Dead End

It is springtime here at Mad Scientist Labs, though you would never know it.  Mother Nature has decided to totally screw us out of our spring.  Snow still covers most of our yard.  I took Buddy for a walk today wearing an old parka and nearly froze to death.

"That sucks", you must be thinking.  You would be thinking wrong.  The weather sucking outside lets me play around inside with little gadgets and gizmos.  The focus of my efforts the last couple of weeks has been trying to get my Stellaris LM4F120 (bought for the princely sum of $4.99 with free shipping) to play nicely with a little RF1100SE CC1101-based board I picked up at Deal Extreme.  The goal here, of course, is to pick up the signal from my Davis Weather Station's Outdoor Sensor Suite (ISS).  In other words, I want to get the Stellaris to tell me how cold it is outside without me having to walk the dog to find out.

Stellaris On Left, RF1100SE On Right, BMP085 Module Hanging In Mid-Air In Middle

"Should be a piece of cake", thought I.  Long ago, I had figured out the mysteries of the ISS wireless transmissions, going so far as to get my Pretty Pink Pager to work as a portable weather station.  Turns out it wasn't so simple.  It might not even be possible.

I've burned a couple of weeks on this trying to get it to work, and when it works, it barely works.  I was seeing all kinds of strange things, and I didn't know if it was my unfamiliarity with the Stellaris board or with the poorly documented RF1100SE CC1101-based module.  At times I would get a bunch of garbage.  At times I would get nothing at all.  But now I think I've come to a conclusion: if you are thinking of picking up one of these RF1100SE boards to work at anything other than 433 MHz, don't to it.  Simple as that.  And here's why.

This picture is taken from the CC1101 datasheet for using the module at 433 MHz.

Recommended CC1101 Schematic for 433 MHz Operation

And this picture is taken from the same datasheet for using the module at 915 MHz, the frequency the ISS operates at.

Recommended CC1101 Schematic for 915 MHz Operation

The two circuits are very different in the connection between the CC1101 and the antenna.  The 433 MHz recommended circuit has four inductors between the chip and the antenna, while the 915 MHz recommends either six or seven inductors between the chip and the antenna.

Now, here is a picture of my module taken from the DX website.

RF1100SE Module With Inductors Circled

Count the inductors circled in red and it becomes pretty obvious pretty fast that these RF1100SE modules are designed for 433 MHz operation.  I feared that this might be the case, and I hoped that getting a proper length antenna for 915 MHz operation might be enough.  It is not.

This is the output from my Stellaris board talking SPI to the RF1100SE with a 915 MHz antenna attached.

This Sucks

This is what I get after tweaking the register settings to within an inch of their life.  You can see all kinds of things here.

• My frequency error (a two's complement value shown on the very right" is about as low as you can get, so I'm dialed right on the signal.
• My RSSI value is terribly low: I'm barely seeing the signal above the background noise.
• As tweaked up as I am, I'm still seeing a lot of bit errors.  Those packets starting with 68 and 78 are corrupted - all of the packets from this ISS should end with zero.
• You can't see it here, but many times the radio misses the packet altogether.

In the process of banging my head against a wall, I went looking for other people using this RF1100SE module at 915 MHz and found none.  People have tried but I haven't come across anybody reporting success.  And this is why.  The CC1101 needs a very different circuit between the 433 and 868 / 915 MHz bands.  This board is designed for the one, and getting it to work at the other isn't going to happen if you expect any kind of performance out of it.

What to do, what to do?  Not quite sure yet.  Any kind of CC11xx module that doesn't already incorporate some kind of processor on it (8051, Atmega) and is cheap seems to be designed for 433 MHz.  What I'm looking for is just the RF chip itself that I can drive directly with the Stellaris board without having to fuss with two different toolchains, processors, etc.  I might have to consider a non-CC11xx type chip instead, like something HopeRF based (like this one, or maybe this new one out soon).

Anyway, this problem was just consuming me for a while.  It wasn't the "not working" part, it was the "why isn't it working" part.  I gave up several times trying to get this working, only to keep going back to it when I realized I couldn't stop thinking about the problem.  Now I know what's wrong.  I haven't been defeated.  I just realize that victory with this module doesn't look to be possible, and I'm OK with that.  You can't do what you can't do.

Mad Scientist Labs: wasting weekends so that you don't have to.