Sensors

Freescale Semiconductor

9

MMA7455L

FEATURES

Self-Test

The sensor provides a self-test feature that allows the verification of the mechanical and electrical integrity of the accelerometer

at any time before or after installation. This feature is critical in applications such as hard disk drive protection where system in-

tegrity must be ensured over the life of the product. When the self-test function is initiated through the mode control register ($16),

accessing the “self-test” bit, an electrostatic force is applied to each axis to cause it to deflect. The Z-axis is trimmed to deflect

1g. This procedure assures that both the mechanical (g-cell) and electronic sections of the accelerometer are functioning.

g-Select

The g-Select feature enables the selection between 3 acceleration ranges for measurement. Depending on the values in the

Mode control register ($16), the MMA7455L’s internal gain will be changed allowing it to function with a 2g, 4g or 8g measurement

sensitivity. This feature is ideal when a product has applications requiring two or more acceleration ranges for optimum perfor-

mance and for enabling multiple functions. The sensitivity can be changed during the operation by modifying the two GLVL bits

located in the mode control register.

Standby Mode

This digital output 3-axis accelerometer provides a standby mode that is ideal for battery operated products. When standby mode

is active, the device outputs are turned off, providing significant reduction of operating current. When the device is in standby

SPI available, but no new measurements can be taken in this mode as all current consuming parts are off. The mode of the device

is controlled through the mode control register by accessing the two mode bits as shown in Table 6.

Measurement Mode

The device can read XYZ measurements in this mode. The pulse and threshold interrupts are not active. During measurement

mode, continuous measurements on all three axes enabled. The g-range for 2g, 4g, or 8g are selectable with 8-bit data and the

g-range of 8g is selectable with 10-bit data. The sample rate during measurement mode is 125 Hz with 62.5 BW filter selected.

The sample rate is 250 Hz with the 125 Hz filter selected. Therefore, when a conversion is complete (signaled by the DRDY flag),

the next measurement will be ready.

When measurements on all three axes are completed, a logic high level is output to the DRDY pin, indicating “measurement data

is ready.” The DRDY status can be monitored by the DRDY bit in Status Register (Address: $09). The DRDY pin is kept high until

one of the three Output Value Registers are read. If the next measurement data is written before the previous data is read, the

DOVR bit in the Status Register will be set. Also note that in measurement mode, level detection mode and pulse detection mode

are not available.

By default all three axes are enabled. X and/or Y and/or Z can be disabled. There is a choice between detecting an absolute

signal or a positive or negative only signal on the enabled axes. There is also a choice between doing a detection for motion

where X or Y or Z > Threshold vs. doing a detection for freefall where X & Y & Z < Threshold.

$16: Mode Control Register (Read/Write)

D7

D6

D5

D4

D3

D2

D1

D0

Bit

--

DRPD

SPI3W

STON

GLVL[1]

GLVL[0]

MODE[1]

MODE[0]

Function

0

0

0

0

0

0

0

0

Default

Table 5. Configuring the g-Select for 8-bit output using Register $16 with GLVL[1:0] bits

GLVL [1:0]

g-Range

Sensitivity

00

8g

16 LSB/g

01

2g

64 LSB/g

10

4g

32 LSB/g

Table 6. Configuring the Mode using Register $16 with MODE[1:0] bits

MODE [1:0]

Function

00

Standby Mode

01

Measurement Mode

10

Level Detection Mode

11

Pulse Detection Mode