Standards differ widely in their adoption and longevity, and 802 is one of the IEEE's most prolific standards families. IEEE 802.15.4, with Zigbee compliance, is a wireless personal-area network (WPAN) communication standard designed for "groups of unattended wireless systems in homes, offices and factories." This teardown highlights the key criteria for Zigbee market adoption and casts the spotlight on two currently available Zigbee offerings: Chipcon's CC2420 and Freescale's MC13192.
Zigbee remains a nascent market today despite bullish analyst predictions, including ABI Research's 2006 shipments forecast of 80 million units, up from 1 million today. Designed for low-power, short-range RF transmission, Zigbee is ideal for applications such as security sensors, light switches, smoke detectors and thermostats that require modest bit rates (250 kbits/second), a multiyear battery life and a low duty cycle (less than 0.1 percent). Zigbee is starting to penetrate the building automation and meter-reading markets, but its killer application remains unknown, causing strategy problems for chip suppliers. One application that is garnering Zigbee traffic is mobile devices: A user could tap Zigbee integration to control the lights in the house, download sensor data or unlock car doors remotely, without having to carry multiple devices.
The total system cost and performance, including power consumption or battery life, will determine whether Zigbee realizes its market potential. The Chipcon and Freescale devices illustrate the inherent cost and performance trade-offs of different product decisions.
See image: Freescale's MC13192 die is twice as small as Chipcon's CC2420 die
The CC2420 has a differential output that is routed almost directly to the antennas.
The Freescale device features two sets of differential outputs, so it requires external components for coupling.
Source: Semiconductor Insights Inc.
Freescale's MC13192 die is twice as small as Chipcon's CC2420 die
The CC2420 has a differential output that is routed almost directly to the antennas.
The Freescale device features two sets of differential outputs, so it requires external components for coupling.
Source: Semiconductor Insights Inc.
Low cost is a definite requirement to enable mass deployment of Zigbee devices. The cost of a Zigbee IC solution (typically one or two chips) plus the necessary discrete components and its associated mechanical components should be less than $15 next year, with a goal of less than $10 the following year, at least for reduced-function devices. According to participants at the recent Embedded Systems Conference, some vendors are already offering sub-$5 components. But cost should not be considered in a vacuum and must also be considered at the network and system levels.
Node types
In a Zigbee-configured network, there are two basic types of devices or nodes: full-function devices (FFDs) and reduced-function devices (RFDs). FFDs act as network coordinators, or routers, with configuration information stored in memory and sufficient logic and processing power to handle self-configuration and application-level tasks. RFDs are relatively simple devices that must be configured in a star topology at the network edge, requiring communication to an FFD. It is possible, although not as cost-effective, to use FFDs as RFDs to minimize network deployment. Enhanced RFD (eRFD) devices create a so-called trimode network but also assume that the eRFDs have low cost and low power. Since RFDs and eRFDs are mainly developed around a highly integrated RF IC, the manufacturer capable of providing the lowest power consumption solution, as well as the lowest cost solution, is positioned to win market share.
System-in-package achieves higher levels of integration and reduces the amount of external components. The SiP also allows more flexibility than the system-on-chip. Freescale's MC13192 is a SiP-type configuration in that it does not implement inductors on the die, limiting the risk of interference in the case of a stacked-dice implementation.
Chipcon has hedged its bets by sending both the CC2420 and the CC2430 to market. The CC2420 RF front end includes memory (128 kbytes of flash, 8 kbytes of RAM) and the microcontroller in a single package. The CC2430 is limited to the functionality set implemented on the die and therefore may be less efficient in cost for a given application. Chipcon's strategy to address a wide range of Zigbee applications compels it to provide a solution that is more generic and, therefore, less cost-optimized.
If a smaller die increases the number of external devices, any cost savings and footprint advantage at the die level may be negated. The microprocessors coupled to the CC2420 and the MC13192 must be clocked. To reduce the bill-of-materials cost, both solutions provide a clock back to the microprocessor using a single crystal for the overall system. Both suppliers used a 16-MHz crystal, but the Freescale device is designed to support a low-resolution crystal, reducing the cost of this component. Although the Freescale solution seems to use more external components than Chipcon's, the external components are less expensive, leaving Freescale with a bill-of-materials cost advantage.
The Freescale MC13192 die is twice as small as the Chipcon CC2420 die, even though they appear to be manufactured with the same process generation (estimated 0.18 micron). A preliminary analysis has revealed that there are similar functional areas on both dice, but there are significant variations in how the functional blocks have been reduced in size. The Freescale device and Chipcon devices differ slightly in architecture. The Chipcon design has a differential output that is routed almost directly to the antennas. The Freescale device features two sets of differential outputs: one for receive and one for transmit. External components are needed to couple and multiplex those signals onto the antennas. The Chipcon device does not need these components, so one would expect the Chipcon die to be larger than the Freescale device-but not twice as large.
