        } 
 
    }
 
    return ((float)count*1000/(ms));
    
}
 
float measure_duty_cycle(uint gpio) {
    // Only the PWM B pins can be used as inputs.
    assert(pwm_gpio_to_channel(gpio) == PWM_CHAN_B);
    uint slice_num = pwm_gpio_to_slice_num(gpio);
 
    // Count once for every 100 cycles the PWM B input is high
    pwm_config cfg = pwm_get_default_config();
    pwm_config_set_clkdiv_mode(&cfg, PWM_DIV_B_HIGH);
    pwm_config_set_clkdiv(&cfg, 100);
    pwm_init(slice_num, &cfg, false);
    gpio_set_function(gpio, GPIO_FUNC_PWM);
    uint16_t count=0;
    pwm_set_counter(slice_num,0);
 
    pwm_set_enabled(slice_num, true);
    sleep_ms(50);
    pwm_set_enabled(slice_num, false);
    count=pwm_get_counter(slice_num);
 
    float counting_rate = clock_get_hz(clk_sys) / 100;
    float max_possible_count = counting_rate *0.05;   // 125M/100*0.05 = 62500 < 65535
 
    return pwm_get_counter(slice_num)/max_possible_count;
}
 
void test_500k_50() {
    gpio_set_function(6, GPIO_FUNC_PWM);
    uint slice_num = pwm_gpio_to_slice_num(6);
    pwm_config cfg = pwm_get_default_config();
    pwm_config_set_clkdiv(&cfg, 12.5);
    pwm_config_set_wrap(&cfg, 19);
    pwm_init(slice_num, &cfg, true);
 
    pwm_set_chan_level(slice_num, PWM_CHAN_A, 10);
    
}
 
int main() {
    stdio_init_all();
    tft_init();
    tft_fill_rect(0,0, TFT_WIDTH-1, TFT_HEIGHT-1, 0xffff);
    printf("\nPWM frequency and duty cycle measurement example\n");
    uint8_t buffer[120];
    float duty, frequency;
    while (1) {
        //sleep_ms(1000);
        frequency = measure_frequency(MEASURE_PIN_1);
        duty = measure_duty_cycle(MEASURE_PIN_1)*100;
        sprintf(buffer, "F1:%.0f", frequency);
        tft_draw_string_withbg(10,10, "            ", 0x001f, 0xffff, &font_ubuntu_mono_24);
        tft_draw_string_withbg(10,10, buffer, 0x001f, 0xffff, &font_ubuntu_mono_24);
        sprintf(buffer, "D1:%02.2f%%", duty);
        tft_draw_string_withbg(10,35, buffer, 0xf800, 0xffff, &font_ubuntu_mono_24);
 
        printf("\nMEASURE 1 == Freq:%07.0f, Duty:%02.2f%%\n", frequency, duty);
       
        frequency = measure_frequency(MEASURE_PIN_2);
        duty = measure_duty_cycle(MEASURE_PIN_2)*100;
        sprintf(buffer, "F2:%.0f", frequency);
        tft_draw_string_withbg(10,65, "            ", 0x001f, 0xffff, &font_ubuntu_mono_24);
        tft_draw_string_withbg(10,65, buffer, 0x001f, 0xffff, &font_ubuntu_mono_24);
        sprintf(buffer, "D2:%02.2f%%", duty);
        tft_draw_string_withbg(10,90, buffer, 0xf800, 0xffff, &font_ubuntu_mono_24);
       
        printf("\nMEASURE 2 == Freq:%07.0f, Duty:%02.2f%%\n", frequency, duty);
        
    }
}