RTC accuracy

VanguardLH wrote:

http://www.maxim-ic.com/view_press_r...elease_id/1101
Depends on how old is your motherboard and its design as to whether it
utilizes a much more accurate (i.e., lower PPM error rate) clock. The
one described here (published in 2005) has a very low error rate that
results in a discrepancy of 2 minutes per year.

I use an NTP client to sync my clock once per hour.

As you experience more and more birthdays, precise time becomes less
compelling. I'm satisfied knowing what month it is!

On Tue, 02 Jun 2009 20:03:42 -0400, Bryce
wrote:

VanguardLH wrote:

http://www.maxim-ic.com/view_press_r...elease_id/1101
Depends on how old is your motherboard and its design as to whether it
utilizes a much more accurate (i.e., lower PPM error rate) clock. The
one described here (published in 2005) has a very low error rate that
results in a discrepancy of 2 minutes per year.

I use an NTP client to sync my clock once per hour.

As you experience more and more birthdays, precise time becomes less
compelling. I'm satisfied knowing what month it is!


It's June... 2007

Bryce wrote:

As you experience more and more birthdays, precise time becomes less
compelling. I'm satisfied knowing what month it is!

When I was much younger and working 12-hours days on a rotating shift, I
was always losing what day of the week it was. Now I'm 30 years older
and nowhere as busy but I'm again losing what day of the week it is.
I'm still doing okay on the months, though.

"VanguardLH" wrote in message
...
pawihte wrote:

My computer RTC is quite accurate as computer clocks go, being
off by no more than a few seconds over a period of several
days
without syncing, whereas I've noticed some computers to be off
by
minutes in 24 hours even with a healthy battery.

I've heard that the accuracy of a computer clock not only
depends
on the oscillator in the RTC hardware, but is also influenced
by
interrupt calls (or something like that) in OS and app
environment. If this is true, do I just happen to have a good
RTC
hardware or does my OS and software installation also have
something to do with it?

Saying you have an RTC component in your system says nothing
about what
devices it actually uses or the design of its circuit (outside
the chip
and what crystal is used).

http://en.wikipedia.org/wiki/Real-time_clock

http://www.intel.com/Assets/PDF/appnote/292276.pdf
Sections 3.3 and 3.4. They don't give actual accuracy
measurements
because so much depends on the circuit design and environmental
conditions. A lot depends on the quality of the crystal used
and its
variation over temperature and its drift (seconds of drift per
month).

http://www.maxim-ic.com/appnotes.cfm/an_pk/504
"An error of 23ppm is about 1 minute per month."

http://www.maxim-ic.com/appnotes.cfm/an_pk/632
"PC clocks are not particularly good at keeping accurate time.
Simple
clocks like a wris****ch and most of the clocks in your home
keep better
time than a standard PC clock."

http://www.maxim-ic.com/view_press_r...elease_id/1101
Depends on how old is your motherboard and its design as to
whether it
utilizes a much more accurate (i.e., lower PPM error rate)
clock. The
one described here (published in 2005) has a very low error
rate that
results in a discrepancy of 2 minutes per year.


I'm not sure I get what you're trying to say here. The point of
my post was that as I seem to have an accurate clock (as computer
clocks go), I wondered if this is entirely due to the hardware or
or partly to my software installations and usage.

I use an NTP client to sync my clock once per hour.

For the level of accuracy I need, the once-a-week auto-sync
provided by WinXP is good enough for me with my current
motherboard. In fact, I think I'll turn off auto-sync and see how
far off it goes.

pawihte wrote:


For the level of accuracy I need, the once-a-week auto-sync
provided by WinXP is good enough for me with my current
motherboard. In fact, I think I'll turn off auto-sync and see how
far off it goes.


