analogWrite not work with my tiva c

I tried many times but it seem some bug with the energia.

void setup()
{
  pinMode(PF_2,OUTPUT);
  analogWrite (PF_2,250);
}

void loop(){
}

When analogWrite value between 0-254, the PF_2 are 0v.

However, when analogWrite value is 255, PF_2 is 3.3v.

I tried with some other pins but still got the same result. Is it a bug or other problems?

board: TM4C1294XL

thanks