Lowering the energy of Bluetooth low energy
Bluetooth low energy is a hallmark feature of Bluetooth v4.0 and has been designed to operate from small batteries with limited capacity. For the first time, Bluetooth low energy allows compact, low duty cycle wireless devices to link to the Bluetooth “ecosystem.”
Although there’s no maximum permissible energy (or power) consumption for a low energy device detailed in the specification, because the operating characteristics of Bluetooth low energy are designed to aid low power operation, the Bluetooth SIG expects manufacturers to design transceivers that can operate from a coin cell such as the ULP RF engineer’s favoured 3 V, 160-to-220-mAhr CR2032 battery. This requirement demands that the Bluetooth low energy transceiver operates at peak currents of less than 30 mA and average currents in the microamp range.
The SIG’s confidence is well placed because a specialist group, including companies with years of experience designing coin cell powered-ULP wireless technology, drew up the specification for Bluetooth low energy. The company I work for, Nordic Semiconductor, was at the forefront of that group since it became a foundation member of Nokia’s Wibree Alliance in 2006 (which merged with the Bluetooth SIG in June 2007).
While the publication of the specification allows any company to attempt the design of a Bluetooth low energy transceiver, only the established RF silicon vendors are likely to try (because RF engineering is a big challenge for the inexperienced). But even accounting for the expertise of these manufacturers and the adoption of a single standard, not all Bluetooth low energy chips will be the same. Some will consume less energy than others – and as the key point of Bluetooth low energy is low power consumption, this distinction will be very important.
If an OEM can select one Bluetooth low energy chip that, in a particular application, enables a battery life of nine months, or another that extends battery life to over a year, it’s likely it’ll chose the latter. Several manufacturers have now released their Bluetooth low energy solutions, and there are some big differences in power consumption even though all the devices are qualified to the Bluetooth v4.0 standard. A glance through the various data sheets shows the peak transmit (TX) and receive (RX) current (when the device uses the most power) of competitive chips from major semiconductor vendors can differ by as much as 14 mA (at 0 dBm output power and 1 Mbps bandwidth). That’s a lot of extra current to take from a coin cell battery and will inevitably shorten its life.
Nordic Semiconductor specializes in ultra-low power wireless connectivity. The recently introduced nRF8001 Bluetooth low energy solution, for example, exhibits 12.5 mA peak RX current and 11 mA peak TX current and connected mode average currents below 12 µA (for one second connection intervals).
Our engineers have worked hard to minimise the power consumption of the company’s Bluetooth low energy solution because customers say that battery life is one of the most important operating parameters for their applications.
Figure 1: Casio’s G-SHOCK Bluetooth Low Energy Watch uses a Nordic chip to keep power consumption down and extend battery life.
Casio, one of Japan’s leading consumer electronics companies, for example, has chosen a Nordic chip to power the wireless connectivity of its G-SHOCK Bluetooth Low Energy Watch. The company says the main reason for this choice was because it wasn’t prepared to compromise on the key customer demand of up to two years of operation from the watch’s CR2032 coin cell.
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