A dedicated sensor controller means the power-hog SoC can be off most of the time

by Jan Frode Bergsø, Business Development Manager at Nanopower Semiconductor

The battery is an awkward component to integrate into wireless sensor systems. Think of the kind of device that monitors the temperature of movable assets in cold-chain logistics, for example, or asset trackers that measure the exposure of fragile goods to shock and vibration. The battery is significant because it’s typically one of the largest components, and it’s supplied in a standard form factor. Often, the battery is the primary factor that determines a product’s shape and size.

Design engineers would like to reclaim their freedom to shape the product to best fit the application, but shrinking the battery also reduces its run-time before becoming fully discharged, and that, in turn, reduces the value of the product to the end user.

A battery with a short life has other undesirable effects, including the additional cost of battery replacement and the increased volume of used batteries shipped to landfills each year.

Now, however, there is a new way to reduce average power consumption in a wireless sensor system by as much as 90%, which is a significant enough reduction to allow designers to shrink the battery or even replace it with harvested energy.

High-power SoCs

The primary issue with battery power in a wireless sensor system is the system-on-chip (SoC), which performs data processing and wireless data transfers via Bluetooth Low Energy (BLE) or another wireless link. In cold-chain logistics, for example, temperature values change slowly.

For most of the time, the high-powered SoC, which is typically a 32-bit microcontroller with a radio integrated into a single chip, simply reads sensor values at a low sampling rate and waits for something significant to happen. This means an SoC is wildly over-specified for this type of idle-mode monitoring function.

Low-power companions

This is why, rather than having the wireless SoC as the direct interface to sensors, Nanopower Semiconductor developed the concept of a sensor controller companion chip called the nPZero sensor controller. This employs a unique silicon technology that operates “sub-threshold,” which means an ultra-low voltage that is unsuitable for the SoC’s high-frequency operations, but fine for simple sensor monitoring.

In monitoring mode, the nPZero takes control, shutting the SoC down completely. The nPZero controls the sensors over a standard I2C or SPI interface. It can duty-cycle the sensors to allow them to be switched off between polling events and monitor the sensor outputs to detect when a value crosses a preset threshold. The nPZero can wake the SoC when a sensor data needs to be processed and transmitted, thereby maintaining the normal functionality of a wireless sensor system.

Demonstration designs show that an average current can fall by as much as 90% when the nPZero is added to the system, compared to a circuit in which the wireless SoC monitors the sensors on its own.

Easy to evaluate and use

The nPZero IC is easy to use; application parameters, such as sensor measurement thresholds and polling intervals, can be set using the nPZero Configurator, a GUI-based configuration tool. Engineers can evaluate the potential for power savings in their applications with the nPZero Evaluation Kit (bit.ly/44Q86DD).

www.nanopowersemi.com