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### Topic: 1hz clock (Read 13043 times)previous topic - next topic

#### baum

##### May 24, 2011, 02:33 am
I am thinking about making a no-microcontroller clock/timer/stopwatch (74 series ICs only + passive components, preferably). To make it as simple as possible I want to have a 1Hz clock as the basis for the system. Any ideas on how to get an accurate 1Hz clock?
I was thinking of either using a 555 timer (is that accurate enough, though?) or a 32.768kHz crystal w/ a frequency divider.

Any thoughts?

Thanks!
baum

#### baum

#1
##### May 24, 2011, 02:55 am
But that looks like a 14-stage divider...
32.768kHz/2^14 = 2Hz!

#### RuggedCircuits

#2
##### May 24, 2011, 02:56 am
Forget the 555 -- it will be subject to the tolerances of the components around it which will never be good enough.

You could use the 60 Hz AC line frequency passed through high-value resistors and some other discretes to generate a pulse every 1/60 of a second, then divide that down to 1 second. Fewer divide stages than 32.768 kHz but slightly more complicated division circuitry.

--
Beat707: MIDI drum machine / sequencer / groove-box for Arduino

#### baum

#3
##### May 24, 2011, 02:58 am
I'd rather not... I'd also like it to be standalone, powered off a 9V battery perhaps?

#### floresta

#4
##### May 24, 2011, 03:11 amLast Edit: May 24, 2011, 03:13 am by floresta Reason: 1
Quote
I am thinking about making a no-microcontroller clock/timer/stopwatch (74 series ICs only + passive components, preferably). To make it as simple as possible I want to have a 1Hz clock as the basis for the system. Any ideas on how to get an accurate 1Hz clock?

I designed one of these about 30 years ago to drive an old 'International' (now IBM) schoolhouse/railroad clock.  It's still in use in the next room.  All of the documentation is in disk, on state of the art 8" double-sided double-density floppy disks.  I suppose I could go down to the garage and dig out my S-100 computer and hope the mice haven't torn the wires out of it or eaten the boot disk.  I'll look around and see if I can find any printed documentation.  As I recall there was a crystal oscillator that used a 'color-burst' crystal (this was before the days of 32 KHz watch crystals) and a chain of 7490's (it was also before the days of inexpensive cmos).

Don

#### floresta

#5
##### May 24, 2011, 03:15 am
Quote
The logical and high-tech solution would be to simply use an RTC which has a precision 1Hz output!

That was my thought as well.  You would have to program it to start the oscillator and enable the 1Hz output - not a big deal.

Don

#### baum

#6
##### May 24, 2011, 03:41 am
Quote
The logical and high-tech solution would be to simply use an RTC which has a precision 1Hz output!  Or, you could use a receiver for the time signal radio service wherever you are on the planet.  Is there some reason you don't even reveal which continent you are on?

1. I was thinking of an RTC, but that would be sort of cheating and an oxymoron. (and expensive) (A very precise clock to drive a homemade not-so-precise clock?)

2. US of A. I don't want the radio. I think I have decided on a crystal.

Send the 32.768kHz to a 12 bit counter, now we have 32,768/2^12 = 8. Then get a decade counter, and tie 'Q7 to reset. Then, when counter reaches 8th output, it "overflows." Would this give me a fairly precise 1Hz clock?

#7
##### May 24, 2011, 04:28 am
If you have a 12 bit counter, why not go all the way to 15?
http://focus.ti.com/lit/ds/scls704a/scls704a.pdf

Line frequency is not that accurate - I have Bose CD clock radio that is always gaining time because it runs from line frequency.  Great sound, always fast.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#8
##### May 24, 2011, 07:56 am
Whenever I put a multimeter frequency counter on the mains here, it has always come back fast,  you can see it doing a slow drift between 60Hz up to about 62.5 if I recall.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### tastewar

#9
##### May 24, 2011, 02:47 pm

Quote
Line frequency is not that accurate

For the purposes of long-term time accuracy, power mains frequency is just as accurate as the NITS Atomic Clock. Because all the utility companies maintain the grid frequency against WWV.

Yeah that 'NITS' atomic clock isn't all it's hyped up to be ;-)

To satisfy my curiosity (which you're obviously under no obligation to do...), can you point to something that documents this? Oh wait, here it is: http://en.wikipedia.org/wiki/Utility_frequency#Long-term_stability_and_clock_synchronization

#### baum

#10
##### May 24, 2011, 10:47 pm
Quote
If you have a 12 bit counter, why not go all the way to 15?
http://focus.ti.com/lit/ds/scls704a/scls704a.pdf

Digikey (where I'm buying from) only sells minimum 880. How about a cheaper 14-bit unit with a toggle (2-bit) connected to the output? 2^14*2 = 32.768kHz.

#### justjed

#11
##### May 25, 2011, 12:37 am

Yeah that 'NITS' atomic clock isn't all it's hyped up to be ;-)

The Knights of NI have a Time Service?

What is the average hz of an unladen swallow's wingbeat?
... it is poor civic hygiene to install technologies that could someday
facilitate a police state. -- Bruce Schneier

#### baum

#12
##### May 25, 2011, 12:54 am
But eight dual flip flops would be 16 flip flops total... 2^16 = 65536. I'm looking for 2^15. If I use seven dual flip flops, what should I use to take the frequency from 2hz to 1hz?

#### baum

#13
##### May 25, 2011, 02:07 am

Doh!

I just hit my self in the head b/c I realized I am being WAY to harsh on myself. I'll just use a 65.536kHz crystal and use a dual 8-bit (16 bits total) binary counter to bring it down to 1 hz.

Thanks for all the help!

#### baum

#14
##### May 25, 2011, 02:28 am
OK. So I found the 74HC590. This is an 8-bit counter, right? So if I daisy-chain 2 of these, I should get the 65.536kHz crystal down to 1Hz?

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