I'm attempting to build the circuit in this RFID chip's white-paper. It left something vague however.
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I can't figure out what these two capacitors by the crystal should be. I've read the data sheet for the crystal from top to bottom and it says nothing to this effect.
I've read up on crystals and learned their history, design and production. This was all very interesting but hasn't helped me with the actual question: What kind of capacitors do I need?
For reference, here is the white-paper for the RFID chip:
http://www.nxp.com/acrobat_download/datasheets/HTRC11001T_2.pdf
EDIT: Ignore this post. Incorrect information used to be here.
What clock crystal are you using? The data sheet for that should tell you what kind of decoupling caps you need.
IIRC as bob said, it depends on the crystal and what frequency you'll be wanting it to run at.
Bob
Free Market Evangelist
I've forgotten that one, and it still make make laugh each time I see it :-)
I searched for crystals and found one that seemed legitimate. Here is the spec sheet for that:
http://www.ecsxtal.com/store/pdf/hc49u.pdf
i think that sheets says a min of 10 pF.
So does that mean it can be anything as long as it's at least 10 pF?
So does that mean it can be anything as long as it's at least 10 pF?
as was said, it depends on what frequency you want the crystal to run at. which decides the frequency the chip itself runs at. you NEED to see what frequency the rfid chip wants and base the calculations for the caps on that.
How do I calculate that? The RFID frequency is 125 kHz but I don't think the crystal would run at the same frequency.
The RFID spec sheet http://www.nxp.com/acrobat_download/datasheets/HTRC11001T_2.pdf says the crystal can be 6, 8, 12 or 16 mhz. It says the chip can figure out which one it is and be okay with that.
So I picked a crystal with 6 8 12 or 16 mhz. But how in the hell do I figure out what the caps need to be? Here is a document that describes how to calculate it.
http://www.mpdigest.com/issue/Articles/2008/Mar/Crystek/Default.asp
It might as well be written in Chinese because I can't even read it.
Okay
Given this:
The total capacitance seen from the crystal looking into the rest of the circuit is called the "load capacitance". When a manufacturer makes a "parallel" crystal, a technician uses a Pierce oscillator with a particular load capacitance (often 18 or 20 pF) while trimming the crystal to oscillate at exactly the frequency written on its package.
To get the same frequency performance, one must then make sure that the capacitances in the circuit match this value specified in the crystal's data sheet. Load capacitance CL can be calculated from the series combination of C1 and C2
[1]
I get this formula:
(20 is the crystal capacitance)
Doing the math you get
So I need two 40pf caps? This seems rather large...
if you find any nice book on this stuff let me know i am working on the same type of stuff
you can find 33pf or 47pf caps in many/any discarded circuit board. It's not that critical, so use whatever you have. ceramic ones (the tan flat discs) will work fine, as will SMT ones, if you are lucky enough to have junk with them marked (resistors use the same numbers, so watch out)