Question on Clock circuit

Technical discussions around xCORE processors (e.g. General Purpose (L/G), xCORE-USB, xCORE-Analog, xCORE-XA).
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rp181
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Question on Clock circuit

Postby rp181 » Sat Dec 25, 2010 5:17 am

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
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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Postby Berni » Sat Dec 25, 2010 10:15 am

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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Postby rp181 » Sat Dec 25, 2010 6:07 pm

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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Postby Berni » Sat Dec 25, 2010 10:46 pm

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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Postby leon_heller » Sun Dec 26, 2010 12:24 am

You'd be better off buying a suitable oscillator module.
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rp181
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Postby rp181 » Sun Dec 26, 2010 4:54 pm

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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Postby leon_heller » Sun Dec 26, 2010 5:06 pm

Why are you so keen on building your own oscillator?
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Postby rp181 » Sun Dec 26, 2010 7:04 pm

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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Postby leon_heller » Sun Dec 26, 2010 9:27 pm

Most of the XMOS boards use oscillator modules, AFAIK. They are quite cheap and very small..
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Postby rp181 » Sun Dec 26, 2010 10:26 pm

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

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