Using realtime clock?

Wolfgang Denk wd at
Thu Jan 11 02:06:45 EST 2001

In message < at> Jerry Van Baren wrote:
> The "proper" way to handle a RTC is to read it on power up and set the
> system clock based on it.  From then on, the system clock will be
> correct and everyone will use the system clock efficiently and accurately.

Well, that's the way  it's  often  done,  but  it's  far  from  being
perfect. On many embedded systems I'm working with the RTC is running
from  a high precision oscillator which provides much better accuracy
than the system clock. Both clocks will drift, and usually there will
be a noticable difference after a few days. This is not really nice.

> On the x86 (PC host), the utility is "hwclock" (man hwclock).  Your
> best approach is to get the source for that, modify it to read your RTC
> hardware (which may be different from the PC RTC hardware, although you
> might get lucky and only have to deal with endian issues).  Then add a
> call to it in your startup scripts, typically in /etc/rc.d/rc.local.

Any idea how to keep the system clock permanently synchronized to the

Wolfgang Denk

Software Engineering:  Embedded and Realtime Systems,  Embedded Linux
Phone: (+49)-8142-4596-87  Fax: (+49)-8142-4596-88  Email: wd at
In an infinite universe all things are possible, including the possi-
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                        - Terry Pratchett, _The Dark Side of the Sun_

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