减法要用 signed 型

　　今天调试一个程序，因为Feedback是电流采样值，Setpoint是PWM值，这两个不可能是负值。所以以为Setpoint和Feedback这两个变量都可以设置为u16型(unsigned int)，结果悲催了，CPU总是跑飞。导致LED暴亮，差点烧掉。。。

　　原因是两个unsigned型数据相减后不能为负值。如： PIdata->Setpoint - PIdata->Feedback

所以，要保证其中有一个是signed 型，系统就默认相减的结果为signed型，程序可以正常运行。

typedef struct {

    u16        Kp;

    u16        Ki;

    u8        OutMax;

    signed long    Setpoint;

    u16            Feedback;

    signed long            Error;

    signed long            Integral;

    signed long            Output;

    u8                Saturated;

    }tPIParams;

tPIParams    Current;

void CalcPI(tPIParams *PIdata)

{

    PIdata->Error = PIdata->Setpoint - PIdata->Feedback;

    // Deadband -- If the magnitude of the error is less than a limit,

    // then don't calculate the PI routine at all.  This saves

    // processor time and avoids oscillation problems.

    if((PIdata->Error > ) || (PIdata->Error < -))

    {

        // Now, calculate the actual PI function here.

        PIdata->Output += (PIdata->Error * PIdata->Kp )/;

        // Perform boundary checks on the PI controller output.  If the

        // output limits are exceeded, then set output to the limit

        // and set flag.

        if(PIdata->Output > PIdata->OutMax)

        {

            PIdata->Output = PIdata->OutMax;

        }else if(PIdata->Output < )

        {

            //PIdata->Saturated = 1;

            PIdata->Output = ;

        }

    }

}