DAC sine wave on an Arduino Uno

// An MCP4725 DAC: a real analog voltage out of an I2C bus.
//
// analogWrite() on an Uno is PWM, a square wave that only averages to a
// voltage. The MCP4725 puts out the voltage itself: VOUT = 5 V x code / 4096,
// in 4096 steps. Here it plays a slow sine, and its output also goes to A0,
// so the sketch can read back what it made.
//
//   MCP4725  SDA, SCL -> SDA, SCL (A4, A5)   VCC -> 5V, GND -> GND
//            VOUT -> A0, and to the scope's channel 1
//   pin 9 -> the scope's channel 2: the same sine as PWM, for comparison
//
// Every 20 ms the sketch writes the next point of a 32-point sine. Every
// eighth point it prints the code, the volts that should come out, and what
// A0 reads. Pin 9 plays the same sine with analogWrite(): on the scope,
// channel 1 is a smooth staircase and channel 2 a square wave whose width
// follows it.
//
// THINGS TO TRY. Change STEP_MS to 1 and the sine is 30 Hz. Or wire A0 on
// the DAC to VCC: it moves to 0x61 and begin() at 0x60 finds nothing.

#include <Wire.h>
#include <Adafruit_MCP4725.h>

const uint8_t DAC_ADDRESS = 0x60;
const unsigned long STEP_MS = 20;

Adafruit_MCP4725 dac;

// One cycle of a sine, 32 points, centered on 2048.
const uint16_t SINE[32] = {
  2048, 2447, 2831, 3185, 3495, 3750, 3939, 4056,
  4095, 4056, 3939, 3750, 3495, 3185, 2831, 2447,
  2048, 1648, 1264, 910, 600, 345, 156, 39,
  0, 39, 156, 345, 600, 910, 1264, 1648,
};

uint8_t i = 0;

void setup() {
  pinMode(9, OUTPUT);
  Serial.begin(9600);
  if (!dac.begin(DAC_ADDRESS)) {
    Serial.println("No MCP4725 at 0x60: check SDA, SCL and A0.");
    while (1) {}
  }
  Serial.println("MCP4725 found at 0x60");
}

void loop() {
  uint16_t code = SINE[i];
  dac.setVoltage(code, false);
  analogWrite(9, code >> 4);    // 0 to 4095 down to 0 to 255
  if (i % 8 == 0) {
    delay(2);
    int a0 = analogRead(A0);
    Serial.print("code ");
    Serial.print(code);
    Serial.print(" = ");
    Serial.print(5.0 * code / 4096, 2);
    Serial.print(" V, A0 reads ");
    Serial.print(a0 * 5.0 / 1023, 2);
    Serial.println(" V");
  }
  i = (i + 1) % 32;
  delay(STEP_MS);
}