Wireless specs expand options for MCUs

With all other forms of data communications going wireless, it is no surprise that control functions might follow suit. Such an effort has, in fact, been in the works for several years in the form of Zigbee and IEEE 802.15.4 standards for wireless personal area networks (WPANs). And now that wireless MCUs and derived products are beginning to enter the market in force; that effort is paying off.

Zigbee and IEEE 802.15.4 are not synonymous, although they are typically mentioned together. The IEEE 802.15.4 standard covers the PHY and foundation media-access control (MAC) layers of a WPAN system. The PHY is a spread-spectrum radio link operating at 88MHz, 915MHz or 2.4GHz while the MAC layer handles flow control, network association and data encryption (AES-128) functions.

The 802.15.4 standard can be used as the basis of many network types, including but not limited to Zigbee.

Zigbee builds on IEEE 802.15.4, augmenting the MAC layer with support for upper levels and adding a networking layer with applications profiles and interfaces. Zigbee creates a self-configuring star, mesh or cluster-tree network and aims at low-data-rate applications such as home automation and industrial sensing and control. The most recent version of the specification, Zigbee 2006, is beginning to find wide adoption.

Integration decisions
Semiconductor vendors have had to decide which network layers to integrate when creating a wireless MCU. Some have chosen to implement only the radio (PHY) layer, leaving the MAC and Zigbee network layers as software in a general-purpose MCU. Others have integrated the radio and MAC layers into one chip, which also requires a second application processor but with reduced performance requirements.

A few companies have created fully integrated devices that implement PHY, MAC and Zigbee protocol layers, as well as having built-in peripherals and headroom for implementing applications software.

Choosing among these approaches is a matter of making trade-offs. Proponents of the separate radio approach point to electromagnetic interference issues and radio performance compromises inherent in mixed-signal chip design.

Meanwhile, the MAC/radio combination is touted as being flexible in that it does not restrict users to Zigbee, yet it frees the applications processor from handling encryption and other MAC operations.

The single-chip approach, for its part, promises lower implementation costs than multichip designs, but memory limitations may impose restrictions on the resources available for applications software. Hardware acceleration for MAC and encryption functions can reduce software overhead for MCU and free some resources.

MCU vendors with MAC/radio and one-chip products typically provide a certified Zigbee stack for their processor. For users choosing the separate radio approach, other companies provide the stack as a standalone software product.

- Richard Quinnell
EE Times