motore passo passo unipolare non funzionate con libreria

I'm trying to run a stepper motor with this program

int freq =3; void setup (){ pinMode (3,OUTPUT); pinMode (4,OUTPUT); pinMode (5,OUTPUT); pinMode (6,OUTPUT);

Serial.begin(9600); }

void loop () {

//passo A digitalWrite (6,LOW); digitalWrite (3,HIGH); digitalWrite (4,LOW); digitalWrite (5,LOW);

delay (freq); //passo B digitalWrite (3,LOW); digitalWrite (4,HIGH); digitalWrite (5,LOW); digitalWrite (6,LOW); delay (freq); //passo C digitalWrite (3,LOW); digitalWrite (4,LOW); digitalWrite (5,HIGH); digitalWrite (6,LOW); delay (freq); //passo D digitalWrite (3,LOW); digitalWrite (4,LOW); digitalWrite (5,LOW); digitalWrite (6,HIGH); delay (freq); the engine is running properly as soon as I try to use the library stepper.h the motor starts to vibrate, and n turns ... someone can tell me why?