May 2004, Issue 166

Radio Roundup

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Start • MaxStream 9XStream • easy-Radio Module • Radiometrix SpacePort • Initium • Sources and PDF

It’s time to work some more RF magic. So get that pointy wizard cap out of the closet and fetch that magic wand hidden in your dial caliper case. You may want to pull out your Western-style Stetson, too—if you have one. 

You already know that the wizard gear is necessary to work with RF. But what do a wizard’s cap, a sorcerer’s wand, and a Stetson cowboy hat have in common? Well, I associate RF with the word “radio.” And, being a 1950s baby, a big old Stetson hat is synonymous with the word “cowboy.” When cowboys get together and bring along horses and cows to poke in a competitive environment, it’s called a “rodeo.” I don’t have any cows and bulls to poke, and I didn’t ride into town on a horse, but for the past month or so I’ve been ridin’ the range and have managed to corral a few embedded data radios along the way. Now that you cowpokes are present, we can saddle up on our microcontrollers, put on our cowboy and wizard hats, and rope, ride, and wrangle some radios in the Circuit Cellar embedded radio rodeo.

I pulled together a collection of embedded radio equipment from various manufacturers to give you an idea of what’s available these days, and I’ll point out the features contained within the new embedded RF technology. A collective representation of all of the radio rodeo participants is shown in Photo 1. Now let’s take a look at all of the embedded radio goodies I rounded up in the Florida room (in alphabetical order).

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Photo 1—Here’s a glimpse of all of the radio participants. Each data radio has its own personality. That’s a good thing because having a variety of differing features allows you to select the right radio for your application.

ABACOM XTR-903-A4

The Abacom Technologies XTR-903-A4’s claim to fame includes its size, ease of use, and programmability. Measuring in at 23 mm × 33 mm, the XTR-903-A4 is almost as thin (5.5 mm top to bottom, excluding the mounting pins) as the PCB it’s built on. The digital heart of the XTR-903-A4 is an Atmel ATmega8(L) microcontroller, which allows the XTR-903-A4 to communicate with an embedded host using simple TTL RS-232 signaling. All of the communication protocol stuff you would normally have to write is already coded into the XTR-903-A4’s Atmel microcontroller. My XTR-903-A4 is a 433-MHz model, but you can get XTR-903-A4s that operate at 868 and 900 MHz.

The XTR-903-A4 operates using frequency modulation (GFSK) at 9600, 19,200, or 38,400 bps, with each data rate range having its own encoding scheme. Hamming and Manchester encoding are present when the XTR-903-A4 is used at 9600 bps. Manchester encoding alone is used when the XTR-903-A4 data rate is upped to 19,200 bps and a scrambling technique is used for 38,400-bps radio links. If your application doesn’t need the speed, the 9600 bps radio link is optimal because it provides a higher level of data protection and longer range (up to 200 m in open air with an omni-directional antenna). 

The data rates are selectable using the XTR-903-A4’s SP1 and SP2 control pins. The XTR-903-A4’s SP1 and SP2 pins can be tied permanently or controlled via a host microcontroller.

While I’m on the subject of configuration pins, the XTR-903-A4 can be put into Power Down mode by the host microcontroller using the PWRDN pin. In this mode, the XTR-903-A4 draws just under 10 µA.

To really keep things simple, the XTR-903-A4 uses a set of unique AT commands to select one of 10 channels between 433 and 434 MHz and set the module’s RF output power. An AT command to monitor channel activity and signal strength is also incorporated.

XTR-903-A4 transmission and reception synchronization is automatic thanks to the embedded ATmega8(L). All you have to do is feed data to the XTR-903-A4’s serial interface without any regard for data length. The XTR-903-A4 doesn’t do any data buffering, nor does it add a CRC or any other form of checksum data to the transmitted data. A preamble is automatically inserted at the beginning of a transmission to allow the remote XTR-903-A4’s receiver to sync up. The inclusion of the preamble induces a data transmission latency of about 20 ms, which means you should allow 20 ms for transmit-and-receive switching in your application code. The logic inside the XTR-903-A4 also automatically appends an end-of-transmission data sequence to the transmitted data packet.

My set of XTR-903-A4 embedded radios didn’t come with an evaluation board. So, using the straightforward technical detail included in the XTR-903-A4 documentation, I easily crafted my own set of XTR-903-A4 evaluation boards, which you can see in Photo 2 and Figure 1.

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Photo 2—I used the PCB construction detail contained within the XTR-903-A4 documentation to fabricate this evaluation board. What you don’t see are all of the SMT transistors, resistors, and capacitors that support the status LEDs.

 

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Figure 1—The XTR-903-A4 hardware interface is dead easy because the blinking lights are optional.

I selected the PIC16F84A as the host microcontroller because it doesn’t require a lot of effort to employ, can operate at the 3-VDC level required by the XTR-903-A4, and is superbly supported on the programming side by PICSTART Plus, MPLAB, and the Custom Computer Services C compiler. As you can see in Listing 1, the inclusion of the Atmel microcontroller in the XTR-903-A4 circuitry makes it easy to code host microcontoller transmit and receive routines.

Listing 1—A simple RS-232-capable microcontroller and some just as simple code is all it takes to put the XTR-903-A4 on the air.