#include <SPI.h>

#define CS 9

/*
Best PWM values
Flight mode 1: 1000
Flight mode 2: 1296
Flight mode 3: 1426
Flight mode 4: 1556
Flight mode 5: 1685
Flight mode 6: 2000
*/

byte buttonPins[] = { 2, 3, 4, 5, 6, 7 };
byte resistanceSettings[] = { 0, 71, 105, 139, 173, 255};

byte activeButtonIndex = 0;

void setup() {
  for (int i = 0; i < sizeof(buttonPins); i++)
    pinMode(buttonPins[i], INPUT_PULLUP);
  pinMode(CS, OUTPUT);
  SPI.begin();
}

void sendValue(byte r) {
  digitalWrite(CS, LOW);
  SPI.transfer(0);
  SPI.transfer(r);
  digitalWrite(CS, HIGH);
}

void loop() {
  for (int i = 0; i < sizeof(buttonPins); i++) {
    if ( ! digitalRead( buttonPins[i] ) )
      activeButtonIndex = i;
  }
  sendValue( resistanceSettings[activeButtonIndex] );
}