user15855: While I was researching ideas for an Arduino clock, I found several clocks that incorporated an RTC (Real Time Clock) module.
I don't know why it is used in some clocks.
So, here are my questions:

How does an RTC module work?
Do I have to use it "like a rule" (is an RTC necessary or unnecessary) in my clock?


user10907: RTC modules are used because they make it easier to keep accurate time. They do this by maintaining time when the system is shut down or rebooted (by way of a battery or some other mechanism for keeping running independently of the host system) and by providing a time base that is, usually, more accurate than that of the host.
Another way to keep very accurate time (potentially better than you can get with all but the best RTC modules) is to get your time from a GPS or from the Internet using NTP. If you're going to use a GPS anyway or need an Internet connection then either of those approaches are very good ways to go. If you don't, then they are considerably more expensive than an RTC module.

user10794: I built a clock designed a while ago by Geoff Graham, who synchronized to a GPS around every 24 hours, and used the internal clock in-between. The GPS corrected for drift in the internal clock (and, of course, gave you the time accurately in the process).
This is more expensive than a RTC, but more accurate. If you also had an RTC it would drift less during the day. Or, you could read from the GPS all the time and not have an RTC.
I made a GPS clock myself which does that - just shows the GPS time.

Higher-contrast image:

Internals:


This is another project that uses an RTC clock chip (indicated on photo):

However if you go for just an RTC chip you can expect it to drift over time. They are more accurate than counting processor ticks (and survive a power outage because of the backup battery) but are not as accurate as the GPS.
Those clock chips are pretty cheap. I got 30 of them for $10 from eBay.

user4143: 
How does a RTC module works?

It counts. As simple as that. Once a second it ticks and counts up. It also remembers - most have a battery backup and will continue ticking when you turn the main power off.

I have to use it "like a rule" (if it is necessary or unnecesary) in my clock?

If you want accuracy and continuation, yes.  An RTC is designed to tick at (as close as reasonably possible) once per second. Trying to do that with just an Arduino is incredibly difficult. The ceramic resonator on an Arduino is not as accurate as the quartz crystals used with an RTC. After a week you may find you are an hour out.

15855: <p>While I was researching ideas for an Arduino clock, I found several clocks that incorporated an RTC (Real Time Clock) module.</p>

<p>I don't know why it is used in some clocks.</p>

<p>So, here are my questions:</p>

<ul>
<li>How does an RTC module work?</li>
<li>Do I have to use it "like a rule" (is an RTC necessary or unnecessary) in my clock?</li>
</ul>
 10907: <p>RTC modules are used because they make it easier to keep accurate time. They do this by maintaining time when the system is shut down or rebooted (by way of a battery or some other mechanism for keeping running independently of the host system) and by providing a time base that is, usually, more accurate than that of the host.</p>

<p>Another way to keep very accurate time (potentially better than you can get with all but the best RTC modules) is to get your time from a GPS or from the Internet using NTP. If you're going to use a GPS anyway or need an Internet connection then either of those approaches are very good ways to go. If you don't, then they are considerably more expensive than an RTC module.</p>
 10794: <p>I built a clock designed a while ago by Geoff Graham, who <a href="http://geoffg.net/GPS_Synchronised_Clock.html" rel="nofollow noreferrer">synchronized to a GPS</a> around every 24 hours, and used the internal clock in-between. The GPS corrected for drift in the internal clock (and, of course, gave you the time accurately in the process).</p>

<p>This is more expensive than a RTC, but more accurate. If you also had an RTC it would drift less during the day. Or, you could read from the GPS all the time and not have an RTC.</p>

<p>I made a <a href="http://www.gammon.com.au/forum/?id=11991" rel="nofollow noreferrer">GPS clock</a> myself which does that - just shows the GPS time.</p>

<p><a href="https://i.stack.imgur.com/aWQyV.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/aWQyV.jpg" alt="GPS clock"></a></p>

<p>Higher-contrast image:</p>

<p><a href="https://i.stack.imgur.com/NhA5R.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/NhA5R.jpg" alt="GPS clock face"></a></p>

<p>Internals:</p>

<p><a href="https://i.stack.imgur.com/r9MOy.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/r9MOy.jpg" alt="Inside GPS clock"></a></p>

<hr>

<p>This is <a href="http://www.gammon.com.au/forum/?id=12106" rel="nofollow noreferrer">another project</a> that uses an RTC clock chip (indicated on photo):</p>

<p><a href="https://i.stack.imgur.com/2EZ5o.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/2EZ5o.png" alt="Temperature sensor"></a></p>

<p>However if you go for just an RTC chip you can expect it to drift over time. They are more accurate than counting processor ticks (and survive a power outage because of the backup battery) but are not as accurate as the GPS.</p>

<p>Those clock chips are pretty cheap. I got 30 of them for $10 from eBay.</p>
 4143: <blockquote>
  <p>How does a RTC module works?</p>
</blockquote>

<p>It counts. As simple as that. Once a second it ticks and counts up. It also remembers - most have a battery backup and will continue ticking when you turn the main power off.</p>

<blockquote>
  <p>I have to use it "like a rule" (if it is necessary or unnecesary) in my clock?</p>
</blockquote>

<p>If you want accuracy and continuation, yes.  An RTC is designed to tick at (as close as reasonably possible) once per second. Trying to do that with just an Arduino is incredibly difficult. The ceramic resonator on an Arduino is not as accurate as the quartz crystals used with an RTC. After a week you may find you are an hour out.</p>
