Variations on XTAL clock frequency

"Phil Allison"


** Correction.

> So, the month to month variation should be maybe  +/- 10ppm or  0.86 
> seconds per day at the worst.



.............  Phil

In comp.arch.embedded,
larwe@larwe.com <larwe@larwe.com> wrote:
>
>The battery-powered RTC clock is frequently VERY inaccurate, though.
>For reasons I've never had explained to me, they almost always have a
>trimcap on that xtal. Their Q/C processes to set that trimcap appear to
>be FUBAR.
>
It's just there because the appnote says to put it there. And it allows
a demanding customer to calibrate the clock himself if he wants to. ;-)

-- 
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

Living your life is a task so difficult, it has never been attempted before.

> ... > [RTC clock drift on a PC ...]

There are clock correction programs that either apply a daily
correction to the time or query the NIST clock in Colorado.

-- 
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer:  Electronics; Informatics; Photonics.
To reply, remove spaces: n o lindan at ix  . netcom . com
psst.. want to buy an f-stop timer? nolindan.com/da/fstop/

"MC" <MC@nonexistant.place> wrote in message 
news:cvf2bo$msb$1@news-02.connect.com.au...

> You're fortunate...
> I've only had one laptop out of 4 that didn't drift in time too much.
> My current one loses about 4 seconds per day., my previous one
> gained about 2 seconds per day.
> The PC battery-backed-up clock system was 'adequate' when it was
> created back in the mid 1980's, but unfortunately that part of
> the PC architecture hasn't changed much since then.

Do they use crystals that are much lower-grade than even the cheapest RF 
crystals?

Ceramic resonators, maybe?

Do they have firmware that is incorrect?  Maybe the problem isn't just 
frequency.

"Phil Allison" <philallison@tpg.com.au> wrote in message 
news:380i5rF5k6g88U1@individual.net...
>
> "Richard Freeman"
>>
>> I would not have expected an un-trimmed Crystal to drift more than 100 
>> Parts per Million or 86.4 mS per day,
>
>
> **  100 ppm  =  1 part in 10,000.
>
> A day has 24 x 60 x 60 seconds or  86,400 seconds.
>
> So a 100 pm error is 8.64 seconds !!!
>
> You are only 1000 times out.

oops You are right .... dunno how I missed that one ? .. actually looks like 
I forgot to multiply by 100 ..... ;-) 

mc wrote:
>>You're fortunate...
>>I've only had one laptop out of 4 that didn't drift in time too much.
>>My current one loses about 4 seconds per day., my previous one
>>gained about 2 seconds per day.
>>The PC battery-backed-up clock system was 'adequate' when it was
>>created back in the mid 1980's, but unfortunately that part of
>>the PC architecture hasn't changed much since then.
> 
> Do they use crystals that are much lower-grade than even the cheapest RF 
> crystals?
> 
> Ceramic resonators, maybe?

Who knows ?. Every manufacturer will use what they want at the time.

> Do they have firmware that is incorrect?  Maybe the problem isn't just 
> frequency.

It's not a firmware issue, more a situation that the hardware timekeeper
is not intended to be super accurate and therefore providing things to
make it more accurate (such as temperature control and accurate
oscillators) can become expensive.

"mc" <mc_no_spam@uga.edu> wrote in message
 news:421b4fdc@mustang.speedfactory.net...
> "MC" <MC@nonexistant.place> wrote in message news:cvf2bo$msb$1@news-02.connect.com.au...
>
>> You're fortunate...
>> I've only had one laptop out of 4 that didn't drift in time too much.
>> My current one loses about 4 seconds per day., my previous one
>> gained about 2 seconds per day.
>> The PC battery-backed-up clock system was 'adequate' when it was
>> created back in the mid 1980's, but unfortunately that part of
>> the PC architecture hasn't changed much since then.
>
> Do they use crystals that are much lower-grade than even the cheapest RF crystals?
>
> Ceramic resonators, maybe?
>
> Do they have firmware that is incorrect?  Maybe the problem isn't just frequency.


The reason that PC clocks drift is that the crystal
is cut for a different mode of vibration than most
higher frequency crystals.  To save power, the
crystal oscillator that runs off of battery in your
PC operates at 32768 Hz (or something nearby).
The crystal vibrates in a flexing mode rather than
the more stable shear modes because that is the
only way to get the frequency down without
having to make a truly huge device that would
require more power to keep going.

-- 
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.