Hijacking Power and Bandwidth from the Mobile Phone’s Audio Interface - Embedded.com

Hijacking Power and Bandwidth from the Mobile Phone’s Audio Interface

The mobile phone is the most pervasive personal communications and computing platform ever created and yet, among its various analog interfaces, only one is truly open: the headset port.. In this paper, we take a closer look at this common interface and assess its utility for augmenting the mobile phone with a range of phone-powered peripherals.

We show that the mobile phone headset port can be used to efficiently power external peripherals and communicate with them, enabling many new phone-centric applications. But why, beyond openness, should we use the headset port?

There are many reasons: it is simple, inexpensive, ubiquitous, and documented.But perhaps even more important is the fact that the headset interface is backward-compatible with most phones in use today, andmany recycled ones too, so existing phones could form the basis for many health and communications applications in developing regions.

This paper explores the feasibility of this interface for power delivery and the generality of the interface for data transfer. Our goal is to understand the design space, enable phone-powered peripherals, and make the phone the center of a new instrumentation ecosystem for developing regions.An affordable and ubiquitous plug-and-play interface for attaching sensors could have many applications.

It could turn the mobile phone into a scientific data collection instrument, a medical device, or an agricultural tool by connecting low-power sensors toa source of connectivity and power. The mobile phone, with itsever improving computing, communications, and graphics capabilities is an obvious choice.

More generally, by leveraging a standard and ubiquitous interface like the headset jack, the phone could alsoserve as a multimeter or oscilloscope – useful tools that otherwise may be out of the reach of most citizens in developing regions, but well within their grasp as a $5 phone accessory.

In this paper we describe how we endowed the mobile phone with a low-cost, open interface thatcan parasitically power external peripherals, and transfer data to and from them, using analog, digital, and serial signaling, using only the existing headset audio port.

This interface, called HiJack, allows the mobile phone to easily integrate with a range of externalsensors, opening the door to new phone-centric sensing applications. In this paper, we characterize the signaling and power delivery capability of the audio jack, design circuits and software totransfer data and harvest energy, and evaluate the performance ofour designs.

We also use the mobile phone’s audio channel to create a layered communications stack that supports low-level, analog signaling and high-level, multiplexed data communications with external devices. Our design supports a single, bi-directional communications channel at a data rate of 8.82 kbps over a Manchester encoded serial stream, using just a few discrete components and the hardware peripherals found in almost any microcontroller.

Our harvester delivers 7.4 mW to a load with 47% efficiency using components that cost $2.34 in 10K volume. Integrating the pieces, we present a combined system for delivering data and power over audio, and demonstrate its use by turning an iPhone into an inexpensive oscilloscope, EKG monitor, and soil moisture sensor, all at price points accessible to most consumers in developing regions.

To read this external content in full, download the complete paper from the author archives at the University of Michigan.

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