This thread has some fun and games with clocks, and notes a couple
applications you might try out. This thread is associated with the
Nforce2 and its problems maintaining anything resembling a correct
software clock.

http://www.nforcershq.com/forum/real...19631-190.html

The ClockMon one has moved here.

http://www.softdevlabs.com/ClockMon/ClockMon.html

For the small pieces of software, I usually throw them into the grinder here.
Seems clean. MD5SUM = 6a05b340f7b9431f68dfd5496f42f079 for ClockMon.2.3.0.291.zip
(MD5SUM is so people can compare the download at a later date.)

http://www.virustotal.com/analisis/c...177-1244019029

Clockmon uses "giveio.sys" for hardware access. That is a way of gaining
access to hardware. When you're finished with Clockmon, you can check
the hidden devices in Device Manager, and it might be in there. On
my other machine, I collected quite a pile of cruft in the hidden
devices, and I don't understand the security implications of having
too much stuff like that.

Have fun,
Paul

pawihte wrote:

I'm not sure I get what you're trying to say here. The point of
my post was that as I seem to have an accurate clock (as computer
clocks go), I wondered if this is entirely due to the hardware or
or partly to my software installations and usage.

The point made by another poster is that the RTC chip is only used to
keep track of time for the hardware and is used when you boot up. It si
NOT used by the operating system. If you severely stress the OS and/or
depending on the accuracy of the software clock therein, you will lose
more time while the host is running than when it is not. The RTC chip
is accurate. The OS is not hence the need to do a periodic time sync.

If you leave your computer powered up 24x7 then, yeah, the RTC clock is
still accurate but it's not getting used.

I use an NTP client to sync my clock once per hour.

For the level of accuracy I need, the once-a-week auto-sync
provided by WinXP is good enough for me with my current
motherboard. In fact, I think I'll turn off auto-sync and see how
far off it goes.

Once per week might be okay if your clock does get off by more than a
minute in that time. SSL connects rely on the two end in a connection
to have nearly the same time. This is used to time out the handshaking
used to establish an SSL session. If your time is too far off from the
other end's clock, you won't be able to make SSL connects to it.

I don't worry much about timestamps on files being super accurate. I do
want to ensure my time is accurate so that I don't run into problems
connecting to SSL-secured sites. I don't recall how much different the
time can be but it is encoded into the hash code using in the SSL
handshaking sequence. SSL connects aren't only for web sites. You
might be using SSL for your e-mail connects. If the times are too far
apart between you and the mail server, your e-mail client might start
reporting vague errors, like "Your outgoing (SMTP) server does not
support SSL-secured connections". That's because the handshaking failed
due to too much difference in timestamps, not because the mail server
does not support SSL connections.

VanguardLH wrote:
Bryce wrote:

As you experience more and more birthdays, precise time becomes
less compelling. I'm satisfied knowing what month it is!

When I was much younger and working 12-hours days on a rotating
shift, I was always losing what day of the week it was. Now I'm
30 years older and nowhere as busy but I'm again losing what day
of the week it is. I'm still doing okay on the months, though.

I have a simpler method of telling time. If the radio starts
spouting BBC news at 5 a.m. it's a weekday. Otherwise it is
Saturday or Sunday. On Saturday it doesn't spout anything until 6
am.

Regardless, I get breakfast and go back to bed until noon. Except
on Monday and Tuesday, when I have an alarm set for 8 am, to go and
play bridge at 9.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: http://cbfalconer.home.att.net
Try the download section.

"pawihte" wrote in message
...
My computer RTC is quite accurate as computer clocks go, being off by no
more than a few seconds over a period of several days without syncing,
whereas I've noticed some computers to be off by minutes in 24 hours even
with a healthy battery.

I've heard that the accuracy of a computer clock not only depends on the
oscillator in the RTC hardware, but is also influenced by interrupt calls
(or something like that) in OS and app environment. If this is true, do I
just happen to have a good RTC hardware or does my OS and software
installation also have something to do with it?




I had one PC that would maintain good accuracy when left running on a Bios
screen or booted from a Linux CD or for that matter if left off.
Time keeping in Windows was just as good until 'one' particular application
was started when it would immediately start loosing 2-3 hours per day.

