FROM EDN EUROPE: Wireless USB aims for lowest of low data rates
By Graham Prophet -- EDN, February 3, 2005
Faced with the problem of designing a low-cost, low-power, low-data-rate wireless data link, designers now have multiple standards from which to choose. Zigbee and Bluetooth are two that have been heavily promoted for use in that applications space. Into the same space comes Cypress Semiconductor's latest implementation of Wireless USB. In fact, Cypress believes that room exists for a wireless-USB standard and product line with an even lower range and data rate than that of either Bluetooth or Zigbee.
The new implementation, Wireless USB N:1, aims to be a wire-replacement technology, predominantly for event-driven, low-duty-cycle applications in which multiple outlying nodes report to a central point. The topology is therefore that of a star network. Maximum bandwidth is 1 Mbps, but maximum throughput is 62.5 kbps; a high degree of processing gain is employed to ensure reliability of data delivery. As with other standards in this domain, W-USB operates in the 2.4-GHz band, using direct-sequence-spread-spectrum (DSSS) radio technology on 79 channels. Range reaches 50m, and the radio design detects occupied channels and avoids them in its automatic channel selection to minimise the effects of interference. A theoretical limit of 65,000 nodes in a system exists. The radio is a single-chip design that requires only 11 passive components and employs a low-cost 13-MHz crystal. Cypress provides a reference design using a pcb-track antenna that designers can adopt by precisely following the layout to avoid the need for any RF-layout-design work.
W-USB N:1 aims at the lowest cost and power. The chip uses less than 0.25 μA in sleep mode; in a typical application, it spends most of its time in this mode (external events awaken it), reporting a measurement and closing down again. Designers can therefore expect years of battery life for such nodes, Cypress says. The company provides a battery-life calculator on its Web site that takes typical application parameters and returns operating time from a specified battery type. A host microcontroller need be only the simplest 8-bit type, and the code for a W-USB node occupies less than 4 kbytes. The radio chip costs less than $3 (100,000). The $795 CY3635 design kit includes five radio modules to allow designers to evaluate a star-topology device network and supports the launch of W-USB (Picture).
Cypress Semiconductor, +44 1256 416969, www.cypress.com.
