#include <SPI.h>
const int slaveSelectPin = 10;

void setup() {
  pinMode (slaveSelectPin, OUTPUT);
  SPI.begin();
}

void loop(){
  for (unsigned int level = 0; level < 1024; level++) {
    digitalPotWrite(0, level);
  }
  delay(100);
}

int digitalPotWrite(unsigned int wiper, unsigned int value){
  //00ww 00 vv vvvv vvvv
  //AAAA CC DD DDDD DDDD
  value &= 0x03FF; // AND off the non-data bits
  wiper &= 0x0003; // AND off high wiper bits
  value |= (wiper << 12); // shift the wiper bits
  digitalWrite(slaveSelectPin,LOW);
  SPI.transfer( (byte)(value >> 0) );
  SPI.transfer( (byte)(value >> 8) );
  digitalWrite(slaveSelectPin,HIGH);
}