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（一）原理图：

PX4飞控的PPM输入捕获由协处理器完成，接在A8引脚，对应Timer1的通道1。

（二）PPM协议：

PPM的每一帧数据间隔为20ms，用两个上升沿之间的时间间隔表示一个通道的值，如图所示是一个6通道信号。在每一帧的结束会有较长时间的持续低电平，两次上升沿时间间隔要大于5ms，可以用于辨别下一帧数据的到来。

（三）相关代码

定时器1配置：

/* timer1 config */
void Bsp_Timer1_Config(void)
{
    TIM_ICInitTypeDef       TIM_ICInitStructure;
    TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;
    NVIC_InitTypeDef        NVIC_InitStructure;
    GPIO_InitTypeDef        GPIO_InitStructure;

    /* Clock Enable */
    RCC_APB2PeriphClockCmd(RCC_APB2Periph_TIM1, ENABLE);
    RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA, ENABLE);

    /* Enable global Interrupt */
    NVIC_InitStructure.NVIC_IRQChannel = TIM1_CC_IRQn;
    NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 1;
    NVIC_InitStructure.NVIC_IRQChannelSubPriority = 1;
    NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
    NVIC_Init(&NVIC_InitStructure);

    /* TIM1 channel 1 pin (PA.08) configuration */
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_8;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_Init(GPIOA, &GPIO_InitStructure);

    /* Time Base configuration, 1us every tick */
    TIM_TimeBaseStructure.TIM_Prescaler = SystemCoreClock / 1000000;
    TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
    TIM_TimeBaseStructure.TIM_Period = 0xFFFF;
    TIM_TimeBaseStructure.TIM_ClockDivision = 0;
    TIM_TimeBaseStructure.TIM_RepetitionCounter = 0;

    TIM_TimeBaseInit(TIM1, &TIM_TimeBaseStructure);

    TIM_ICInitStructure.TIM_Channel = TIM_Channel_1;
    TIM_ICInitStructure.TIM_ICPolarity = TIM_ICPolarity_Rising;
    TIM_ICInitStructure.TIM_ICSelection = TIM_ICSelection_DirectTI;
    TIM_ICInitStructure.TIM_ICPrescaler = TIM_ICPSC_DIV1;
    TIM_ICInitStructure.TIM_ICFilter = 0x0;

    TIM_ICInit(TIM1, &TIM_ICInitStructure);

    /* TIM enable counter */
    TIM_Cmd(TIM1, ENABLE);

    /* Enable the CC1 Interrupt Request */
    TIM_ITConfig(TIM1, TIM_IT_CC1, ENABLE);
}

PPM处理程序：

#define GET_GAP(x, y)	(x > y ? (x - y) : (0xFFFF - y + x))

ST_PPM machine_param;

static uint8_t PPM_Recv = 0;        /* ppm received flag */ 
static uint8_t PPM_Send_Freq = 0;   /* freq of sending ppm to fmu */ 

/* status turn to the next one */ 
static void ppm_next_status(ST_PPM *param)
{
    static uint8_t max_ch = STA_EDGE_9 + 1;

    param->status = (param->status + 1) % max_ch;
}

/* reset the ppm machine state */
void reset_machine_state(ST_PPM *param)
{
    param->status = STA_EDGE_1;
    param->bad_frame = 0;
    param->last_ic = 0;
}

/* the function will be called back by the timer1 interrupt */
void App_PPM_Status_Machine(uint16_t ic)
{
    static uint16_t temp_val[8];
    uint16_t gap;

    gap = GET_GAP(ic, machine_param.last_ic);

    /* each ppm frame has a 5.5ms start puls, each tick is 1us */
    if (gap > 5000)
    {
        reset_machine_state(&machine_param);
    }
    else if (gap < 800 || gap > 2200)
    {
        /* bad signal, drop frame */
        machine_param.bad_frame = 1;
    }

    if (machine_param.status == STA_EDGE_1)
    {
        /* do nothing */
    }
    else if (machine_param.status == STA_EDGE_9)
    {
        uint8_t i;

        temp_val[STA_EDGE_9 - 1] = gap;
        /* we know next we have a 5.5ms gap, so we can do some time-cost task here */
        /* If it's bad frame, we drop it */
        if (!machine_param.bad_frame)
        {
            for (i = 0; i < 8; i++)
            {
                machine_param.ppm_val[i] = temp_val[i];
            }
            PPM_Recv = 1;
        }
    }
    else
    {
        temp_val[machine_param.status - 1] = gap;
    }

