Regards,
Nagaraj
> 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.
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.
--
Jonathan Bromley, Consultant
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Peter Alfke, Xilinx Applications
====================
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.c...@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)
> > 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.
>
> 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.
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.
>Glen Herrmannsfeldt wrote:
>> 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.
>
>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.
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.
http://www.foxonline.com/jitomain.htm
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
Nagaraj wrote:
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