MMA6525KW datasheet(24/56 Pages) FREESCALE

24 / 56 page

Sensor

Freescale Semiconductor, Inc.

MMA65xx

3.6

Self Test Interface

When self test is enabled, the self test interface applies a voltage to the g-cell, causing a deflection of the proof mass. Once

enabled, offset cancellation is suspended and the deflection results in an acceleration which is superimposed upon the input ac-

celeration.

The resulting acceleration readings can be compared either against absolute limits, or the values stored in the Self Test De-

flection Registers (Reference Section 3.1.2). The self test interface is controlled through SPI write operations to the DEVCFG_X

and DEVCFG_Y registers described in Section 3.1.7 only if the ENDINIT bit in the DEVCFG register is cleared. A diagram of the

self test interface is shown in Figure 10.

Figure 10. Self Test Interface

3.6.1

Raw Self Test Deflection Verification

The raw self test deflection can be directly verified against raw self test limits listed in Section 2.4.

3.6.2

Delta Self Test Deflection Verification

The raw self test deflection can be verified against the ambient temperature self test deflection value recorded at the time the

device was produced. The production self test deflection is stored in the STDEFL_X and STDDEFL_Y registers such that the

minimum stored value (0x00) is equivalent to

ΔST

MIN, and the maximum stored value (0xFF) is equivalent to ΔSTMAX. The Delta

Self Test Deflection limits can then be determined by the following equations:

where:

Y-AXIS

g-CELL

SELF TEST

VOLTAGE

GENERATOR

ENDINIT

ST_Y

ST_X

X-AXIS

g-CELL

ENDINIT

ΔST

ACC

The accuracy of the self test deflection relative to the stored deflection as specified in Section 2.4.

ΔSTDEFLx

CNTS

The value stored in the STDEFL_X or STDEFL_Y register.

ΔST

MIN

The minimum self test deflection at 25C as specified in Section 2.4.

ΔST

MAX

The maximum self test deflection at 25C as specified in Section 2.4.

ΔST

ACCMINLIMIT

FLOOR

ΔST

MIN

ΔSTDEFLx

CNTS

255

------------------------------------------

ΔST

MAX

ΔST

MIN

–

[]

⎝⎠

⎛⎞

ΔST

ACC

–

()

⋅

ΔST

ACCMAXLIMIT

CEIL

ΔST

MIN

ΔSTDEFLx

CNTS

255

------------------------------------------

ΔST

MAX

ΔST

MIN

–

[]

⎝⎠

⎛⎞

ΔST

ACC

()

⋅

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