March 2005, Issue 176

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

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DIFFERENT CORE CHIPSETS

Now let’s review some of the transceivers implementing 802.15.4 and ZigBee standards. I’ll concentrate on describing each of the different core chipsets rather than commenting on each implementation.

More than a few companies use the Chipcon 2420. I’m sure there are good and bad implementations out there. So, the following review will be beneficial if you want to muck in at the chip level rather than the module level. The advantage is that if you place one of the chips on the PCB, you’re looking at $15 for the entire transceiver in low quantities, assuming you are already doing your own PCB for the project.

I’ll also describe some alternatives to implementing a strict ZigBee solution on the transceiver ICs. The topic of this article is cutting edge. The ZigBee Alliance ratified the upper layers of the ZigBee standard in October 2004. The 802.15.4 portion is more firmly established. There are alternatives to the upper layers of ZigBee if you want an easier ride until ZigBee is more freely available to people who are on a budget.

Let’s pick up where I left off by focusing on alternative protocols that can run on the CC2420. Remember, chips like the CC2420 only give you the physical and MAC support layers. Chipcon is going to package the CC2420 with a ZigBee software stack for $2.30 per unit in large quantities. I’m also excited about the upcoming launch of the CC2430, which will include a microcontroller, RAM, and flash ROM in the same package as the RF transceiver.

An alternative to ZigBee is to choose your own simplified protocol or buy one off the shelf. One such alternative comes from Moteiv, a company founded by three Berkeley students who worked on the open-source TinyOS wireless network system. Their Telos product with a Texas Instruments MSP430 microcontroller doesn’t require any programming boards or development software. It has integrated USB and the tools for compiling. It’s approximately $130, but offers an instant on approach backed by the availability of open source code.

Ember’s EM2420 was co-developed with Chipcon. It’s the same IC, but it isn’t currently available to the general public in low quantities. According to the datasheet, the chip is only available with a licensed Ember networking stack. It’s targeted to approved 8-bit processors.[3] This means Ember only has relationships with serious developers. It can’t sell small quantities of hardware for casual developers to play around with. Still, it may ship one to you if you can come up with $13,950 for the EM1020 developer kit (plus shipping!).

The 802.15.4 hardware in the form of the EM2420 and CC2420 was ready long before the ZigBee standard was ratified. This didn’t stop companies from developing their own ZigBee-like transport/network layer. Ember’s is called EmberNet.

If you want ZigBee included without running the ZigBee layers on your own controller, then Oki Electric Industry Co. might be able to help. Last May it announced the first IEEE802.15.4 and ZigBee single-chip solution. This leaves just the application layer for you to implement. CompX sells PCBs based on this IC.

A key member of the Zigbee Alliance, Freescale recently released the MC13192 2.4-GHz RF transceiver data modem. The 13192DSK evaluation kit is similar in principle to the Chipcon CC2420DBK. You get two nodes for $199. Each node has the MC13192 transceiver and MC9S08GT60 low-power microcontroller. The microcontroller is preloaded with simple MAC (SMAC) software with which you can establish simple point-to-point or star proprietary network topologies. You can also download the 802.15.4 MAC source code for free and use it with higher-level routines. Metrowerks’s CodeWarrior development studio for HCS08 microcontrollers is included. The on-board peripherals include two accelerometers, some switches, LEDs, and an RS-232 port for monitoring and flash memory programming.

The MC13192’s lack of an on-chip transmit/receive switch is a potential disadvantage. The evaluation kit overcomes this with no increase in the bill of materials by using separate transmit and receive antennas. This less integrated approach results in a smaller package size, but you’d need an external transmit/receive switch in applications requiring a single antenna. The advantage of excluding the internal transmit/receive switch on-chip is that an external power amplifier can be used to boost the signal for greater range. However, both the Freescale and Chipcon ICs already have a transmission power of 1 mW, which is twice the minimum required by the ZigBee specification.

Chipcon, Atmel, and Freescale will bundle their hardware with a ZigBee software stack developed by Figure 8 Wireless. ZMD is another main player with silicon. Its ZMD44101 is in the 868- to 928-MHz bands. ZMD is optimizing for a lower data rate of 40 kbps, which should make it a good choice for devices that require the most reliable communications at the maximum range.

According to William Craig, program manager for wireless communications at ZMD, the IEEE 802.15.4 standard provides for one channel (868.3 MHz) in Europe because of bandwidth limitations. The sub-1-GHz band is desirable for RF characteristics, where range and attenuation are at issue. Even more desirable is an increased frequency range providing more channels for the IEEE 802.15.4. ETSI may consider this requirement, William said. The ZMD44101 Fractional-N RF_PLL design provides for software-controlled frequency selection that will accommodate emerging frequency standards. There is a 1% duty cycle restriction in Europe for 868 MHz, which means that it’s currently suitable only for RFD end devices.

Atmel is also initially focusing only on the low bands with its AT86RF210 Z-Link transceiver and matching AT86ZL3201 Z-Link controller. It will offer a 2.4-GHz version of the transceiver IC before developing a single-chip solution.

As with Freescale, the ZigBee craze seems to be an excuse to hawk general-purpose components relabeled as “ZigBee-compatible.” There are a few special features on these microcontrollers such as hardware AES encryption and a specialized random number generator. However, transceiver chips such as the CC2420 already do this in hardware, thereby allowing you to choose a true general-purpose microcontroller or reuse one from an existing application.

One advantage is that the Atmel two-chip solution turns into a one-stop package for an entire ZigBee solution. The transceiver and microcontroller, including 802.15.4 in ROM and a ZigBee protocol stack sharing the 32-KB flash memory with your application, are sold together for $6.75 in large quantities. Figure 8 Wireless supplies this ZigBee protocol software to Freescale, Chipcon, and Atmel. This will surely be one of the easiest ways to implement a ZigBee device at home. With the controller available in a 64-lead TQFP, or QFN, and the transceiver in a 48-pin QFN, it’s still feasible to place these ICs on your own hand-soldered board.

Refer to Table 1 for a comparison of the four aforementioned transceivers. Some criteria (e.g., sensitivity) cannot be compared directly when considering transceivers supporting different PHY frequencies.