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Comparing BLE and proprietary RF for HID apps

Posted: 11 Oct 2012     Print Version  Bookmark and Share

Keywords:human interface devices  Bluetooth Low Energy  interference 

As the array of available wireless technologies expands, more sophisticated human interface devices (HID) are entering the market with integrated wireless technology (wireless keyboards, wireless mice, etc). While wireless standards provide the benefit of interoperability, they also introduce complexity and overhead that an application may not require, resulting in a higher system cost. On the other hand, proprietary protocols give developers flexibility to customise applications at the expense of requiring developers to take on the development process.

This article compares the Bluetooth Low Energy technology with the proprietary protocols in the HID market.

The key RF requirements for selecting a PC HID are cost, power consumption, reliability/security, speed/throughput, and ease-of-design. In general, standards win in the market over proprietary implementations because of interoperability between devices. However, in case of PC HIDs until 2009, only proprietary protocols have dominated the market. This can be attributed to the lack of any wireless standard optimised for the PC HID market in terms of cost, power, and efficiency.

Enter Bluetooth Low Energy, aimed at low power applications and the PC HID market. Both Basic Rate (BR) and Low Energy (LE) Bluetooth support device discovery, connection establishment, and connection mechanisms. Bluetooth LE also includes features designed to enable products that require lower current consumption, lower complexity, and lower cost than BR.

The wireless HID market is extremely cost-sensitive, and the MCU serving as the RF base band controller needs to have enough Flash to hold the wireless protocol stack. As Bluetooth LE is a standard, stack code size is far greater than for proprietary protocols, thereby requiring more Flash and increasing cost. The wireless HID market has been dominated by proprietary protocols because of their light-weight protocol stacks.

Proprietary networks, however, require an external bridge to be connected to the PC/Host so that they can talk to the other devices in the network. As Bluetooth is in most of the PCs and mobile phones, these devices will have an integrated Bluetooth bridge with dual mode support (BR and LE) in the future which eliminates the need for an external bridge.

Power consumption
Unlike Basic Rate Bluetooth, Bluetooth LE is an "always-off" technology and specifies that an application should not consume more than 20mA of peak current and 15mA in coin cell applications for a maximum of 3 ms data transfer. This allows small devices like watches and sports sensors to achieve years of battery life through the use of small coin-cell batteries. Dual-mode implementations supporting Bluetooth and Bluetooth LE, however, will share the existing Bluetooth physical radio and antenna and so maintain the same power consumption as classic Bluetooth technology.

Comparing power consumption can be challenging, given that proprietary RF chip manufacturers typically do not disclose power consumption on datasheets. Developers must therefore obtain their own power consumption data using experimental board set-ups and their respective firmware test environments.

Reliability and security
Robustness to interference in the 2.4GHz world means the ability to reliably co-exist with 802.11b/g, Bluetooth, Wireless USB, and a host of cordless phones and microwave ovens. While some proprietary radio devices like the CYRF6936 can employ DSSS (Direct-Sequence Spread Spectrum) along with FHSS (Frequency-Hopping Spread Spectrum) transmission schemes, Bluetooth LE uses only adaptive frequency hopping technology common to all versions of Bluetooth technology. DSSS ensures data robustness while FHSS allows the wireless signal to hop to new channels once interference becomes too great.

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