Question on Clock circuit

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rp181
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Question on Clock circuit

Post by rp181 »

I am new to using clocks for microcontrollers, and wanted to verify what I had planed for the XS1-L1. This is exactly what I am planning:
clk.JPG
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clk.JPG
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Will this work, with CLK connected to the CLK pin of the XMOS chip? Could someone explain what is going on in this? I would like to understand it rather than just using it. The unbuffered inverter used is this:
http://search.digikey.com/scripts/DkSea ... 04M5XCT-ND


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Berni
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Post by Berni »

Thats a basic cirucit used to drive crystals on all sorts of stuff. The crystal is basically a LC tank circuit that resonates at a certain frequency. So when you turn it on first it picks up noise and starts wiggling its output to put some energy in to the LC circuit. One its osculating the inverter just keeps it going by switching the output pin and keeping the crystal osculating and that pin is also the frequency output

There is also a similar inverter circuit that uses a resistor and cap to generate a frequency.
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rp181
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Post by rp181 »

Crystal datasheets list a load capacitance, is this what the two caps connecting from the crystal to ground should be? If it says 18pf, should these two be 18pf?
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Post by Berni »

Well those depend on the parasitic capacitances. But generally sticking something between 10 and 20 pF is fine.It also works no problem if you leave the caps out.
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Post by leon_heller »

You'd be better off buying a suitable oscillator module.
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Post by rp181 »

I think I am going to try this with this crystal:
http://search.digikey.com/scripts/DkSea ... -9871-1-ND
and 33pf capacitors to ground.

What does the output on the clk pin look like? I have a 20mhz analog scope, so I may or may not be able to look at it.
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leon_heller
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Post by leon_heller »

Why are you so keen on building your own oscillator?
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Post by rp181 »

Can you provide a link to an oscillator? All the modules I find are rather expensive.

If those are better to use, why do the xmos dev kits and the xtag use crystals?

Im not set on using one specific thing, as long as it is small and cheap. I need a L2 (Maybe two L1 depending on the soldering issue) to fit on a 50mmx30mm pcb.
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Post by leon_heller »

Most of the XMOS boards use oscillator modules, AFAIK. They are quite cheap and very small..
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Post by rp181 »

Oh, ok. I was under the impression that oscillators are the same as crystals. Thank you!
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