File content as of revision 8:45a0205a298f:

#include <mbed.h>
#include  "log.h"
#include  "esp.h"
#include "wifi.h"
#include "time.h"
#include   "io.h"
#include   "at.h"
#include  "cfg.h"

#define GET_TIME_INTERVAL  600
#define    RETRY_INTERVAL   60

#define ID 0

#define SIXTEENTHS_TO_ADD 1

static Timer timer;

struct Packet {
/*
    LI:  00 no warning; 01 last minute has 61 seconds; 10 last minute has 59 seconds; 11 alarm condition (clock not synchronized)
    VN:   3 = 011
    Mode: 3 = 011 for client request; 4 = 100 for server reply
       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |LI | VN  |Mode |    Stratum    |     Poll      |   Precision   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0 0 0 1|1 0 1 1|
    
    Mode: If the Mode field of the request is 3 (client), the reply is set to 4 (server).
          If this field is set to 1 (symmetric active), the reply is   set to 2 (symmetric passive).
          This allows clients configured in either client (NTP mode 3) or symmetric active (NTP mode 1) to
          interoperate successfully, even if configured in possibly suboptimal ways.
          
    Poll: signed integer indicating the minimum interval between transmitted messages, in seconds as a power of two.
    For instance, a value of six indicates a minimum interval of 64 seconds.
    
    Precision: signed integer indicating the precision of the various clocks, in seconds to the nearest power of two.
    The value must be rounded to the next larger power of two;
    for instance, a 50-Hz (20 ms) or 60-Hz (16.67 ms) power-frequency clock would be assigned the value -5 (31.25 ms),
    while a 1000-Hz (1 ms) crystal-controlled clock would be assigned the value -9 (1.95 ms).
    
    Root Delay (rootdelay): Total round-trip delay to the reference clock, in NTP short format (16bitseconds; 16bit fraction).

    Root Dispersion (rootdisp): Total dispersion to the reference clock, in NTP short format (16bitseconds; 16bit fraction)..
*/
    union
    {
        uint32_t FirstLine;
        struct
        {
            unsigned Mode : 3;
            unsigned VN   : 3;
            unsigned LI   : 2;
            uint8_t  Stratum; 
             int8_t  Poll;
             int8_t  Precision;
        };
    };
    uint32_t RootDelay;
    uint32_t Dispersion;
    uint32_t RefIdentifier;
    
    uint64_t RefTimeStamp;
    uint64_t OriTimeStamp;
    uint64_t RecTimeStamp;
    uint64_t TraTimeStamp;
};
static struct Packet packet;

static uint64_t ntohll(uint64_t n) {
    int testInt = 0x0001; //Big end contains 0x00; little end contains 0x01
    int *pTestInt = &testInt;
    char *pTestByte = (char*)pTestInt;
    char testByte = *pTestByte; //fetch the first byte
    if (testByte == 0x00) return n; //If the first byte is the big end then host and network have same endianess
    
    union ull
    {
        uint64_t Whole;
        char Bytes[8];
    };
    union ull h;
    h.Whole = n;
    
    char t;
    t = h.Bytes[7]; h.Bytes[7] = h.Bytes[0]; h.Bytes[0] = t;
    t = h.Bytes[6]; h.Bytes[6] = h.Bytes[1]; h.Bytes[1] = t;
    t = h.Bytes[5]; h.Bytes[5] = h.Bytes[2]; h.Bytes[2] = t;
    t = h.Bytes[4]; h.Bytes[4] = h.Bytes[3]; h.Bytes[3] = t;
    
    return h.Whole;
}
uint64_t getTimeAsNtp()
{
    uint64_t ntpTime = 2208988800ULL << 32;
    ntpTime += TimeGet16ths() << 28;
    return ntpTime;
}
void setTimeAsNtp(uint64_t ntpTime)
{
    ntpTime -= 2208988800ULL << 32;
    uint64_t time16ths = (ntpTime >> 28) + SIXTEENTHS_TO_ADD;
    TimeSet16ths(time16ths);
}
void NtpInit()
{
    EspIpdReserved[ID] = true;
}
int preparePacket()
{        
    memset(&packet, 0, sizeof(packet));
    packet.LI   = 0;
    packet.VN   = 1;
    packet.Mode = 3; //Client
    packet.TraTimeStamp = ntohll(getTimeAsNtp());    
    return 0;
}
int handlePacket()
{        
    
    //Handle the reply  
    char leap    = packet.LI;
    char version = packet.VN;
    char mode    = packet.Mode;
    char stratum = packet.Stratum;
    if (leap    == 3) { LogF("Remote clock has a fault\r\n");                     return -1; }
    if (version  < 1) { LogF("Version is %d\r\n", version);                       return -1; }
    if (mode    != 4) { LogF("Mode is %d\r\n", mode);                             return -1; }
    if (stratum == 0) { LogF("Received Kiss of Death packet (stratum is 0)\r\n"); return -1; }
    
/*
    See http://www.eecis.udel.edu/~mills/time.html for timestamp calculations
        Ori
    ----t1---------t4---- RTC
          \       /
    -------t2---t3------- NTP
           Rec  Tra
    offset (RTC - NTP) = (t1 + t4)/2 - (t2 + t3)/2 ==> [(t1 - t2) + (t4 - t3)] / 2
    delay              = (t4 - t1)   - (t3 - t2)
*/

    //Set the RTC
    setTimeAsNtp(ntohll(packet.RecTimeStamp));
    
    return 0;
}
int NtpIdConnectStatus = AT_NONE;
static void outgoingMain()
{
    if (AtBusy()) return;
    if (!WifiStarted()) return;
    
    static int firstAttempt = true;
    static int result = AT_NONE;
         
    if (NtpIdConnectStatus == AT_SUCCESS)
    {
        int retryAfterFailure =  timer.read() > RETRY_INTERVAL && result != AT_SUCCESS;
        int repeat            =  timer.read() > GET_TIME_INTERVAL;
        
        if (firstAttempt || retryAfterFailure || repeat)
        {
            preparePacket();
            AtSendData(ID, sizeof(packet), &packet, &result);
            firstAttempt = false;
            timer.reset();
            timer.start();
        }
    }
    else
    {
        AtConnectId(ID, "UDP", CfgNtpIp, 123, &packet, sizeof(packet), &NtpIdConnectStatus);
    }
}
static void incomingMain()
{
    if (EspDataAvailable == ESP_AVAILABLE && EspIpdId == ID)
    {
        if (EspIpdLength == sizeof(packet)) handlePacket();
        else LogF("Incorrect NTP packet length of %d bytes", EspIpdLength);
    }
}
int NtpMain()
{
    outgoingMain();
    incomingMain();
    return 0;
}