In a common network one should have the same time on all machines – be them servers, workstations or small controllers, like Pis for real time control as described, e.g. in this in publication.
The rationale: You want

  • comparable file dates and
  • log file entries that can be put in order over several systems.   You have
  • distributed controller with co-ordinated timed actions.

These time dependent features would fail if internal time of the systems differs too much. Hence, we have a hard or soft real time requirement on the synchronisation of those system times. And as ever, “real time” does nothing say on absolute values. The maximum tolerable system time difference of two of your systems might be something between 50ns and 2min – it depends. Anyway, if after a system after reboot does not know hour nor date abandon all hope.

NTP server

The proven way to synchronous system times is having all systems use the same NTP server or the same ordered set of servers. Such NTP server must be in site i.e. in the local (W)LAN. It may get its time from an external server.

Reasons for a common (first) NTP server being internal are (probably) more stable transmission times and reachability for all. Not all computers or embedded systems should have access to outside servers.

The NTP server role is usually given to domain controllers usually having the internal IP p.r.v.1 to p.v.r.4.
In private homes or small companies DNS (and DHCP) roles are often given the provider’s or private router. In Germany this will often be a fritz.box, IP 192.168.178.1.

NTP failure

If NTP fails in our local site drift will start. A system affected rebooting would start with gaga time. To avoid this total de-synchronisation when NTP servers or the connections to them fail a redundant time source is necessary.

Real time clocks

Therefore often so called “real time clocks” are used. They are battery powered quartz clocks holding date also without hands but with an interface to the processor/controller to be read and set. As long as the setting by NTP fails they drift as every over non-synchronised clocked and if the battery gets low they drift faster or fail totally.

If by design such “real time clock” is incorporated in the hardware and known to the OS without any configuration (as is with standard PCs) use it.
But think twice before fitting such thing to an (embedded) controller serially without. One might get troubles to get/keep the thing working from beginning or after updates. And checking/replacing a battery in some places where those little systems have to dwell might be a nightmare.

DCF77 receiver

A DCF77 receiver is usable in large parts of central Europe. Its AM signal gives an exact second clock, and these pulses yield a low frequency (1 bit /s) data stream. This gives the exact CET/CEST date and time information as well as DST on or off and leap seconds announcement. Hence within a minute after start of receiving or reboot one has the atomic standard time. Properties:
  o   needs one digital port of the controller
  -   usually no OS support   -   own DCF77 software / (sudo) application needed (not rocket science)
  +   no battery needed   +   cheap but reliable DCF77 AM receiver modules available
  -   those (cheap) modules only demodulate the AM and not DCF77’s PM modulation (less subject to EMI).   +   the module might be assembled together and supplied from the controller (a Raspberry Pi e.g.)
  +   the module might as well be put several meters away from the controller.
  +   the slow (1 bit/s) signal from one receiver might easily be distributed to many controllers.
And the biggest +: Any NTP server in Europe with any sense will in the end PTB’s atomic clocks and hence DCF77 time. A systems with time set by DCF will be in quite good sync with a NTP used later, i.e. will never have make big jumps to correct time.

The good accordance of DCF77 and NTP does not hold on an hour or even a day before a leap second if the NTP server uses the so called leap second smearing (as most do) meaning intentionally delivering wrong time. As DCF77 delivers an announcement in the last hour before leap second xx:59:60 one knows it without having read IERS bulletin C.

Sofar we shared we shared the considerations for choosing DCF77 instead of battery powered “real time clocks” as second time source for our embedded/distributed controller projects mostly with Raspberry Pis.

DCF77 implementation with a Pi

Connecting a DCF77 receiver module to a Raspberry Pi and implementing the decoding in C will be reported on later – here or in a separate publication.