Hi, I want to interface a crystal to a Xilinx FPGA. Earlier Xilinx FPGA devices like XC2000 series had crystal input provision which is not there in Spartan 30K gate device which I am using for my design. I want to know how to build an oscillator circuit (amplifier) for the crystal in FPGA. Any references?
Regards, Nagaraj
Don't mess around. Buy a packaged oscillator. The price of the oscillator is about three orders of magnitude lower than the value of the headaches you will get from not using one.
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Don't do it! In XC3000 we had a single-stage amplifier between two pins, and that worked kind of ok with most crystals. But there were problems with very low frequencies (like 32 kHz) and some overtone designs. We (i.e.I) got dragged into many complicated analog discussions, so we never offered such a circuit again. If you implement an inverter from one input pin to an output pin, you actually have more than half a dozen stages cascaded (that's the way we build our input and output buffers) and that is not a healthy arrangement for a reliable oscillator that always starts and never goes into spurious odcillations. It might work in the lab, but is not good for production. Buy a complete oscillator can. You can buy them retail for
mmm many people are paranoid about xtal oscillators it seems. Still I agree the small integrated DIL ones are cheap and easy, but for the ones who MUST use an xtal when all shops are closed, this solution has always worked for me:
+5V | drain |---- ----------------------->| BF245 B | | | |---- | | === C |-----------0 to TTL level input | | |-------------| | | | |--- | | | | | = | | | | |
--- | |100k | | | 1k5 |XTAL | === 2 C | | | | | -----------------------------------------0
It can drive a HCT7404 or such directly. Stability is very very good. The caps ratio is 1 to 2 C, values like 22 and 44 pF (2 22 in parallel) I use.
If the drawing looks distorted use a fixed spacing font perhaps. I have uses this at 20MHz, 8.8MHz, 6 MHz, 100kHz, etc... Always works. Decauple the +5. The output looks like a sine wave, bit flatted, but very clean. If you want total separation, you can use a drain resistor and take the signal from there. Add a varicap with a small series cap parallel to the XTAL to make a voltage controlled oscillator for a PLL etc.. (For example color subcarrier lock). JP
The oscillator that I used to know used three CMOS inverting gates in series with the crystal wrapped around them. Possibly some resistors, too. Usually one more gate to buffer and shape the result.
Though I remember people having a hard time sometimes with the 32kHz crystals, it worked well for everything else.
-- glen
But what is the advantage over an oscillator unless you are trying to squeze every penny out of the design? The difference between an oscillator and a crystal is less than $.50.
-- Rick "rickman" Collins rick.collins@XYarius.com Ignore the reply address. To email me use the above address with the XY removed. Arius - A Signal Processing Solutions Company Specializing in DSP and FPGA design URL http://www.arius.com 4 King Ave 301-682-7772 Voice Frederick, MD 21701-3110 301-682-7666 FAX
(snip about a crystal oscillator for use with FPGA's)
series
Some people just don't like them, but otherwise I agree.
-- glen
I wouldn't know what there is not to like. An oscillator unit is smaller, simpler and works better than a crystal circuit you can design in just a few hours without extensive testing. If the cost difference is not an issue (such as production volumes below 10,000) I can't see how it would pay to design your own oscillator. Even with higher volume production, I bet the lower failure rate would make a self design not worth the effort.
-- Rick "rickman" Collins rick.collins@XYarius.com Ignore the reply address. To email me use the above address with the XY removed. Arius - A Signal Processing Solutions Company Specializing in DSP and FPGA design URL http://www.arius.com 4 King Ave 301-682-7772 Voice Frederick, MD 21701-3110 301-682-7666 FAX
Off the shelf oscillators for standard frequencies are one thing.
But when I was looking for custom frequencies I found manufacturers were happy to make small batch or even one-off crystals at a very respectable price; at the time, they didn't seem to be tooled up to make one-off oscillator modules - or at least they wanted much more money for the job.
Maybe this has changed in the last few years.
- Brian
Fox has custom frequency oscillators availably at reasonably cost and reasonably quickly.
I expect there are others.
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Digi-Key carries several brands of off the shelf oscillators that they can program to your custom frequencies (qty = 1).
Daniel Lang
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