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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
register 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 .
SELF-TEST
VOLTAGE
GENERATOR
g-CELL
ENDINIT
ST
ENDINIT
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
Δ ST ACCMINLIMIT = FLOOR ? ? Δ ST MIN + ------------------------------------------ × [ Δ ST MAX – Δ ST MIN ] ? × ( 1 – Δ ST ACC )
Δ ST ACCMAXLIMIT = CEIL ? ? ? Δ ST MIN + ------------------------------------------ × [ Δ ST MAX – Δ ST MIN ] ? ? × ( 1 + Δ ST ACC )
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 register such that the minimum stored value
(0x00) is equivalent to Δ ST MIN , and the maximum stored value (0xFF) is equivalent to Δ ST MAX . The Delta Self Test Deflection
limits can then be determined by the following equations:
Δ STDEFLx CNTS
? 255 ?
Δ STDEFLx CNTS
255
where:
Δ ST ACC
Δ STDEFLx CNTS
Δ ST MIN
Δ ST MAX
The accuracy of the self test deflection relative to the stored deflection as specified in Section 2.4 .
The value stored in the STDEFL register.
The minimum self test deflection at 25C as specified in Section 2.4 .
The maximum self test deflection at 25C as specified in Section 2.4 .
MMA655x
Sensor
Freescale Semiconductor, Inc.
23
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