March 2005, Issue 176

Zeroing in on ZigBee (Part 2)
Chipsets and Source Code

---

CC2420 802.15.4 KIT

Chipcon boasts that the CC2420 is the first transceiver IC to meet both the IEEE 802.15.4 and ZigBee standards. Of course, there’s meeting the standards, and then there’s the ability to produce a workable cost-effective product. After using the evaluation kit, I can say that Chipcon has produced both in the first version of silicon.

Chipcon’s hardware support and software tools keep you ahead of the complexity curve. The single-chip implementation limits the RF circuitry to just the antenna and its matching network (see Figure 1). The PC-based SmartRF Studio software makes configuring and using the evaluation kit a breeze.

(Click here to enlarge)

Figure 1—Sure, you say, here comes another one of those single-chip transceivers that somehow has managed to morph itself into a complicated, messy RF jungle by the time it makes its way from the sales brochure to the application diagram. With the CC2420, however, this promise is almost entirely kept.[1]

Fortunately, soldering the CC2420 (although fairly tiny) is just under that critical threshold where doing it yourself is still doable. The CC2420 QLP-48 package with a 0.5-mm lead pitch is a challenge, but it’s one I’ve been able to cope with. Products like PCB iron-on artwork transfer paper has allowed amateur PCB makers like me to easily produce tracks and clearances with 10-mil (0.01") track widths and clearance. A friend of mine regularly does 7 mils with a clothes iron and a slosh of etching acid!

As well as the physical challenge of the intricate surface-mount packages, software drivers are the other big impediment to experimenting at home. Often, a lack of information in the datasheet and the need to program 100 registers consigns hopeful efforts to the waste bin. When success is achieved and the results are posted on a web site, the person responsible is rightly proclaimed a hero.

ZigBee, or even a full 802.15.4 link, would take more than an afternoon to develop your own software for, but you’d be able to get something useful fairly quickly. To prove the RF link, simply use the serial port to confirm two-way serial communication with the CC2420. A simple register setting then places it in Transmit or Receive Test mode, so you can check if the hardware works.  

Making a simple one-way, point-to-point link is the next step. Then, a simple two-link is accomplished by implementing mode control by manipulating the CC2420 state machine. If you need more than a master/slave relationship, add a simple MAC protocol such as aloha (time slots) or check for a clear channel before transmission. You can add more features at this point. Encryption, link quality indication, and dynamic power output level control would be nice. But wait. This is starting to sound like 802.15.4! Why don’t you just download the free code from the Chipcon web site?

Most of what you need is included with the CC2420DBK demonstration board kit. You can download the files from Chipcon to make your own PCB. The schematic, BOM, and PCB layout are all there. You can use the free source code for the IEEE 802.15.4 layer. If you don’t want to make the PCBs yourself, they’re available on the web site for $50. The software isn’t a full implementation of the IEEE 802.15.4 MAC, but it’s enough for basic message sending and acknowledgment. The full IEEE 802.15.4 MAC software is free if you sign a licensing agreement. Although both versions were written in C for the Atmel ATmega128L microcontroller, they port easily to other microcontrollers. 

The easiest approach would be to copy the PCB exactly and use the free GNU AVR gcc cross-development tool. Chipcon strongly recommends that you copy the reference design as closely as possible. If you want to develop a two-layer PCB, you should keep the ground via positions as close as possible to the location on the original board. Of course, FCC licensing is another story. The first stage is to get to a working prototype!

The documentation for the CC2420DBK demonstration board kit states that practical testing outdoors with line of sight (LOS) resulted in a range of 170 m at 250 kbps (zikes!). The simplicity of this information surprised me, because IC vendors usually provide ambiguous range performance information.

I actually used the CC2420DK development kit for range checking. It consists of a pair of motherboards (CC2400EB) and their respective transceiver plug-in boards (CC2400EM), as you can see in Photo 1.

(Click here to enlarge)

Photo 1—Your next 802.15.4 project will look this simple. The IC is 7 mm × 7 mm. Chipcon encourages you to copy the PCB layout and adapt it for your own needs. You need a four-layer PCB to guarantee the quoted performance.[2]

I was initially discouraged by the presence of a Xilinx Spartan II FPGA on the motherboard. Why so complex? But my disappointment vanished when I discovered that the FPGA is only used to configure the evaluation board in a variety of different setups. The FPGA routes signals so that the on-board microcontroller controls the plug-in transceiver board. In another configuration, the plug-in transceiver board connections are routed via the headers to your processor board when prototyping with your own microcontroller.

The process of setting up, including the installation of the SmartRF Studio evaluation software on the PC, took less than 20 min. I found that it wouldn’t work on some of my USB ports. Apart from this, things went extremely well. Getting the two boards set up for a range check was as simple as selecting Normal View, Packet TX, Start Packet Transmit on one of the boards, and Start Packet Receive on the other. I then checked for the lights that flash after a reception. Link quality statistics were displayed in real time on the PC. The CRC and packet count information was great, but the RSSI didn’t work. A separate piece of software can be used for packet sniffing.

I unplugged the board configured for reception, performed a walk-around range check, and continued to look for the flashing of the LEDs. Using this technique, I found reliable reception everywhere in my 1,100-square-foot, single-level wood house. Reliable transmission occurred when transmitting through three walls with the doors closed, even when partially shielding the receiver antenna with my hand and placing it right on top of other operating electronic equipment.

Taking it outside, I found reliable reception 90' away through one wooden wall out to the backyard. Walking down the street, I got intermittent operation as far as 100 yards away from the transmission end inside my house. I even got reliable reception from 12' away after disconnecting the antenna on the receiver!

The version of the IC I tried suffered from excessive current draw in Shutdown mode when the internal 1.8-V regulator was in use. This was corrected in a later silicon revision, along with an address recognition problem.

Now let’s take a look at a simple software application example. A pair of the CC2420DB evaluation kits running the example program will establish a point-to-point RF link, sending data packets containing a 5-byte payload (see Listing 1). The payload will be transmitted when the potentiometer is turned, or the switch, S2, is held down. The first byte of the payload contains the potentiometer value, which controls the PWM duty cycle of the LED at the receiving node. This example isn’t a full-blown ZigBee application, or even a proper 802.15.4 implementation, but it illustrates the basics of doing something useful with the hardware. (Well, assuming blinking LEDs enthrall you!)

Listing 2 shows how pressing the button and noting the status of the yellow and green LEDs can perform a simple range check. This is a simple but useful example. To keep things simple, only a small subset of the IEEE 802.15.4 standard is implemented in this example. A pair of CC2420DBs running this program will establish a point-to-point link on a single channel. The frame format conforms to 802.15.4, but it doesn’t implement any other MAC properties. The full IEEE 802.15.4 MAC software can be obtained for free after signing a licensing agreement.