Crystal Oscillator for 74HCXX family Digital Clock

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con-!

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Hi all, I am trying to design a crystal oscillator to give my digital clock a frequency of 32.768kHz. I have tried many design such as the 4061 IC, Pierce Oscillator, Series RLC circuit but none of them work. Does anyone know how I can design my final part or at least give me some help?? Please find attached my digital clock design**broken link removed**... I must connect my oscillator instead of the temporary clock connected to pin 5 of the 74HC192
 

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Already tried that

I have already encountered that website... the simulation gives me multiple errors and therefore doesnt work for my design. As i have mentioned before I have already tried the Pierce Oscillator.
 
Did you actually build it in real life or have you just played a computer game?

Some simpulators can be a bit flakey when it comes down to this kind of thing.
 
I have run the simulation using Proteus. I have arranged the error it was giving me. he error was brought up because of problems with the NOT gate since it was of a different family. However the oscillations are not produced. The output from the oscillator always remain low and therefore nothing is driven. I cannot afford to build the circuit and hope it works as I am going to built it on a PCB. There must be a combination of how to interface a crystal oscillator with the 192...
 
There shouldn't be any need to simulate the oscillator, this type of oscillator has been used for years my many without any porblems.

If you're really worried about it then, why not just build the part your having trouble with on a piece of breadboard or stripboard? That way you can test it and ensure it works before you make it for real.

I don't use Proteus but from what I've gathered it seems more digital than analogue and the Pierce oscillator relies on the analogue properties of a gate to work.

EDIT:
Have you tried using a different simulator for the oscillator?

LTSpice should work, it's free to download and you can get models for most logic gates from the Yahoo group.
 
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I agree with you that proteus is more digital... however is the design is correct, proteus should be able to run it without any problems. the only problem with proteus is that most of the time it does not consider component values but as long as the design is correct it will run. I have made many simulations using proteus and always worked properly. Hope there is some other way of doing this apart from the pierce oscillator... Is there a way of designing the crystal oscillator with an ic and taking the output from the ic isntead??
 
Sorry, I think you're putting too much faith in your simulator program which isn't good.

I don't think you should worry about it not working in a silly simulator program. I can probably think of other circuits which will work if you build them but not in your simulator. I understand that you need some assurance it works before you build the whole thing but that can be solved by a testing the oscilator which is giving you trouble.
 
Oscillators are always difficult to simulate. You may have to insert a pulse (use a current generator to provide the pulse) at the start of the simulation to help achieve oscillation.

But as Hero stated, you don't really need to simulate an oscillator since they are quite standard circuits.
 
i have simulated the oscillator... now the only thing i have to do is to find exact values for capacitance and even resistors since the accuracy of the crystal oscillator can be adjusted by these values. I have not used a pierce oscillator. I have used a combination of an RC circuit with a schmitt trigger... Please tell me if you need to see the circuit I built.
 
Yes, please post the schematic.

Also bear in mind that there are some circuits which work well on a simulator but either don't work or perform extremely poorly in real life.
 
What's the value of C4?

You need to be sure that it's the crystal is actually doing its job and resonating otherwise you could've just designed a glorified RC oscillator which I think is the case here.

The resistor and capacitor values should make little difference to the frequency of a crystal oscillator, try changing them and it shouldn't make much difference, for example, if increasing the value of C4 by 20% greatly reduces the frequency then the crystal isn't doing anything.
 
He's already tried that but it won't work in his silly simulation problem so he doesn't want to do it. I know that it not working in the simulator doesn't matter and everyone else can see that but him.

Here's a couple of papers giving more detail on oscillators. I suggest you sod the simulator and use one of those.
**broken link removed**
http://www.icbase.com/pdf/add/ti/AN-101-00039en.pdf
 
My company makes crystal oscillators and I design them. I've only tried simulators a couple of times, and at best they show you that an oscillator will oscillate. If you are lucky, it will tell you the frequency.

What it won't tell you is the load capacitance of the crystal that is needed. You can forget startup-time or pulling range (for VCXOs). The model of a crystal as a series combination of an inductor, a capacitor and a resistor, with as capacitor in parallel with the lot, doesn't simulate spurious response or overtone oscillation.

Oscillator start up is often thermal noise that is amplified by the active element and filtered by the crystal. That is very, very difficult to simulate, and needs a very small time resolution and a very good voltage and current resolution, and the simulation needs to run for the time of many thousands of cycles. In real life, 32.768 kHz watch crystals take up to about 3 seconds to start.

Simulators are good for digital circuits. They can be used for analogue circuits with a bit of care, but they are not designed for working with crystal oscillators.
 
with reply to hero999, i havent replied because i am trying to find a different way... i am not ignoring you if that is what you're saying... i know what you meant when you said that the oscillator will also work without the crystal... but pls note that thats a bug from my somulator since the problem also occurs when removing the crystal from the 8051 microcontroller... it will still work. I have tried what wade told me but the 74HCU04 is not available in the simulator... now... for hero 999... if i set up the circuit using the pierce oscillator or the circuit wade has showed me... are you saying that in practice it will work where as in the simulator it will not??
 
to diver300... do you think that in reall life, the pierce oscillator using a 74HCU04 will work in praactice when i connect it to my clock???
 
con-! said:
if i set up the circuit using the pierce oscillator or the circuit wade has showed me... are you saying that in practice it will work where as in the simulator it will not??
Click to expand...
Yes, that's what I've being saying all along.

And if you're really worried, build the oscillator on a small piece of stripboard first but make sure you shorten the strips as much as possible by cutting the traces.
 
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