    machine_param.last_ic = ic;
    ppm_next_status(&machine_param);
}

/* received a wholed ppm frame */
uint8_t App_PPM_Ready(void)
{
    static uint32_t prev_time = 0;
    uint32_t time = Sys_Tick_Time;

    if (!PPM_Send_Freq)
    {
        return 0;
    }

    if (time - prev_time > 1000 / PPM_Send_Freq && PPM_Recv)
    {
        prev_time = time;
        return 1;
    }

    return 0;
}

/* copy the ppm frame to the buff when the ppm frame will be used */
void App_Get_PPM_Value(uint16_t ppm[8])
{
    memcpy(ppm, machine_param.ppm_val, 8);
    PPM_Recv = 0;
}

/* set the frequency of sending ppm buff to fmu */
uint8_t App_Set_PPM_Send_Freq(uint8_t freq)
{
    if (freq <= 50)
    {
        PPM_Send_Freq = freq;
        return 1;
    }
    else
    {
        return 0;
    }
}	(x > y ? (x - y) : (0xFFFF - y + x))

ST_PPM machine_param;

static uint8_t PPM_Recv = 0;        /* ppm received flag */ 
static uint8_t PPM_Send_Freq = 0;   /* freq of sending ppm to fmu */ 

/* status turn to the next one */ 
static void ppm_next_status(ST_PPM *param)
{
    static uint8_t max_ch = STA_EDGE_9 + 1;

    param->status = (param->status + 1) % max_ch;
}

/* reset the ppm machine state */
void reset_machine_state(ST_PPM *param)
{
    param->status = STA_EDGE_1;
    param->bad_frame = 0;
    param->last_ic = 0;
}

/* the function will be called back by the timer1 interrupt */
void App_PPM_Status_Machine(uint16_t ic)
{
    static uint16_t temp_val[8];
    uint16_t gap;

    gap = GET_GAP(ic, machine_param.last_ic);

    /* each ppm frame has a 5.5ms start puls, each tick is 1us */
    if (gap > 5000)
    {
        reset_machine_state(&machine_param);
    }
    else if (gap < 800 || gap > 2200)
    {
        /* bad signal, drop frame */
        machine_param.bad_frame = 1;
    }

    if (machine_param.status == STA_EDGE_1)
    {
        /* do nothing */
    }
    else if (machine_param.status == STA_EDGE_9)
    {
        uint8_t i;

        temp_val[STA_EDGE_9 - 1] = gap;
        /* we know next we have a 5.5ms gap, so we can do some time-cost task here */
        /* If it's bad frame, we drop it */
        if (!machine_param.bad_frame)
        {
            for (i = 0; i < 8; i++)
            {
                machine_param.ppm_val[i] = temp_val[i];
            }
            PPM_Recv = 1;
        }
    }
    else
    {
        temp_val[machine_param.status - 1] = gap;
    }

    machine_param.last_ic = ic;
    ppm_next_status(&machine_param);
}

/* received a wholed ppm frame */
uint8_t App_PPM_Ready(void)
{
    static uint32_t prev_time = 0;
    uint32_t time = Sys_Tick_Time;

    if (!PPM_Send_Freq)
    {
        return 0;
    }

    if (time - prev_time > 1000 / PPM_Send_Freq && PPM_Recv)
    {
        prev_time = time;
        return 1;
    }

    return 0;
}

/* copy the ppm frame to the buff when the ppm frame will be used */
void App_Get_PPM_Value(uint16_t ppm[8])
{
    memcpy(ppm, machine_param.ppm_val, 8);
    PPM_Recv = 0;
}

/* set the frequency of sending ppm buff to fmu */
uint8_t App_Set_PPM_Send_Freq(uint8_t freq)
{
    if (freq <= 50)
    {
        PPM_Send_Freq = freq;
        return 1;
    }
    else
    {
        return 0;
    }
}

定时器1中断程序：

/* timer1 interrupt */
void TIM1_CC_IRQHandler(void)
{
    uint16_t ic = 0;

    if (TIM_GetITStatus(TIM1, TIM_IT_CC1) == SET) 
    {
        /* Clear TIM1 Capture compare interrupt pending bit */
        TIM_ClearITPendingBit(TIM1, TIM_IT_CC1);

        ic = TIM_GetCapture1(TIM1);
        App_PPM_Status_Machine(ic);
    }
}