As it was a till monitoring system it needed to be running most of the day.
I wasn't convenient to reinstall the OS and the software so I just set it to
get it's time from another PC on the LAN that wasn't lumbered with the
problem software.
Eventually a new PC was installed and the problem went away.

So all that goes to confirm software can interfere with the RTC.

Best
Paul.

PeeCee wrote (on Fri, 12 Jun 2009 16:36:18 +1200):

I had one PC that would maintain good accuracy when left running on a Bios
screen or booted from a Linux CD or for that matter if left off.
Time keeping in Windows was just as good until 'one' particular application
was started when it would immediately start loosing 2-3 hours per day.

As it was a till monitoring system it needed to be running most of the day.

Which means the RTC was *not* being used to maintain the clock. You are
using the OS clock at that point, NOT the real-time chip on the mobo
that is used to keep track of time while the host is powered down.

I wasn't convenient to reinstall the OS and the software so I just set it to
get it's time from another PC on the LAN that wasn't lumbered with the
problem software.
Eventually a new PC was installed and the problem went away.

So all that goes to confirm software can interfere with the RTC.

No, you just proved the point that intensive applications running under
the OS can affect the OS clock. That does nothing regarding the time
being recorded in the RTC. If you had a host whose system clock was way
off and powered it down and back up, it syncs back to the time that is
being separately tracked by the RTC.

The "system clock" in Windows is Windows maintaining that counter. It
doesn't poll the RTC to find out what it says is the current time. The
RTC is used to generate a Real-Time Clock Interrupt that can update the
OS counter. Programs can issue stop clock interrupts plus the
instructions for the applications are being processed by the same CPU
receiving the interrupts (which get queued as to when they are handled).
RTC interrupts to the CPU can be missed (because they were ignored when
stopped). The OS' "system time" is based on the clock interrupts from
the CPU that it sees, not the ones that are missed, stopped, nor from
the RTC itself. That the RTC sends an interrupt that the OS can use is
NOT the same as what the RTC tracks for time. An interrupt simply tells
the CPU to tick away another increment of time. It does not report the
recorded time.

Think of it this way: once per minute someone nudges you in the side to
remind you that one minute has elapsed. They don't report the time to
you. They just give you a nudge at 1-minute intervals. They get
distracted, need a bathroom break, need a 2nd cup of coffee to wake up,
get a phone call while trying to recover from their computer crashing
while their boss is standing over them demanding a status report while a
hot cup of coffee spilled on their lap, or they get called to a meeting
so your nudging gets put on hold, so they don't get around to nudging
you for awhile. Not until you get another nudge do you increment your
counter again. The next day you arrive and your friend tells you what
time is on his watch and the 1-minute but possibly interrupted nudging
begins again. You start with a known time but thereafter all you have
to track the passage of time is to know when those 1-minute nudges
happen but which may be longer than a minute apart at times.

http://en.wikipedia.org/wiki/Intel_8254

The time gets updated when the system powers up and uses the RTC.
Thereafter, clock ticks (interrupts) are used to monitor the passage of
time. The RTC still knows what time it is tracking. Everyone else
thereafter (i.e., the OS) has to use a counter based on the clock ticks.
Also, it is possible to disable interrupts from devices in the system,
including the clock interrupt from the timer; see

http://www.arl.wustl.edu/~lockwood/c...17/CH17-3.html.

http://www.intel.com/technology/itj/...nt_results.htm
"The OS uses a periodic clock interrupt to keep track of time, trigger
timer objects, and schedule application threads. While the default
interrupt rate is set by the OS, applications can increase the interrupt
rate to any desired frequency (as low as 1ms)."

So it is by the nudges that the OS tracks time, not by getting the
reported time tracked by the RTC itself. With 1-minute nudges, how much
time has elapsed after getting 10 nudges? Maybe 10 minutes. Could be
longer if that nudging was stopped or delayed for awhile.