- Real-time clock module (RTC) – RTC is a module that keeps time and serves as a primary source of actual time for the clock. The clock can read the time from the module when it needs to and update the time on the RTC when it receives fresh time from the internet. RTC module keeps running even when the clock is powered off, thanks to the supercapacitor power backup – providing time instantly after startup of the clock before connecting to the internet. The time stored in the RTC module will drift over time, so it needs to be synchronized periodically, otherwise, the time displayed on the clock will not be accurate. This is normally done automatically when the clock syncs the time from the internet. In offline mode, it needs to be done manually.
- Supercapacitor power backup – If the clock is powered off, the RTC needs an alternate energy source. For this purpose, the clock uses a supercapacitor that can provide energy for several days, which is enough for common power outages. If the clock is powered off for a longer time, the supercapacitor is eventually drained and the time in the RTC is lost. The supercapacitor is fully charged within a few minutes after the startup of the clock. There is no wear on this part, so it will not deteriorate with use and will never need to be replaced. The supercapacitor provides energy only for the RTC module, not for the nixie tubes or for the main microprocessor – the nixie tubes are off and the clock is reported as offline in the apps when powered off.
Timekeeping in online mode
The NX.2 Clock in online mode periodically synchronizes time from the internet. As long as the internet is available, the clock updates its time. If the internet is not available for a long time (days, weeks), the clock keeps running, but its time will eventually drift. The time is kept for several days even when the clock is powered off thanks to the RTC module with supercapacitor power backup.
It first tries to obtain the current time from its internal real-time clock module (RTC) when the clock starts up. If the time is unavailable (the clock was powered off for more than a couple of days and the supercapacitor backup is discharged), the clock assumes 00:00 and will show zeros at the display. A few seconds after the start, the clock will attempt to connect to a known wifi network. If it succeeds, it sets the time from the internet and stores it also in the RTC. That initial time accuracy is somewhere around 1 second. You can observe that on accurate time sources, e.g. https://time.is. After another 10 seconds, the clock will start NTP time synchronization from an NTP server closest to your location. Using the NTP service, the clock will increase its accuracy to tens of milliseconds off from the actual time. The time is periodically updated via NTP in longer intervals during the operation of the clock.
Timekeeping in offline mode
The clock in offline mode keeps time in RTC as well, the time can only be synchronized via Direct connect from mobile apps (iOS / Android). We are working on adding that feature to the “configuration website:https://docs.daliborfarny.com/nx-2/1/en/topic/configuration-website as well. When you synchronize the time from the mobile device, the clock is supplied with the time from the device.