Magnetic sensor makes electronic compass design easy

Originally posted on Freescale’s Smart Mobile Devices Embedded Beat Blog

Contributed by Mike Stanley


In a past life, while working on Freescale’s digital signal controllers, my coworkers and I considered including Magnetoresistive Random Access Memory (MRAM) for use in one of our DSCs. At the time, MRAM was trumpeted as a possible “universal memory,” with write speeds approaching those of SRAM and densities approaching those of flash memory. Since our DSCs included both RAM and flash, we thought we might be able to simplify the architecture by using a single memory to address both dynamic and static code and data storage.

We met with the company’s MRAM technical team to review the technology and consider tradeoffs. After crunching the numbers, we concluded the timing was not optimal for a change in architecture. Move the calendar forward a few years, and here we are in 2011. I’m now part of the Freescale sensors’ operation, and again looking at MRAM technology. But this time around, we’re using Tunneling Magnetoresistive Technology (TMR) as a basis for a new family of devices for magnetic sensing.

At CES last week, Freescale announced the first member of that family, the MAG3110 3-axis magnetometer. This product introduction is the next logical step in Freescale’s plans to offer a comprehensive set of sensors in support of navigation and gaming markets.

Before digging into specs, let’s have a brief review of our units of measurement: the SI unit of magnetic field B is the tesla (abbreviated T). Wikipedia will tell you that “a particle carrying a charge of 1 coulomb and passing through a magnetic field of 1 tesla at a speed of 1 meter per second experiences a force of 1 newton.” In equation form: 1 T = 1 N / (A m).  Another commonly used unit of measure is the gauss (abbreviated G). The two are easily converted from one to another:

1 T = 10,000 G
1 G = 1 x 10-4 T = 100 microTesla

Some typical values of magnetic field that you might expect to encounter would be:

Item being measured cgs Units* SI Units
Earth magnetic field at zip code 85284 (Tempe, Arizona) on 1/1/2011 0.482 gauss** 48.2 microTesla
typical refrigerator magnet 50 gauss 5,000 microTesla
small iron magnet 100 gauss 10,000 microTesla
small neodymium-iron-boron (NIB) magnet 2000 gauss 200,000 microTeslas

* Some of the magnetic field source values come from Wikipedia.
The strength of the earth’s magnetic field varies from less than 30 microTeslas to over 60 microTeslas around the magnetic poles.

With that behind us, let’s review the MAG3110 preliminary specs:

Parameter Value
Dynamic Range +/- 10 Gauss (+/- 1000 microTesla)
Resolution 0.1 microTesla
Output data rate 2.5 to 80 Hz
Core Supply (VDD) 1.95V to 3.6V
I/O Supply 1.62V to VDD
Package 2mm x 2mm x 0.85mm 10 pin microDFN
Interface I2C

From a block diagram perspective, the MAG3110 includes a 3-axis TMR sensor, associated drive circuitry and analog-to-digital converter, and state-machine-based controller.  Communications with a host controller are via standard 400KHz I2C supplemented by a smart interrupt output signal from the MAG3110 to the host controller.

More detailed specifications are available to select customers under NDA.  But just from the above, you can already see that the MAG3110’s tiny size, extended sensor range, high resolution and flexible supply options make it an ideal choice to add magnetic sensing to just about any consumer product.  Freescale is supporting the MAG3110 with dedicated development boards, canned application code, and detailed application notes. Connect the MAG3110 to the Freescale MMA9550L intelligent 3-axis controller (see previous posts: Evolving Intelligence with Sensors and The Zen of Sensor Design), add software drivers supplied by Freescale, and you have a very intelligent, very economical electronic compass.

The MAG3110 is being sampled to select customers today, and is expected to sell for $1.47 (USD) in 10,000-piece quantities starting in Q2 2011. I’ll be discussing MAG3110 technology and application topics in more detail in future posts. Until then, please check the Freescale web site for more information on sensors.

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