S2：下到 MCU——从「有操作系统」到「只有一行无限循环」

// 第一遍：库函数。看得懂，但不知道它干了什么
#define LED_PIN 4

void setup() {
    pinMode(LED_PIN, OUTPUT);
}

void loop() {
    digitalWrite(LED_PIN, HIGH);
    delay(1000);
    digitalWrite(LED_PIN, LOW);
    delay(1000);
}

// 第二遍：寄存器。慢，但每一行都能解释
#include <stdint.h>

#define GPIO_ENABLE_REG   (*(volatile uint32_t *)0x3FF44020)
#define GPIO_OUT_REG      (*(volatile uint32_t *)0x3FF44004)
#define GPIO_OUT_W1TS_REG (*(volatile uint32_t *)0x3FF44008)  // Write 1 To Set
#define GPIO_OUT_W1TC_REG (*(volatile uint32_t *)0x3FF4400C)  // Write 1 To Clear

#define LED_BIT (1u << 4)   // GPIO4

static void busy_delay(volatile uint32_t n) {
    while (n--) { __asm__ volatile("nop"); }
}

void app_main(void) {
    GPIO_ENABLE_REG |= LED_BIT;          // ① 把该脚设为输出

    for (;;) {
        GPIO_OUT_W1TS_REG = LED_BIT;     // ② 置 1（点灯）
        busy_delay(2000000);
        GPIO_OUT_W1TC_REG = LED_BIT;     // ③ 清 0（灭灯）
        busy_delay(2000000);
    }
}

// 错误示范：轮询 + 阻塞延时
void loop() {
    if (digitalRead(BTN_PIN) == LOW) {
        do_something();
    }
    delay(500);          // ← 这 500ms 里，按键完全被忽略
}

// 正确做法：中断里只做一件事——置标志
#include <stdint.h>
#include <stdbool.h>

#define BTN_PIN   25
#define DEBOUNCE_MS 20

static volatile bool     g_btn_flag = false;
static volatile uint32_t g_btn_tick = 0;

// 中断服务程序（ISR）：越短越好，不要在这里做业务
static void IRAM_ATTR on_button_isr(void) {
    uint32_t now = xTaskGetTickCountFromISR();
    // 简单时间窗去抖：距上次触发不足 20ms 的直接丢掉
    if (now - g_btn_tick < DEBOUNCE_MS) return;
    g_btn_tick = now;
    g_btn_flag = true;         // ← 只做这一件事
}

void app_main(void) {
    gpio_config_t io = {
        .pin_bit_mask = 1ULL << BTN_PIN,
        .mode         = GPIO_MODE_INPUT,
        .pull_up_en   = GPIO_PULLUP_ENABLE,   // ← 必须开，否则悬空
        .intr_type    = GPIO_INTR_NEGEDGE,    // 下降沿：按下时从 1 → 0
    };
    gpio_config(&io);
    gpio_install_isr_service(0);
    gpio_isr_handler_add(BTN_PIN, on_button_isr, NULL);

    for (;;) {
        if (g_btn_flag) {
            g_btn_flag = false;    // 先清标志，再做业务
            do_something();        // ← 业务放这里，慢一点也没关系
        }
        vTaskDelay(pdMS_TO_TICKS(10));
    }
}

#include <stdio.h>
#include "driver/i2c.h"

#define I2C_PORT  I2C_NUM_0
#define SDA_PIN   21
#define SCL_PIN   22

void i2c_scan(void) {
    i2c_config_t conf = {
        .mode             = I2C_MODE_MASTER,
        .sda_io_num       = SDA_PIN,
        .scl_io_num       = SCL_PIN,
        .sda_pullup_en    = GPIO_PULLUP_ENABLE,
        .scl_pullup_en    = GPIO_PULLUP_ENABLE,
        .master.clk_speed = 100000,      // 100kHz 标准模式
    };
    i2c_param_config(I2C_PORT, &conf);
    i2c_driver_install(I2C_PORT, conf.mode, 0, 0, 0);

    printf("scanning I2C bus...\n");
    for (uint8_t addr = 1; addr < 127; addr++) {
        i2c_cmd_handle_t cmd = i2c_cmd_link_create();
        i2c_master_start(cmd);
        i2c_master_write_byte(cmd, (addr << 1) | I2C_MASTER_WRITE, true);
        i2c_master_stop(cmd);
        esp_err_t ret = i2c_master_cmd_begin(I2C_PORT, cmd, pdMS_TO_TICKS(50));
        i2c_cmd_link_delete(cmd);
        if (ret == ESP_OK) {
            printf("  found device at 0x%02X\n", addr);
        }
    }
    printf("scan done\n");
}

