Can I use one shared crystal?

There are two problems to solve to use a single crystal to drive two chips. The two inputs may not settle to the same DC bias voltage. And the extra load of the second chip may slightly affect the first. You solve the first problem by coupling the output of the first oscillator to the input of the second. You will have to find out by trial whether the chip actually connected as an oscillator is loaded too much by having its output coupled to the input of the second chip.

A problem free way to drive both chips from a single crystal would be to build an independent oscillator with the crystal and a cmos inverter, or use a canned oscillator. Use the output of either of these to drive both clock inputs. If the single inverter oscillator, I would capacitively couple to the inputs to let each chip find its own bias point. If the canned oscillator with a logic level output, direct connection will work fine.

Donald ( snipped-for-privacy@dontdoithere.com) ha scritto:

::: Can I use one parallel resonant shared crystal for both ::: devices?

:: Yes, but you can not connect the two crystal pins together :: on the two processor.

Then I can't connect both input (and so output) pins of the devices together to one lead of the crystal?

:: Connect the crystal to one chip, and with a TinyLogic device :: like NC7WZ17 from Fairchild or TI, connect the output of the :: device with the crystal to the input of the other. ( leave the :: output side open ) :: :: This will isolate the two oscillator circuits.

Do I really need to isolate the two oscillator circuits or can I connect them as I have written above?

If I would have to use another logic chip, then I may use 2 crystal for an even better solution, don't I?

Thank you.

Yes, but you can not connect the two crystal pins together on the two processor.

Check the data sheets of each part. One pin will be an input, the other an output.

Connect the crystal to one chip, and with a TinyLogic device like NC7WZ17 from Fairchild or TI, connect the output of the device with the crystal to the input of the other. ( leave the output side open )

This will isolate the two oscillator circuits.

good luck

donald

Because the signal swing is large enough to saturate each input, regardless of the exact zero input bias voltage of either. The bias voltage is important only if the signal swing is small and must be centered on that bias point to achieve a full swing output.

Why can you connect the output of the canned oscillator directly to each oscillator input without coupling caps?

What he ( John ) said.

If you don't like that answer given, please try it and let us know what you've found.

donald

Too quick on the SEND button.

This is not a unknown problem.

I am sure many here have tried this before as well.

I tried it with 6502 processors years ago. The osc circuit would work with each one seperatly but together, nothing, dead, natha.

I had an experienced friend look at my circuit, and he laughed his butt off.

He ask me if I have a clue what I was doing, I said "well, it should work" and he laughed again.

So, the answer is "don't do it".

But please try it for your self.

donald

John Popelish ( snipped-for-privacy@rica.net) ha scritto:

:: There are two problems to solve to use a single crystal to :: drive two chips. The two inputs may not settle to the same :: DC bias voltage. And the extra load of the second chip may :: slightly affect the first.

Exaustive explanation and good advices, thank you.

Donald ( snipped-for-privacy@dontdoithere.com) ha scritto:

:: I tried it with 6502 processors years ago. The osc circuit would work :: with each one seperatly but together, nothing, dead, natha. :: :: I had an experienced friend look at my circuit, and he laughed his butt off. :: :: He ask me if I have a clue what I was doing, I said "well, it should :: work" and he laughed again.

:-) Thank you.

I dont see why one cant drive the other, a usual arangement for one IC wich seem to have a cmos inverter between the two pins is to have ~ 20pf on both input and output pins with the xtal between the two, If the output of the IC doing the oscilating is conected to the other IC input via a 100pf decoupling capacitor, what can it do but load it by 10pf or so, this can be compensated for by reducing the 20pf capacitor, ofc the output of the 2nd IC is just left open.

you might find one ic or the other is better at driving the other. and you might have to play around a bit with capacitances etc. ofc you would need to make sure the track length to conect them is Very short. If its for even low volume production you would have to look into it a bit deeper.

Colin =^.^=

colin ( snipped-for-privacy@ntworld.NOSPAM.com) ha scritto:

:: I dont see why one cant drive the other,

Thank you Colin, can you draw a picture which explains this situation, please?

I can't understand how to connect both devices to the single crystal... :-(

As told before: A crystal oscillator has an input and an (inverting) output. A crystal connected in be input and output will cause the whole thing to oscillate at the crystal's resonant frequency.

If you want to use one crystal for 2 devices, you should place the devices close together. Connect the crystal between the crystal oscillator pins of device A, connect the crystal oscillator output from device A to the oscillator input from device B with a small capacitor (say 1nf).

The datasheets should tell you more about such a setup and which pins are input and output.

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Reply to nico@nctdevpuntnl (punt=.)
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The biggest risk may be other signals that capacitively couple into the trace going to the second chip and contaminate the wave being fed to the crystal. If this trace is more than an inch long, I would probably shield it with a pair of ground traces (grounded at the oscillator end, not the receiver end).

Like this ??

+------+ | | | | | | __|__ --- 0 \\ / [ ] / \\ \\ / --- / \\ 0 | ----- | | | | | | +------+----()---+ 1nf CPU A CPU B

I don't think this will work.

I think the drive of CPU A output will be loaded too much to drive the crystal and the input of CPU B.

Let me know how it works.

donald

Yes.

The OP should give it a try. Most modern CMOS chips have a very small input capacitance.

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I think there's still a miscommunication here; the chip can accept an external oscillator, OR oscillate a connected crystal, and what is recommended is that ONE CPU oscillates a crystal while the OTHER uses an external oscillator.

So, one poster thinks both CPUs are in oscillate-a-crystal connection (and wants to connect two wires on each of the two), while the respondent only connects two wires to the crystal-oscilator chip, and one wire to the uses-external- oscillator chip.

The TinyLogic device is just used as a buffer, so the (possibly sensitive) crystal wiring isn't connected to an antenna of interconnect wire. Any available gate would serve.

whit3rd ( snipped-for-privacy@gmail.com) ha scritto:

:: The TinyLogic device is just used as a buffer, so the :: (possibly sensitive) crystal wiring isn't connected to :: an antenna of interconnect wire. Any available gate :: would serve.

BTW, I need to use a crystal with nominal frequency of 16 MHz. So the best soultion should be to use a crystal for each device, shouldn't it?

Possibly the easiest and least expensive anyway. Unless you have some special need for them to run _exactly_ at the same frequency and phase.

My "little voice" is also telling me that I'd use much less than

1 nf for the cap - maybe more like 33 ~ 120 pf.

Thanks, Rich