Sensors

24

Freescale Semiconductor

MMA690xQ

3.3

REPRESENTATION

3.3.1

Overrange Response

Positive acceleration levels which exceed the full-scale range of the device fall into two categories: overrange and overflow.

Overrange conditions exist when the signal level is beyond the full-scale range of the device but within the computational limits

of the DSP. An overflow condition occurs if the output of the low-pass filter equals or exceeds the maximum digital value which

can be output from the sinc filter. Sinc filter saturation will occur before the internal data path width is exceeded. At 25

°C and

OVLD = 0, the sinc filter will not saturate at sustained acceleration levels with the range of

±200g. The DSP operates predictably

under all cases of overrange, although the signal may include residual high frequency components for some time after returning

to the normal range of operation due to non-linear effects of the sensor. If an overflow condition occurs, the signal is internally

clipped. The DSP will recover from an overflow condition within a few sample times after the input signal returns to the input range

of the DSP. Due to internal clipping within the DSP, some high-frequency artifacts may be present in the output following an over-

flow condition.

For negative acceleration levels, corresponding underrange and underflow conditions are defined.

3.4

CAP/HOLD INPUT

The CAP/HOLD input provides a system-level synchronization mechanism. When driven high, transfer of acceleration results

from the DSP to the SPI buffers does not occur. The DSP continues its normal operation regardless of the state of CAP/HOLD.

Data read from the device when CAP/HOLD is high will reflect the last values available from the DSP at the time of the signal

transition.

Table 3-10. Nominal 11-bit Acceleration Data Values

11-bit Unsigned

Digital Value

Nominal Acceleration

3.5g Range

5.0g Range

2047

Critical/Reset Exception Value

2046

Invalid Axis Selection

2045

Overflow

2044

Overrange

2043

+3.50g

+5.00g

2042

+3.50g

+5.00g

2041

+3.49g

+4.99g

•

•

•

•

•

•

•

•

•

1027

+10.3 mg

+14.7 mg

1026

+6.87 mg

+9.81 mg

1025

+3.43 mg

+4.91 mg

1024

0g

0g

1023

-3.43 mg

-4.91 mg

1022

-6.87 mg

-9.81 mg

1021

-10.3 mg

-14.7 mg

•

•

•

•

•

•

•

•

•

7

-3.49g

-4.99g

6

-3.50g

-5.00g

5

-3.50g

-5.00g

4

Underrange

3

Underflow

2

SPI Clock Fault

1

0

Reserved