Freescale has made significant reductions in the area occupied by the receive circuitry, realizing a 68 percent size advantage over Chipcon for this block. Freescale's transmit circuitry is also significantly smaller, but that reduction is consistent with the reduction in total die area. The die area occupied by the frequency synthesizer has been reduced the least of any of the major analog functional blocks. Based on our analysis, Freescale appears to have performed the most innovative die-area reductions in the receive path.
Another notable difference between the devices is the absence of inductors on the Freescale die. That allows Freescale to to implement a die-on-die solution more easily than with the Chipcon solution, in which die interference would be difficult to manage.
The CC2420 was a very early market entrant, and it is clear that Chipcon's design focus was on time-to-market and low power consumption, with cost optimization being secondary. The next generation of Chipcon devices (the CC2500 family) seems to indicate significant die size improvement that may be as much as 50 percent.
The Freescale device is also a first-pass design, but Freescale focused more on cost vs. power as a development strategy.
Performance and power
To minimize power consumption and realize a multiyear battery life, Zigbee devices spend most of their time in sleep vs. active mode. In active mode, the devices are more often listening to, or receiving information from, the network than providing information, or transmitting. To remain in line with Zigbee's power consumption objectives, semiconductor suppliers need to focus on receive-, idle- and sleep-mode power consumption.
Mode by mode, the Freescale solution consumes more power than the Chipcon solution. With Zigbee solutions, however, duty cycles are important parameters. In addition, latency should be taken into consideration very carefully, since it could dramatically modify the duty cycle of a device. Depending on the application, the time to wake up from "doze" or "hibernate" mode will define how long the device can sleep when required by the network. With regard to latency, Chipcon seems to be better than Freescale; but here, again, a full analysis within a specific application is required, since some Zigbee devices may not need such performance.
The sweet spot for Zigbee applications seems to be around three years of battery operating lifetime using commercially available battery solutions, such as AA or AAA. If this requirement holds, and depending on the application duty cycle, the power consumption difference between Chipcon and Freescale ends up being minimal, although applications with low duty cycle would benefit from a Chipcon solution.
Low-duty-cycle applications can often be attributed to RFD devices. Chipcon's additional offering of the CC2430 single-chip solution clearly indicates its desire to dominate this market. On the other hand, Freescale's low-power solution, although also applicable to RFDs devices, seems to be more appropriate to eRFD- and FFD- type devices.
It is worth noting at this stage that lower power consumption allows the use of smaller batteries and therefore lowers the system cost. This is an important consideration for Zigbee devices, since the battery cost becomes a major contributor to the final bill of materials.
Both manufacturers have recognized the importance of providing a solution beyond the Zigbee minimum requirements. With a higher output power, approximately 5 dBm, the Freescale device provides additional range, potentially enabling customers to achieve additional network cost optimization (by reducing the number of FFDs).
The performance and robustness of the link between devices is another important criterion, not only because those devices live in the busy, 2.4-GHz ISM band but also because a robust link that is immune to interference will limit the number of retransmissions. This will reduce power consumption compared with a less-robust solution and therefore enable a reduced cost.
The CC2420 and the MC13192 are both designed in 0.18-micron CMOS process. Zigbee devices, whether single-chip or multichip, are not complex. The digital content implemented on the die is small, so moving those designs to 130- and 90-nm process technology will not generate great die size savings. Any cost saving in the future should be achieved via architectural innovations.
A commercial requirement for success in Zigbee is the provision of a full system solution. The acquisition of Figure 8 Wireless by Chipcon is a clear sign of this strategy-as is the Freescale suite of evaluation and development kits.
In addition, both manufacturers are providing solutions for RFDs, eRFDs and FFDs devices.
The next generation
The course of the Zigbee market's evolution will determine which vendors will prove to have hit upon the right product strategy for market success. Both Chipcon and Freescale are presenting strong arguments with their first silicon, and given the large number of applications and the complexity of mesh-networking requirements, both suppliers might well end up winners in this market.
The industry should carefully watch for the second-generation devices from Chipcon and Freescale. Further power consumption reductions and component cost reductions should be the respective priorities for Freescale and Chipcon, as the companies look to increase deployment of their Zigbee devices.
Meanwhile, suppliers such as Ember, Atmel and Oki will have competitive products available in the future. Both Freescale and Chipcon are coming from large and proven RF experience, and the new entrants are watching to see how each addresses the Zigbee market.
