[Linux-based NVRs] RTC mechanism and default time setting

Question

In an environment where there is only internal network, it is common that no NTP (network time protocol) server exists, thus customers sometimes have to set system time manually. This article will explain RTC (real-time clock) mechanism and default time setting of our Linux-based NVRs so that we can quickly classify the the issue if customer reports any.

Answer

This article will explain the time-related issues in three sections. First, the relation between RTC and system time of Linux will be mentioned. Second, the manufacturing process relating default time-setting will be mentioned. Last, will join the 1 and 2 sections to give an overall picture of how the time going in Linux-based NVRs

1. RTC (real-time clock) and system time:

A real-time clock (RTC) is a computer clock (most often in the form of an integrated circuit) that keeps track of the current time. The system time is maintained by Linux kernel.

All Linux-based NVRs use cell battery for RTC. For example,

Model name	Cell battery	Quantity	Theoretical lifetime
ND8322P	CR2032	210mAh	Over 24 years
ND9322P	CR1220	40mAh	Over 11 years

The cell battery with the smallest quantity used in Linux-based NVR is CR1220. Below pictures show where the cell battery locates in an ND9541 and how it looks like:

2. The default time-setting of Linux-based NVR:

When producing NVRs, there is a program that tests the functionality of the NVR, including the RTC and system time check. The RTC check will set the system time to 2013-01-01 00:00:00 (UTC+0). After that, it will become 2013-01-01 08:00:00 (UTC+8). The system time check will write the current time of the computer to the NVRs.

 

The first time a user starts the NVR (factory default), the system will first read the time from the RTC and then send an NTP request. If the NVR succeeds in getting the time from the NTP server, the system time and RTC will be changed simultaneously.

 

If there are some problems with the RTC so that the system cannot read the time saved in it, it will be set to 2010-01-01 08:00:00 (UTC+8).

3. With 1 and 2 in mind, now we can draw the picture of how the time in the NVR changes after its production as below: