#include <Adafruit_MCP4728.h>
#include <Wire.h>

Adafruit_MCP4728 mcp;


long int last_


void setup(void) {
  Serial.begin(115200);
  while (!Serial)
    delay(10); // will pause Zero, Leonardo, etc until serial console opens

  if (!mcp.begin(0x64)) {
    Serial.println("Failed to find MCP4728 chip");
    while (1) {
      delay(10);
    }
    Serial.println("MCP4728 initialised");
  }

  mcp.setChannelValue(MCP4728_CHANNEL_B, 0);  
}


void loop() {
  int pot = analogRead(A0);
  float rate = 2000 * (float)pot / 1024.0 + 0.01;
  long int t = millis();
  float v = 0.5 + 0.5 * sin((float)t / rate);
  mcp.setChannelValue(MCP4728_CHANNEL_A, round(v * 4095));  
}