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'[EE]: Anyway to create stable 115.2khz pulse'
2001\01\07@211016 by William

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Hi!

Now, I am connecting a crystal (14.7456Mhz) to CD74HC4060 (binary
counter), and
get a signal of 115.2khz for 4060 pin 6.

And I like to know is there anyway to use a 555 timer to generate a
stable, clean (sharp
waveform) of 115.2khz.

Thank you.

rgds,
William

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2001\01\08@021920 by Roman Black

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William wrote:
>
> Hi!
>
> Now, I am connecting a crystal (14.7456Mhz) to CD74HC4060 (binary
> counter), and
> get a signal of 115.2khz for 4060 pin 6.
>
> And I like to know is there anyway to use a 555 timer to generate a
> stable, clean (sharp
> waveform) of 115.2khz.


No! 555 are very unstable even one cycle to the
next. For simple apps you can run the 555 into a
divide by 16 count, and with 16 or more division
per-cycle stability is ok (I have device that uses
this system).

Then you still get temperature/freq drift, maybe
you can work with that though?? Look for ceramic
resonators, there are a lot of cheap plastic
box types in the 100+ kHz range, maybe you can
get one?
-Roman

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2001\01\08@030717 by William

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Hi, Roman. Thanks for your reply and advice.

I just look thru RS contalogue, and those resonators are not suitable for

my 115.2khz.

Actually, I'm going to use 115.2khz for serial communication, so it
should
be quite precious and waveform needs to be clean and uniform.

Late, I think I need to stay to use a crystal and binary counter to get
my
required frequency!

Thank you.


Roman Black wrote:

{Quote hidden}

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2001\01\08@090507 by Olin Lathrop

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> And I like to know is there anyway to use a 555 timer to generate a
> stable, clean (sharp
> waveform) of 115.2khz.

Yes, but accuracy will be an issue if you want to use this for RS-232
timing.  RS-232 can only tolerate a 5.8% mismatch between the sending and
receiving baud rates at 8 bits per characters.  Of course the mismatch must
be below that in practise because this is the "guaranteed to fail" value.  I
try to stick to half that, or 2.9%.  It will be nearly impossible to
maintain that kind of accuracy with a 555 timer and the associated R/C
components.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, spam_OUTolinTakeThisOuTspamembedinc.com, http://www.embedinc.com

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2001\01\08@120017 by Bill Westfield

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   It will be nearly impossible to maintain that kind of accuracy (2-5%)
   with a 555 timer and the associated R/C components.

Why do you say that?  Aren't the primary variations introduced by
temperature variations of the RC component values?  How much temperature
change is necessary for a 5% value shift in a typical component, anyway?
Doesn't it tend to related to absolute temperature (ie room temp = 300K,
5% of which is 15 degrees, which is a LOT for any sort of temperature
controlled environment (say, indoors where people work), assuming your
circuit doesn't use enough power to changes its own temperature much.

FWIW, I made a rs232 format/speed converter (6bit 75bps to 8 bit 300bps)
for a newswire system some years ago from a couple uarts using a 555 as
the clock source.  I just stuck an LED on the "framing error" output of
the uart, and adjusted a pot in the 555 circuit to the middle of the
range where the LED didn't light.  In theory, this was "to be replaced
with a more stable source if necessary", but it was never necessary.
(This DID run inside a temperature controller mainfram machine room...)

BillW

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2001\01\08@123000 by Olin Lathrop

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>     It will be nearly impossible to maintain that kind of accuracy (2-5%)
>     with a 555 timer and the associated R/C components.
>
> Why do you say that?  Aren't the primary variations introduced by
> temperature variations of the RC component values?  How much temperature
> change is necessary for a 5% value shift in a typical component, anyway?
> Doesn't it tend to related to absolute temperature (ie room temp = 300K,
> 5% of which is 15 degrees, which is a LOT for any sort of temperature
> controlled environment (say, indoors where people work), assuming your
> circuit doesn't use enough power to changes its own temperature much.

First of all, I was talking about 2.9%.  You added the 2-5% comment in
parenthesis.  I wouldn't want to run RS-232 with 5% clock mismatch.

The problem is not as much stability as initial accuracy.  It will be very
difficult to find a resistor and capacitor that together have no more than a
3% error.  Most of this error will come from initial part tolerances, but
you should probably allow 1% or so for drift due to time and temperature.
This leaves 2% for the resistor plus the capacitor tolerance.

Yes, you could use a pot and tweak it at production time, but that will be
MUCH more expensive and less reliable than just using a crystal or ceramic
resonator in the first place.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, .....olinKILLspamspam@spam@embedinc.com, http://www.embedinc.com

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2001\01\08@200021 by William

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If 555 timer is unstable for generating uniform pulse, then is there any other
timer
that can preform well, which can generate 115.2khz?

I hope I can get an alternative cheaper way to generate the pulse.

Thanks


Olin Lathrop wrote:

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2001\01\08@212951 by J Nagy

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>Hi!
>
>Now, I am connecting a crystal (14.7456Mhz) to CD74HC4060 (binary
>counter), and
>get a signal of 115.2khz for 4060 pin 6.
>
>And I like to know is there anyway to use a 555 timer to generate a
>stable, clean (sharp
>waveform) of 115.2khz.
>

       No. A 555 isn't stable enough, and doesn't give full swing unless
you use a CMOS version. If you use that same crystal with a PIC, and count
exactly 32 cycles you'll have 115,200 Hz though.


       J. Nagy
       Elm Electronics
 ICs for Experimenters
http://www.elmelectronics.com/

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