MMA8652FC datasheet(14/65 Pages) NXP

14 / 65 page

MMA8652FC

Sensors

Freescale Semiconductor, Inc.

5.3

Internal FIFO data buffer

MMA8652FC contains a 32-sample internal FIFO data buffer, which helps minimize traffic across the I2C bus. The FIFO can also

save system power, 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 are options for accessing the full 12-bit data or for accessing only the 8-bit data.

When access speed is more important than high resolution, the 8-bit data read is a better option.

The FIFO contains four modes (Fill Buffer mode, Circular Buffer mode, Trigger mode, and Disabled mode), which are described

in F_SETUP Register 0x09.

•

Fill Buffer mode collects the first 32 samples and asserts the overflow flag when the buffer is full and another sample arrives.

It does not collect any more data until the buffer is read. This benefits data logging applications where all samples must be

collected.

•

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.

•

Trigger mode will hold the last data up to the point when the trigger occurs, and can be set to keep a selectable number of

samples after the event occurs.

The MMA8652FC FIFO Buffer has a configurable watermark, allowing the processor to be triggered after a configurable number

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

5.4

Low power modes vs. high resolution modes

The MMA8652FC can be optimized for lower power modes or for higher resolution of the output data. One of the oversampling

schemes of the data can be activated when MODS = 10 in Register 0x2B, which will improve the resolution of the output data

only. The highest resolution is achieved at 1.56 Hz.

There is a trade-off between low power and high resolution. Low power can be achieved when the oversampling rate is

reduced. When MODS = 11, the lowest power is achieved. The lowest power is achieved when the sample rate is set to 1.56 Hz.

……

…

1000 0000 0001

–1.999 g

–3.998 g

–7.996 g

1000 0000 0000

–2.0000 g

–4.0000 g

–8.0000 g

Table 9. Accelerometer 8-bit output data

8-bit Data

Range ±2 g

(15.6 mg/LSB)

Range ±4 g

(31.25 mg/LSB)

Range ±8 g

(62.5 mg/LSB)

0111 1111

1.9844 g

+3.9688 g

+7.9375 g

0111 1110

1.9688 g

+3.9375 g

+7.8750 g

……

…

0000 0001

+0.0156 g

+0.0313 g

+0.0625 g

0000 0000

0.000 g

0.0000 g

1111 1111

–0.0156 g

–0.0313 g

–0.0625 g

……

…

1000 0001

–1.9844 g

–3.9688 g

–7.9375 g

1000 0000

–2.0000 g

–4.0000 g

–8.0000 g

Table 8. Accelerometer 12-bit output data (Continued)

12-bit data

Range ±2 g (1 mg/LSB)

Range ±4 g (2 mg/LSB)

Range ±8 g (4 mg/LSB)

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