arduino問題..麻煩幫解惑

float x=60,speed=80,kp=1.2,p,y,i,pwm1,pwm2;
int pp[128]={0};
setup(){
pinMode(9,OUTPUT);//PWM1
pinMode(10,OUTPUT);//PWM2
}
loop(){
for(i=1;i<128;i++){
pp[i]=analogRead(A0);
if (i==10 || i==110){
if (pp[i]<55){
y=i;
p=(float)(x*5-y*5);
value=(float)(p*kp);
pwm1=speed-value;
pwm2=speed+value;
if(pwm1>125)pwm1=125;
if(pwm1<0)pwm1=0;
if(pwm2>125)pwm2=125;
if(pwm2<0)pwm2=0;
}
}
analogWrite(9,pwm1);
analogWrite(10,pwm2);
}
}
目前以此做馬達轉速..但不知為何..馬達運算為負數應該為0..看UART數據也為0..但是馬達在看UART為0時..竟會正反轉
可是不管我直接給pwm12正值或負值皆為正轉..實在不懂為何反轉??