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Joined: Jan 2003
Posts: 4,391
iwire Offline OP
[Linked Image]

As I was sitting at my desk not writing the checks I should be, I was staring at my $2.00 battery powered LCD clock counting out the seconds and a thought crossed my mind.

How does this cheaply made device 'know' how long 1 second is? [Linked Image]

Grandfather clocks have pendulums, some watches have quartz timing, the clock on the wall may be controlled by the AC cycles.

But what is it in a silicon chip that can tell time?


Bob Badger
Construction & Maintenance Electrician
Joined: Jul 2004
Posts: 9,791
Likes: 14
Digital watches use a quartz oscillator. A small rock that has a characteristc frequency it vibrates at and they use "divide by" circuitry to count the oscillations down to one second. A desk clock is just a watch with a bigger display

[This message has been edited by gfretwell (edited 03-19-2005).]

Greg Fretwell
Joined: Jul 2002
Posts: 8,432
Likes: 3
Good call Greg,
A clock uses a 38kHz crystal to provide the synchronisation.
The actual crystals themselves are what we would call Rochelle salts, they oscillate at a given frequency when applied with a bias voltage.
One thing I'm short of knowledge on, is how the oscillating frequency is set.
Is it the size, type of crystal, like the size of a capacitor plate vs dielectric?. [Linked Image]

Joined: Jul 2003
Posts: 394
I believe the frequency is determined by the physical size of the crystals. When I was in two-way, we would order crystals "cut" to a specific frequency.

Joined: Mar 2002
Posts: 582
Ron Offline
A battery model has either a crystal oscillator as mentioned, or an integrated circuit that generates a timing signal (such as a digital 555 timer operating in an astable mode).
An a/c powered clock uses a synchronous motor.

Joined: Jul 2004
Posts: 9,791
Likes: 14
555s may be OK for something like a microwave timer but they drift too much for a decent clock. I am using one as a spa timer (along with a 14 bit counter) but that is not too critical.

Greg Fretwell
Joined: Oct 2000
Posts: 2,722
Broom Pusher and
Is there a way to stablize a 555, to be used as a "Tolerable Clock"? - like within 1% tolerance?

I have built a 555 based flasher (drives a 20mA LED), which is in close proximity of a "Kit-Bashed" AC powered Clock (Kit-Bashed being a normal alarm clock w/ digital LED segmented display, removed from original enclosure and placed inside an O-27 scale Caboose).

The normal "½ Second Flashing" of the "Colon" is directed to a Railroad Crossing Crossbuck - which controls alternating flashing LEDs via op-amp.

The 555 based flasher pulse cadence is at least 3x faster, yet does not keep a predictable sync. with the crossbuck.

Just curious if anyone has info.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Apr 2001
Posts: 109
so what do you do if the clock on your computer gains 3 min. every day?

Jon Niemeyer
Joined: Jul 2004
Posts: 9,791
Likes: 14
Replace the system board. Not an attractive option but the TOD clock is soldered on the board.

There is a chance this could be fixed with a BIOS flash but that would only be if lots of people have the same problem and it is some strange microcode problem. Check the manufacturer's web site. They will have the latest BIOS update (if any)

[This message has been edited by gfretwell (edited 04-07-2005).]

Greg Fretwell
Joined: Sep 2001
Posts: 806
A few older motherboards had a trimmer capacitor to tweak the RTC oscillator frequency. You can either do it using a frequency counter (recommended), or turn it a bit at a time, and see if the clock gains or loses time. The usual oscillator frequency is 32.768 KHz.

If you have decent soldering skills , you could try replacing the RTC chip (may be socketed!) or oscillator crystal (if not integrated into the RTC).

A software workaround if you are running Windoze XP is to go into the time settings and enable automatic synchronization, and set it to do so every hour or 2, so the time drift becomes less noticeable.

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