Monday, April 11, 2011

Crunching The Numbers

In my last post, I showed how I had used my $50 logic analyzer to sniff the initialization sequence of the Davis VP2 weather console's CC1021 RF chip.  This was an important step in figuring out how to build a compatible receiver.  The one problem was that there were a lot of 1's and 0's in there but not a lot of real numbers.  Figuring out the numbers requires digging into the datasheet, looking at the formulas, and seeing if what gets spit out makes sense.  Generally, they do.

The values for the various registers are taken from my last post.  The datasheet references for each formula used in the calculation are shown in the third column.  Values in grey are important.  Why?  Because it is those values that you can plug into SmartRF Studio7 to get compatible register definitions for other chips, such as the CC1110F that lies at the heart of the Pretty Pink Pager.

But "hey", I hear you thinking.  "What about that row in there colored yellow?"  Good question.  Glad you asked.  If you look at the Davis FCC certification report for the Vantage Vue that I discussed here, you'll see a spectrum picture that looks like this.
The center frequency of this carrier is supposed to (nominally) be 902.355 MHz.  But if you look at the numbers I calculated during the initialization, I get 902.382375 MHz.  That is a 27 kHz error, and that is quite a bit for a narrowband device.  So I'm not exactly sure right now what is going on.  I am speculating right now that this is just a test signal and doesn't have anything to do with the normal hop sequence, but I don't know that for sure.  There will be one way to find out: write some code for the Pretty Pink Pager and see if I can see it.

Got time for one more picture?  Great.
Hmmmmm... #2
Most people, yours truly included, thought that the separation between the frequency hops would be a nice even multiple of 500 kHz.  Well, 914.9 MHz - 902.35 MHz is not an even multiple of 500 kHz.  So it would seem that they jostle the spacing around somewhat.  That is what I was seeing when I first uncovered the frequency hopping sequence, but I wasn't sure if that was really what was going on or if I was doing the calculations wrong.  Looks to be the former.

Now here is a different issue.  Winter is finally coming to a close.  That means I have to get off my butt and get some work done outside on weekends before the snow flies once again.  And that is going to mean a lot less time for weather station hacking.  I'll try to get some work done now and then over the summer, but don't be too surprised to see the frequency of posts here drop off a bit.  Such is life.

Having said all that, I'd be ecstatic is somebody were to jump in and pick this up.  Progress would certainly be quicker with an extra set of hands helping out.  I suspect I'll get there otherwise though, but it will just take a little longer.


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