【求助】摇杆控制单电机代码问题

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我的目的是希望用摇杆的y轴正半轴控制电机正传，负半轴控制电机反转，但是程序写好以后串口调试的时候，发现正半轴PWM可以输出0~255，负半轴却是0~-255。。。y轴的中间位置是508，508~1023映射给0~255 没有问题，508~0映射给0~255就不行，而且电机始终一个方向在转，希望大神们能帮我看看哪里出了问题，搞了几天，焦头烂额啊，{:soso_e136:} 谢谢。。代码如下：
int JoyStick_X = 0;
int JoyStick_Y = 1;
int INC = 8;
int IND = 9;
int motor = 11;
int up, down,a;
int x,y;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(motor, OUTPUT);
  pinMode(INC, OUTPUT);
  pinMode(IND, OUTPUT);
  
}
void loop() {
  // put your main code here, to run repeatedly:
  x=analogRead(JoyStick_X);
  y=analogRead(JoyStick_Y);
if ( 508<y<=1023 ){
up = map(y,508,1023,0,255);
Serial.println(up);
digitalWrite(INC,HIGH);
digitalWrite(IND,LOW);
analogWrite(motor,up);
} else {
down = map(y,0,508,255,0);
Serial.println(down);
digitalWrite(INC,LOW);
digitalWrite(IND,HIGH);
analogWrite(motor,down);
}
}