This is a read-only archive of an earlier blog posting. Reasons for the
change are at http://blog.sensicomm.com.
The permanent version of this post - with comments (if any) - is at
Linux system clock and hardware clock
This is a followup to the "my Debian Tweaks"
post on on Jan 21. Briefly, my PC tends to spend a lot of time
disconnected from the Internet, and the default clock management as
implemented in /etc/init.d/hwclock.sh
doesn't work very well in that case.
timekeeping background Linux on Intel-based PC's has two
timekeepers, often referred to as the system clock and the hardware
clock. The system clock is based on a high frequency crystal
oscillator and is mainly intended for timing and scheduling
programs and such. It doesn't run when the system is turned off, so
it needs to be initialized with the current time when the PC is
powered on. The hardware clock is a battery-backed circuit that
keeps time even when the PC is off; it provides a value to
initialize the system clock at power up and is rarely used for
anything else. Debian systems assume the system clock is more
accurate than the hardware clock, and they update the hardware
clock every time the system is powered down (see /etc/init.d/hwclock.sh).
So how accurate are the crystal oscillators for
system and hardware clocks?Relative accuracies of these
clocks depend on the specific PC. The only spec I found in a quick
search is 500 parts per million (PPM) in the Intel
HPET timer spec software-developers-hpet-spec-1-0a.pdf.
The system clock may or may not be related to the HPET timer.
Timing crystals are usually spec'ed at 100 PPM or better even for
cheap crystals, so 500PPM sounds like a really loose spec.
The PC hardware clock typically uses a 32.768 kHz watch
crystal and a battery-backed timing circuit (you can see the coin
cell battery in the photos in my Dec 24 2012 posting). Watch
crystals are readily available with 20 PPM accuracy specs, and the
frequency is generally stable with time and temperature. For
example, the CFS206 available from Digikey and other
distributors is 20PPM accuracy, 3PPM per year max drift and .034
PPM per degree C temperature sensitivity. So timekeeping accuracies
of a few PPM should be feasible with some calibration.
The US National Institute of Standards and Technology did an
interesting study of wristwatches, tf.nist.gov/general/pdf/2276.pdf,
and found that even a cheap ($30) modern watch can achieve
better than 1 PPM timekeeping accuracy. Older quartz watches were
orders of magnitude worse, which implies that manufacturers have
started calibrating the crystals in the last few years. So a PC's
hardware clock should be capable of similar performance.
How good are the clocks in my PC?Here's a
quickie experiment with my main PC, an Acer Aspire 5253
notebook with hardware clock updating disabled in
hwclock.sh. I set the hardware clock on Feb 13, 2013, and
it hasn't been touched since. The hardware clock lost 6.227 seconds
in 4609045 seconds (about 53 days), which is an average frequency
offset of 1.35 parts per million (PPM). In contrast the Linux
system clock lost 0.44 seconds in 9103 seconds (about 2.5 hours),
which is 48 PPM.
I checked a couple of older systems and
their hardware clocks are in the 6 to 10 PPM range. So newer PC's
are probably being calibrated like newer watches are. I suspect
that the hardware clock is still more accurate than the system
clock on the older PC's, but the difference should be less
hardware clock in the Acer Aspire is more accurate than the system
clock by a factor of 35, so that's why I don't let hwclock.sh
update the hardware clock on shutdown; instead I manually set the
time using ntpdate-debian every month or so, and manually update
the hardware clock at that time. That keeps me within 3 seconds or
so of true time, which is more than adequate for my purposes.
There are routines (adjtimex and such) that attempt to improve
the calibration of the hardware clock and/or the system clock. They
generally assume the oscillators are off by a fixed percentage from
the true rate and that frequency changes due to temperature and
drift are insignificant. In the 1 PPM range I doubt that that's
true, so I don't bother with adjtimex and friends.
Side Notes I checked my Timex DataLink
USB watch at the same time, and it lost about 2 seconds over the
same interval. So it's averaging around 0.5 PPM.
if you want to spend a few thousand dollars, you can get a Grand
Seiko SBGX059 which is spec'ed at less than 10 seconds
per year, or about 0.3 PPM. Nice, but I think I'll stick with my
Comments on blogspot.com
To make or view comments, see the original post at