// SHT30 单次测量、高重复性、无时钟拉伸
static esp_err_t sht30_read(float *temp_c, float *rh_pct) {
    const uint8_t addr = 0x44;
    const uint8_t cmd[2] = {0x24, 0x00};   // 单次测量命令
    uint8_t buf[6];

    // ① 发命令
    i2c_cmd_handle_t c = i2c_cmd_link_create();
    i2c_master_start(c);
    i2c_master_write_byte(c, (addr << 1) | I2C_MASTER_WRITE, true);
    i2c_master_write(c, cmd, sizeof(cmd), true);
    i2c_master_stop(c);
    esp_err_t ret = i2c_master_cmd_begin(I2C_PORT, c, pdMS_TO_TICKS(100));
    i2c_cmd_link_delete(c);
    if (ret != ESP_OK) return ret;

    vTaskDelay(pdMS_TO_TICKS(20));         // ② 等测量完成（手册要求 15ms）

    // ③ 读 6 字节：温度(2)+CRC(1) 湿度(2)+CRC(1)
    c = i2c_cmd_link_create();
    i2c_master_start(c);
    i2c_master_write_byte(c, (addr << 1) | I2C_MASTER_READ, true);
    i2c_master_read(c, buf, 6, I2C_MASTER_LAST_NACK);
    i2c_master_stop(c);
    ret = i2c_master_cmd_begin(I2C_PORT, c, pdMS_TO_TICKS(100));
    i2c_cmd_link_delete(c);
    if (ret != ESP_OK) return ret;

    // ④ 换算（公式来自数据手册）
    uint16_t raw_t = (buf[0] << 8) | buf[1];
    uint16_t raw_h = (buf[3] << 8) | buf[4];
    *temp_c = -45.0f + 175.0f * (float)raw_t / 65535.0f;
    *rh_pct = 100.0f * (float)raw_h / 65535.0f;
    return ESP_OK;
}

static uint8_t crc8(const uint8_t *data, int len) {
    uint8_t crc = 0xFF;
    for (int i = 0; i < len; i++) {
        crc ^= data[i];
        for (int b = 0; b < 8; b++)
            crc = (crc & 0x80) ? (crc << 1) ^ 0x31 : (crc << 1);
    }
    return crc;
}
// 用法：if (crc8(buf, 2) != buf[2]) return ESP_ERR_INVALID_CRC;

for (int i = 0; i < 3; i++) {
    if (sht30_read(&t, &h) == ESP_OK) break;
    vTaskDelay(pdMS_TO_TICKS(50));
}

// 深睡 60 秒后自动醒来
esp_sleep_enable_timer_wakeup(60ULL * 1000000ULL);
// 或者：按键唤醒
// esp_sleep_enable_ext0_wakeup(GPIO_NUM_25, 0);
esp_deep_sleep_start();      // 这一行之后，程序重新从 app_main 开始

// STM32F103 寄存器版点灯（最小示例）
#include <stdint.h>

#define RCC_APB2ENR   (*(volatile uint32_t *)0x40021018)
#define GPIOC_CRL     (*(volatile uint32_t *)0x40011000)
#define GPIOC_ODR     (*(volatile uint32_t *)0x4001100C)

#define IOPCEN        (1u << 4)     // GPIOC 时钟使能位

void led_init(void) {
    RCC_APB2ENR |= IOPCEN;          // ① 第一步永远是开时钟

    // ② PC13 配成推挽输出、50MHz
    GPIOC_CRL &= ~(0xFu << 20);     // 清掉 PC13 的 4 位配置
    GPIOC_CRL |=  (0x3u << 20);     // 0011 = 输出模式 50MHz
}

void led_toggle(void) {
    GPIOC_ODR ^= (1u << 13);        // ③ 翻转 PC13
}