MMA8450QT datasheet(12/57 Pages) FREESCALE

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Sensors

Freescale Semiconductor

MMA8450Q

Terminology

3.1

Sensitivity

Sensitivity describes the gain of the sensor and can be determined by applying a g acceleration to it, such as the earth's

gravitational field. The sensitivity of the sensor can be determined by subtracting the -1g acceleration value from the +1g

acceleration value and dividing by two.

3.2

Zero-g Offset

Zero-g Offset (TyOff) describes the deviation of an actual output signal from the ideal output signal if no acceleration is present.

A sensor in a steady state on a horizontal surface will measure 0g in X-axis and 0g in Y-axis whereas the Z-axis will measure 1g.

The output is ideally in the middle of the dynamic range of the sensor (content of OUT registers 0x00, data expressed as 2's

complement number). A deviation from ideal value in this case is called Zero-g offset. Offset is to some extent a result of stress

on the MEMS sensor and therefore the offset can slightly change after mounting the sensor onto a printed circuit board or

exposing it to extensive mechanical stress.

3.3

Self-Test

Self-Test checks the transducer functionality without external mechanical stimulus. When Self-Test is activated, an

electrostatic actuation force is applied to the sensor, simulating a small acceleration. In this case the sensor outputs will exhibit

a change in their DC levels which are related to the selected full scale through the device sensitivity. When Self-Test is activated,

the device output level is given by the algebraic sum of the signals produced by the acceleration acting on the sensor and by the

electrostatic test-force.

Modes of Operation

Figure 5. MMA8450Q Mode Transition Diagram

All register contents are preserved when transitioning from Active to Standby mode. Some registers are reset when

transitioning from Standby to Active. These are all noted in the device memory map register table. For more detail on how to use

the Sleep and Wake modes and how to transition between these modes, please refer to the functionality section of this document.

Table 7. Mode of Operation Description

Mode

I2C Bus State

VDD

Function Description

OFF

Powered Down

<1.5 V

<VDD+0.3V

The device is powered off.

SHUTDOWN

I2C communication ignored

EN = Low

All analog & digital blocks are shutdown.

STANDBY

I2C communication possible

EN = VDD

Standby register set

Only POR and digital blocks are enabled.

Analog subsystem is disabled.

Registers accessible for Read/Write.

Device configuration done in this mode.

ACTIVE

I2C communication possible

EN = VDD

Standby register reset

All blocks are enabled (POR, digital, analog).

VDD = OFF

Mode

SHUTDOWN

Mode

STANDBY

Mode (00)

WAKE

Mode (01)

OFF

= V

= GN

EN = GND

D =

OFF

SLEEP

Mode (10)

EN = VDD & VDD = ON

MMA8450QT Datasheet(PDF) 12 Page - Freescale Semiconductor, Inc

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