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Utilize interrupt features of a MEMS accelerometer

Posted: 26 Dec 2011 ?? ?Print Version ?Bookmark and Share

Keywords:MEMS? accelerometers? linear acceleration?

Many of the 3-axis digital-MEMS accelerometers recently released on the market can generate interrupts for a number of applications. Some of the applicationssuch as screen rotation, wakeup, freefall, single click and double click recognitionhave been very popular and implemented in various devices.

A digital MEMS accelerometer usually has one or two interrupt-output pins available that can be interfaced with the I/O pins of an external host processor. The accelerometer can be configured to monitor acceleration or motion in the background while the host processor could perform other tasks or simply stay in low power sleep mode. When an interrupt is detected by the accelerometer, the host processor can then wake up to check whether the interrupt needs to be served or not.

This article describes some of the popular interrupt-based applications including data ready, freefall, wakeup, single and double click interrupts.

Data-ready interrupt
A digital MEMS accelerometer can be configured to generate a signal to indicate when a new set of measurement data is available to be read. This signal is usually represented by the data-ready bit in the status register of the accelerometer which is accessible to the user. The signal can be routed to an interrupt pin by enabling the data-ready-interrupt bit in one of the control registers, and its polarity can be set to active-low or active-high through the edge-level bit in another control register. By default, when the accelerometer is powered up, the interrupt pin is active-high.

The data-ready interrupt signal goes high when a new set of acceleration data has been generated in the data registers of the accelerometer. The interrupt goes low when the high bytes of the data registers of all enabled axes have been read. If the high bytes haven't been read during 1/ODR (Output Data Rate) time period, then the data ready interrupt will stay high and the overrun bit in the status register becomes "1", which means that data overrun has occurred and the previous data samples have been overwritten.

The data ready interrupt feature can be used to synchronize the continuous data acquisition to prevent data loss. By default, the interrupt pin is a push-pull topology which can be tied directly to an I/O port of the host processor that has interrupt function. When new data is not ready, the host processor can perform other tasks or simply stay in low-power sleep mode.

Once the data ready interrupt occurs, the host processor can wake up at the rising edge of the I/O port to enter the interrupt service subroutine and then read and process the acceleration data. This can help reduce the overall system power consumption as the host processor is not required to keep polling the data-ready bit in the status register.

During the evaluation of the accelerometer, the data ready interrupt feature can be used to check the frequency of the interrupt signal wave form at different ODRs by using an oscilloscope.

Freefall interrupt
The freefall interrupt feature of the accelerometer has been widely used in hard disk protection. When, for example, a laptop computer is falling, the accelerometer embedded in the laptop can generate an interrupt so that the hard disk can park the magnetic head of the disk in a safe place, to prevent the mechanical damage of the disk.

In medical applications, the accelerometer freefall interrupt can be integrated with the altitude detection by a MEMS pressure sensor to detect if a patient has fallen onto the floor. If the fall event is confirmed, then the host processor inside the medical device will send an alarm to the hospital personnel through a wireless link. The latest accelerometers and pressure sensors with small size and ultra-low-power consumption have enabled such an application (known as man-down application), as these medical devices are battery operated and therefore power sensitive.

By definition, the freefall stands for the state where there is no external force to be applied to the accelerometer, except for Earth gravity. Theoretically, therefore, a 3-axis accelerometer simultaneously outputs zero-g measurements about all it sensing axes during freefall. This is the specific characteristic of the freefal,l meaning that other motions will not be able to fake the freefall event.

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