MMA8452低功耗三轴加速度传感器：12位分辨率与智能功能

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Sensors

Freescale Semiconductor 10

MMA8450Q

3 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 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.

4 Modes of Operation

Figure 6. 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

C Bus State

VDD EN Function Description

OFF

Powered Down <1.5 V <VDD + 0.3 V The device is powered off.

SHUTDOWN

C communication ignored

EN = Low All analog & digital blocks are shutdown.

STANDBY

C 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

C 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

EN = GND

FS = 0

EN = GND

VDD = OFF

VDD = ON

SLEEP

Mode (10)

EN = VDD & VDD = ON

Sensors

Freescale Semiconductor 11

MMA8450Q

5Functionality

The MMA8450Q is a low-power, digital output 3-axis linear accelerometer packaged in a QFN package. The complete device

includes a sensing element and an IC interface able to take the information from the sensing element and to provide a signal to

the external world through an I

C serial interface. There are many embedded features in this accelerometer with a very flexible

interrupt routing scheme to two interrupt pins including:

• 8-bit or 12-bit data, high pass filtered data, 8-bit or 12-bit configurable 32 sample FIFO

• Low power and Auto-Wake/Sleep for conservation of current consumption

• Single and double pulse detection 1 channel

• Motion detection and Freefall 2 channels

• Transient detection based on a high pass filter and settable threshold for detecting the change in acceleration above a

threshold

• Flexible user configurable portrait landscape detection algorithm addressing many use cases for screen orientation

All functionality is available in 2g, 4g or 8g dynamic ranges. There are many configuration settings for enabling all the different

functions. Separate application notes have been provided to help configure the device for each embedded functionality.

5.1 Device Calibration

The IC interface is factory calibrated for sensitivity and Zero-g offset for each axis. The trim values are stored in Non Volatile

Memory (NVM). On power-up, the trim parameters are read from NVM and applied to the circuitry. In normal use, further

calibration in the end application is not necessary. However, the MMA8450Q allows the user to adjust the Zero-g offset for each

axis after power-up, changing the default offset values. The user offset adjustments are stored in 6 volatile registers. For more

information on device calibration, refer to Freescale application note, AN3916.

5.2 8-bit or 12-bit Data

The measured acceleration data is stored in the OUT_X_MSB, OUT_X_LSB, OUT_Y_MSB, OUT_Y_LSB, OUT_Z_MSB, and

OUT_Z_LSB registers as 2’s complement 12-bit numbers. The most significant 8-bits of each axis are stored in OUT_X (Y,

Z)_MSB, so applications needing only 8-bit results can use these 3 registers and ignore OUT_X(Y, Z)_LSB.

When the full-scale is set to 2g, the measurement range is -2g to +1.999g, and each LSB corresponds to 1g/1024 (0.98 mg)

at 12-bits resolution. When the full-scale is set to 8g, the measurement range is -8g to +7.996g, and each LSB corresponds to

1g/256 (3.9 mg) at 12-bits resolution. The resolution is reduced by a factor of 16 if only the 8-bit results are used. For more

information on the data manipulation between data formats and modes, refer to Freescale application note, AN3922. There is a

device driver available that can be used with the Sensor Toolbox demo board (LFSTBEB8450Q) with this application note.

5.3 Internal FIFO Data Buffer

MMA8450Q contains a 32 sample internal FIFO data buffer minimizing traffic across the I

C bus. The FIFO can also provide

power savings of the system by allowing the host processor/MCU to go into a sleep mode while the accelerometer independently

stores the data, up to 32 samples per axis. The FIFO can run at all output data rates. There is the option of accessing the full 12-

bit data for accessing only the 8-bit data. When access speed is more important than high resolution the 8-bit data flush is a better

option.

The FIFO contains three modes (Fill Buffer Mode, Circular Buffer Mode, and Disabled) described in the F_SETUP Register

0x13. Fill Buffer Mode collects the first 32 samples and asserts the overflow flag when the buffer is full. It does not collect anymore

data until the buffer is read. This benefits data logging applications where all samples must be collected. The Circular Buffer Mode

allows the buffer to be filled and then new data replaces the oldest sample in the buffer. The most recent 32 samples will be stored

in the buffer. This benefits situations where the processor is waiting for an specific interrupt to signal that the data must be flushed

to analyze the event.

The MMA8450Q FIFO Buffer also has a configurable watermark, allowing the processor to be interrupted after a configurable

number of samples has filled in the buffer (1 to 32).

For details on the configurations for the FIFO Buffer as well as more specific examples and application benefits, refer to

Freescale application note, AN3920.

5.4 Low Power Mode

The MMA8450Q can be set to a low power mode to further reduce the current consumption of the device. When the Low Power

Mode is enabled, the device has access to all the configurable sampling rates and features as is available in the Normal power

mode. To set the device into Low Power Mode, bit 0 in the System Control Register 2 (0x39) should be set (1) (this bit is cleared

(0) for Normal Power Mode). Low Power Mode reduces the current consumption by internally sleeping longer and averaging the

data less. The Low Power Mode is an additional feature that is independent of the sleep feature.The sleep feature can also be

used to reduce the current consumption by automatically changing to a lower sample rate when no activity is detected.

For more information on how to configure the MMA8450Q in Low Power Mode and the power consumption benefits of Low

Power Mode and Auto-Wake/Sleep with specific application examples, refer to Freescale application note, AN3921.

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