Crystal Voltage ?

Areal Person

I've hooked up a 5MHz crystal circuit, but I must ramp the input voltage up to 12vdc
to get it to 4998815 i.e 5MHz (using the FrequencyCount.spin program) which
uses the Propeller counters.

I place the output signal on pin 7 of the propeller demo board.

Will this damage my Propeller ? I'm not putting any other connections to the propeller
just the output signal of the crystal circuit.

Also, is 12vdc a normal voltage to run a simple 5MHz crystal at ?

Whats the max voltage for the 5MHz crystal on the Demo board if it was not connected to the propeller? (seperate from the demo board)

-Areal

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I have a tree growing out of my head, but

what do you expect ? I'm a programmer.

Mike Green

1) Without a circuit diagram, I really can't answer your questions.

2) Depending on your circuit, it might be possible to damage your Propeller

3) A lot of crystal oscillators are run at 12V, but a crystal oscillator can be designed for pretty much any voltage.
The Propeller runs its crystal oscillator down to 2.7V

4) Vacuum tube crystal oscillators can run at several hundred volts. It depends on the circuit used.

hippy

It's not so much the voltage the crystal oscillator circuit runs at but what voltage you are putting into the Propeller and through what circuit.

Yes; it's entirely possible to damage or destroy your Propeller Chip but without a circuit diagram of your oscillator and interface circuits it's impossible to say if you are likely to.

David B

My experience with crystal oscillators is that even tiny amounts of stray capacitance can make all the difference in how they work. You might find that adding or removing even 5 or 10 pf in the right place can vastly improve its operation, and may enable yours to run from a lower voltage. But I can't tell you from where - this is where the art of design comes in. Two circuits that look identical on paper will have different amounts of stray capacitance in the components, wiring and board layout, and will affect the oscillator operation.

Once you get the oscillator working reliably, why not feed the oscillator signal to the propeller through a large series resistor? That will help protect the propeller's inputs and also reduce the load on the oscillator, which should help it maintain its design frequency, and allow you to continue to run the oscillator itself at 12V, or whatever it needs in order to run reliably.

Beau Schwabe

David B,
·
As you mention, "...even tiny amounts of stray capacitance can make all the difference..."
·
Likewise,·if you were to··"...feed the oscillator signal to the propeller through a large series resistor..." , you tend to create a an RC low pass filter, which would block or·reduce the
signal you are trying to detect.
·
·
·
The best thing at this point is to "see"·a circuit diagram from a person that's real· (aka - Areal Person)
·


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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Areal Person

Here is the circuit I'm using, the output is going into pin 7 of the propeller

at 12vdc, please advise me if this is not safe. The chart showes the size components

I'm using, as my crystal is 5MHz

http://www.radio-electronics.com/info/circuits/transistor_crystal_oscillator/crystal_oscillator.php

thanks,

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I have a tree growing out of my head, but

what do you expect ? I'm a programmer.

Beau Schwabe

Areal,

Nothing against the Colpitts oscillator, but is there a·reason you are using descrete components for your oscillator?· I would not use that particular circuit, even though the capacitor on the output will provide some sort of current "buffering".· The main reason is actually because of the capacitor.· The way that it is configured will cause the voltage to swing above and below 0V... the Propeller would not like too much of the below 0V part.· Another reason of concern, would be temperature stability.

Depending on your specific application, there might be a better solution.· For example, a single gate Schmitt inverter might be all that you need, but it's hard to tell because we don't know the exact details of your application.

http://focus.ti.com/lit/ds/symlink/sn74ahc1g14.pdf··

By connecting the crystal directly across the input and output of the inverter, and driving pin 7 of the Propellar as per your description, that should do the trick.· That particular Schmitt inverter will work down to 2V (5V max).

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Alex Mackinnon

Umm... if it's cheap and simple you are after I use little epoxy moulded 3-pin resonators with my Propellers - they are much cheaper than crystals, stable enough to support the VGA outputs and all serial speeds without any glitches or hiccups. I think they are made by Hosonic - I pay 17p each for 100 off (that's about 35 cents).

The middle pin is for grounding the 2 built-in capacitors and the outer two pins connect to the crystal terminals of the prop.

If you need an independant clock source for several chips then I use the Seiko clock modules (there are other makes in 8 and 14 pin)·which are·self-contained 8-pin oscillator modules - much more expensive but a no-brainer to use.

Alex.