红外遥控 NEC 解码为什么必须用输入捕获：一次轮询丢码的排查

while (1) {
    if (HAL_GPIO_ReadPin(GPIOA, GPIO_PIN_0) == GPIO_PIN_RESET) {
        uint16_t us = 0;
        while (HAL_GPIO_ReadPin(GPIOA, GPIO_PIN_0) == GPIO_PIN_RESET) {
            HAL_Delay(1);   /* 1ms 粒度，还想量 560us */
            us += 1000;
        }
        /* 用 us 判断这是引导码还是数据位 */
    }
    OLED_Refresh();
    printf("key=%d\r\n", last_key);
}

/* PA0 -> TIM2_CH1；初始化时捕获极性设为下降沿 */
typedef enum { S_IDLE, S_LEADER_HIGH, S_BIT_FALL, S_BIT_RISE } ir_state_t;

static volatile ir_state_t s_state = S_IDLE;
static volatile uint16_t   s_last;      /* 上一次捕获的 CCR */
static volatile uint32_t   s_frame;
static volatile uint8_t    s_bits;
static volatile uint8_t    s_idle_ms;
volatile uint8_t  ir_ready;
volatile uint32_t ir_frame;

static void ir_pol(uint32_t p) { __HAL_TIM_SET_CAPTUREPOLARITY(&htim2, TIM_CHANNEL_1, p); }

static uint8_t rev8(uint8_t x)
{
    x = (uint8_t)((x & 0xF0) >> 4 | (x & 0x0F) << 4);
    x = (uint8_t)((x & 0xCC) >> 2 | (x & 0x33) << 2);
    return (uint8_t)((x & 0xAA) >> 1 | (x & 0x55) << 1);
}

void HAL_TIM_IC_CaptureCallback(TIM_HandleTypeDef *htim)
{
    if (htim->Instance != TIM2) return;

    uint16_t ccr = HAL_TIM_ReadCapturedValue(htim, TIM_CHANNEL_1);
    uint16_t dt  = (uint16_t)(ccr - s_last);   /* 16 位无符号相减，回绕自动正确 */
    s_last = ccr;
    s_idle_ms = 0;

    switch (s_state) {
    case S_IDLE:                       /* 任意下降沿都当作一次帧的起点 */
        ir_pol(TIM_ICPOLARITY_RISING);
        s_state = S_LEADER_HIGH;
        break;

    case S_LEADER_HIGH:                /* 上升沿，dt = 引导码低电平宽度 */
        ir_pol(TIM_ICPOLARITY_FALLING);
        if (dt > 8000 && dt < 10000) { s_frame = 0; s_bits = 0; s_state = S_BIT_FALL; }
        else                         { s_state = S_IDLE; }
        break;

    case S_BIT_FALL:                   /* 下降沿，dt = 上一位的高电平宽度 */
        ir_pol(TIM_ICPOLARITY_RISING);
        s_state = S_BIT_RISE;
        if (dt > 3000) break;          /* 这是引导码的 4.5ms 高电平，跳过 */
        if      (dt > 400  && dt < 800)  s_frame = (s_frame << 1) | 0u;
        else if (dt > 1400 && dt < 1900) s_frame = (s_frame << 1) | 1u;
        else { s_state = S_IDLE; break; }          /* 位宽不像 NEC，整帧作废 */
        if (++s_bits == 32) {
            s_state = S_IDLE;
            uint8_t a = rev8(s_frame >> 24), ai = rev8(s_frame >> 16);
            uint8_t c = rev8(s_frame >> 8),  ci = rev8(s_frame);
            if ((uint8_t)(a ^ ai) == 0xFF && (uint8_t)(c ^ ci) == 0xFF) {
                ir_frame = s_frame;            /* 主循环里再解析 */
                ir_ready = 1;
            }
        }
        break;

    case S_BIT_RISE:                   /* 上升沿，dt = 这一位的低电平，约 560us */
        ir_pol(TIM_ICPOLARITY_FALLING);
        s_state = (dt > 300 && dt < 900) ? S_BIT_FALL : S_IDLE;
        break;
    }
}

/* 放在 1ms 的 SysTick 回调里：帧间空闲超时复位，等下一帧 */
void IR_Tick1ms(void)
{
    if (s_state != S_IDLE && ++s_idle_ms > 20) {
        s_state = S_IDLE;
        ir_pol(TIM_ICPOLARITY_FALLING);
    }
}