MMA8451Q datasheet(13/53 Pages) FREESCALE

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Sensors

Freescale Semiconductor

MMA8451Q

Terminology

3.1

Sensitivity

The sensitivity is represented in counts/g. In 2g mode the sensitivity is 4096 counts/g. In 4g mode the sensitivity is 2048 counts/

g and in 8g mode the sensitivity is 1024 counts/g.

3.2

Zero-g Offset

Zero-g Offset (TyOff) describes the deviation of an actual output signal from the ideal output signal if the sensor is stationary. A

sensor stationary 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 6. MMA8451Q 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. The SLEEP and WAKE

modes are ACTIVE modes. For more information 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

VDDIO

Function Description

OFF

Powered Down

<1.8 V

VDDIO Can be > VDD

The device is powered off. All analog and digital blocks

are shutdown. I2C bus inhibited.

STANDBY

I2C communication with

MMA8451Q is possible

VDDIO = High

VDD = High

ACTIVE bit is cleared

Only digital blocks are enabled.

Analog subsystem is disabled. Internal clocks disabled.

ACTIVE

(WAKE/SLEEP)

I2C communication with

MMA8451Q is possible

VDDIO = High

VDD = High

ACTIVE bit is set

All blocks are enabled (digital, analog).

SLEEP

WAKE

STANDBY

OFF

ACTIVE

MMA8451Q Datasheet(PDF) 13 Page - Freescale Semiconductor, Inc

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