SNTP Module

Since Origin / Contributor Maintainer Source
2015-06-30 DiUS, Johny Mattsson Johny Mattsson sntp.c

The SNTP module implements a Simple Network Time Procotol client. This includes support for the "anycast" NTP mode where, if supported by the NTP server(s) in your network, it is not necessary to even know the IP address of the NTP server.

When compiled together with the rtctime module it also offers seamless integration with it, potentially reducing the process of obtaining NTP synchronization to a simple sntp.sync() call without any arguments.


Attempts to obtain time synchronization.

For best results you may want to to call this periodically in order to compensate for internal clock drift. As stated in the rtctime module documentation it's advisable to sync time after deep sleep and it's necessary to sync after module reset (add it to init.lua after WiFi initialization). Note that either a single server can be provided as an argument (name or address), or a list (table) of servers can be provided.


sntp.sync([server_ip], [callback], [errcallback], [autorepeat]) sntp.sync({ server1, server2, .. }, [callback], [errcallback], [autorepeat])


  • server_ip if non-nil, that server is used. If nil, then the last contacted server is used. This ties in with the NTP anycast mode, where the first responding server is remembered for future synchronization requests. The easiest way to use anycast is to always pass nil for the server argument.
  • server1, server2 these are either the ip address or dns name of one or more servers to try.
  • callback if provided it will be invoked on a successful synchronization, with four parameters: seconds, microseconds, server and info. Note that when the rtctime module is available, there is no need to explicitly call rtctime.set() - this module takes care of doing so internally automatically, for best accuracy. The info parameter is a table of (semi) interesting values. These are described below.
  • errcallback failure callback with two parameters. The first is an integer describing the type of error. The module automatically performs a number of retries before giving up and reporting the error. The second is a string containing supplementary information (if any). Error codes:
  • 1: DNS lookup failed (the second parameter is the failing DNS name)
  • 2: Memory allocation failure
  • 3: UDP send failed
  • 4: Timeout, no NTP response received
  • autorepeat if this is non-nil, then the synchronization will happen every 1000 seconds and try and condition the clock if possible. The callbacks will be called after each sync operation.



Info table

This is passed to the success callback and contains useful information about the time synch that just completed. The keys in this table are:

  • offset_s This is an optional field and contains the number of seconds that the clock was adjusted. This is only present for large (many second) adjustments. Typically, this is only present on the initial sync call.
  • offset_us This is an optional field (but one of offset_s and offset_us will always be present). This contains the number of microseconds that the clock was adjusted.
  • delay_us This is the round trip delay to the server in microseconds. Thie setting uncertainty is somewhat less than this value.
  • stratum This is the stratum of the server.
  • leap This contains the leap bits from the NTP protocol. 0 means that no leap second is pending, 1 is a pending extra leap second at the end of the UTC month, and 2 is a pending leap second removal at the end of the UTC month.


-- Best effort, use the last known NTP server(s) (or the NTP "anycast" address initially)
-- Sync time with some servers from the NTP pool and print the result, or that it failed
sntp.sync({ '', '', '' },
  function(sec, usec, server, info)
    print('sync', sec, usec, server)

See also



Sets the offset between the rtc clock and the NTP time. Note that NTP time has leap seconds in it and hence it runs slow when a leap second is inserted. The setoffset call enables explicit leap second tracking and causes the rtc clock to tick more evenly -- but it gets out of step with wall clock time. The number of seconds is the offset.




  • offset The offset between NTP time and the rtc time. This can be omitted, and defaults to zero. This call enables the offset tracking.




Gets the offset between the rtc clock and the NTP time. This value should be subtracted from the rtc time to get the NTP time -- which corresponds to wall clock time. If the offset returned has changed from the pervious call, then there has been a leap second inbetween.


offset = sntp.getoffset()


The current offset.