Arduino: Ethernet NTP Time Sync

Code

#include <Time.h>
#include <SPI.h>
#include <Ethernet.h>
#include <EthernetUdp.h>

#define SD_CS 4

// Ethernet library configuration
byte mac[] = { 0x24, 0x7B, 0xA2, 0x4A, 0x52, 0x10 };
IPAddress timeServer(128, 138, 140, 44 ); // time.nist.gov

/* ALTER THESE VARIABLES AT YOUR OWN RISK */
unsigned int localPort = 8888;
const int NTP_PACKET_SIZE= 48;     
byte packetBuffer[NTP_PACKET_SIZE]; 
EthernetUDP Udp;                   

void setup() {               

  // disable SD SPI Chip Select
  pinMode(SD_CS,OUTPUT);

  Serial.begin(9600);
  Serial.println("Wait for DHCP Server");
  if (Ethernet.begin(mac) == 0) {  // start Ethernet by DHCP conf.
    Serial.println("DHCP config failed! ");
  }
  Serial.print("NTP Server: ");
  Serial.println(timeServer);

  setSyncInterval(5); // Set seconds between re-sync (5s for test only) 
  Udp.begin(localPort);
}
 
void loop() {
  setSyncProvider(getNtpTime);
  digitalClockDisplay();
  delay(1000);
}

void digitalClockDisplay(){
  // digital clock display of the time
  Serial.print(day());
  Serial.print(".");
  Serial.print(month());
  Serial.print(".");
  Serial.print(year());
  Serial.print(" ");
  Serial.print(hour());
  printDigits(minute());
  printDigits(second());
  Serial.println(" ");
}

void printDigits(int digits){
  // utility function for digital clock display: prints preceding
  // colon and leading 0
  Serial.print(":");
  if(digits < 10)
    Serial.print('0');
  Serial.print(digits);
}

/*-------- NTP code ----------*/

unsigned long getNtpTime()
{
  sendNTPpacket(timeServer); // send an NTP packet to a time server
    delay(500);
   
    if ( Udp.parsePacket() ) {
     Udp.read(packetBuffer,NTP_PACKET_SIZE);  // read packet into buffer  

     //the timestamp starts at byte 40, convert four bytes into a long integer
     unsigned long hi = word(packetBuffer[40], packetBuffer[41]);
     unsigned long low = word(packetBuffer[42], packetBuffer[43]);
     // this is NTP time (seconds since Jan 1 1900
     unsigned long secsSince1900 = hi << 16 | low; 
     // Unix time starts on Jan 1 1970
     const unsigned long seventyYears = 2208988800UL;     
     unsigned long epoch = secsSince1900 - seventyYears;  // subtract 70 years, add Daylight Saving and Timezone adjust
     return epoch;
  }
  return 0; // return 0 if unable to get the time
}

// send an NTP request to the time server at the given address
unsigned long sendNTPpacket(IPAddress& address)
//sendNTPpacket(IPAddress address)
{
  memset(packetBuffer, 0, NTP_PACKET_SIZE);  // set all bytes in the buffer to 0 

  // Initialize values needed to form NTP request
  packetBuffer[0] = B11100011;   // LI, Version, Mode
  packetBuffer[1] = 0;     // Stratum
  packetBuffer[2] = 6;     // Max Interval between messages in seconds
  packetBuffer[3] = 0xEC;  // Clock Precision
  // bytes 4 - 11 are for Root Delay and Dispersion and were set to 0 by memset
  packetBuffer[12]  = 49;  // four-byte reference ID identifying
  packetBuffer[13]  = 0x4E;
  packetBuffer[14]  = 49;
  packetBuffer[15]  = 52;

  // send the packet requesting a timestamp:
  Udp.beginPacket(address, 123); //NTP requests are to port 123
  Udp.write(packetBuffer,NTP_PACKET_SIZE);
  Udp.endPacket();
}