Which part of the Android source code determines initial Android time when there is no time source?

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I am an engineer currently working with the Android P's system time setting/synchronization.

When we build Android and load it for the very first time we can see an initial time has already been set(whether it is correct time or not). I have no idea where to look at for getting the initial time setting without any time source activated(wifi, GPS, etc.). Rough guess is that maybe this time reference can be obtained from the linux time info, but as I said it is a just a guess and I do not know how I can prove/check the code for this also.

Can someone give me an enlightenment? Thanks.

android-9.0-pie embedded-linux time c++ java
2 Answers

As an electronics engineer, I will try to answer this question in more realistic way.

There is a special part in main board (called RTC) which is designed just to keep track of time. As @Lie Ryan stated, it has its own battery and relatively larger capacitor. In addition there is a special chip, which basically functions as a counter. You can initialize it, load new data, etc. That chip is powered by the special battery (usually very small). The chip can get input from a crystal and CPU. Once the system is assembled and the battery is placed, even the main system has no battery, it starts counting. The value stored in it keeps increasing. It takes crystal frequency as a reference frequency, makes it a clock signal, and uses it as its reference clock. The value of the counter increases at each fixed number of crystal oscillations (clock signal). When you power on the system, assuming the system is set to read the real time from RTC, it reads the value stored in that chip and decodes it in a format the system was set.

If the value you get is wrong, after the time is set from internet or manually, you load new data, and the chip continues counting from that new reference. Because of the imperfections of the real world system (crystal oscillation frequency is given with percentage error), if you don't set the time for long enough period, the time of your system might not be synchronized with real time (as you might have practiced). Or if you wait long enough, the battery gets out of power and the system does not work.

user_94907770

Pretty much any modern CPU/motherboard have an RTC (real time clock), this is a component that maintains time independently of any external time source. In absence of an external time synchronisation source, this is the source of truth about time.

The RTC is usually powered by its own battery/supercapacitor, so it will keep running even if you disconnect the main battery.

In the underlying Linux device filesystem, this device is usually exposed as /dev/rtc*. Documentation.

user_3094120
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