MMA8652FC datasheet(30/77 Pages) FREESCALE

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MMA8652FC

Sensors

Freescale Semiconductor, Inc.

6.5.2

0x09: F_SETUP FIFO Setup register

The FIFO mode can be changed while in the Active mode. The Active mode must first be disabled (F_MODE = 00), before the

mode can be switched between Fill mode, Circular mode, and Trigger mode.

A FIFO sample count exceeding the watermark event does not stop the FIFO from accepting new data. The FIFO update rate is

dictated by the selected system ODR.

•

In ACTIVE mode, the ODR is set by the DR bits (CTRL_REG1 register).

•

When Auto-SLEEP is active, the ODR is set by the ASLP_RATE field (CTRL_REG1 register).

When a byte is read from the FIFO buffer, the oldest sample data in the FIFO buffer is returned (and also deleted from the front of

the FIFO buffer), while the FIFO sample count is decremented by one. It is assumed that the host application will use the I

2C multi-

byte read transaction to empty the FIFO.

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

F_MODE1

F_MODE0

F_WMRK5

F_WMRK4

F_WMRK3

F_WMRK2

F_WMRK1

F_WMRK0

Figure 23. 0x09 F_SETUP: FIFO Setup register (Read/Write)

Table 17. F_SETUP register

Bit(s)

Field

Description

7–6

F_MODE[1:0]((1))((2))

1. Bit field can be written in ACTIVE mode.

2. Bit field can be written in STANDBY mode.

FIFO buffer overflow mode

00 FIFO is disabled. (default)

01 FIFO contains the most recent samples when overflowed (circular buffer).

The oldest sample is discarded and replaced by a new sample.

10 FIFO stops accepting new samples when overflowed.

11 Trigger mode. The FIFO will be in a circular mode up to the number of samples in the watermark. The

FIFO will be in a circular mode until the trigger event occurs, after which the FIFO will continue to accept

samples for 32-WMRK samples and then stop receiving further samples. This allows data to be collected

both before and after the trigger event, and it is definable by the watermark setting.

• The FIFO is flushed whenever the FIFO is disabled, during an automatic ODR change (Auto-WAKE/

SLEEP), or transitioning from STANDBY mode to ACTIVE mode.

• Disabling the FIFO (F_MODE = 00) resets the F_OVF, F_WMRK_FLAG, F_CNT to zero.

• A FIFO overflow event (i.e., F_CNT = 32) will assert the F_OVF flag and a FIFO sample count equal to the

sample count watermark (i.e., F_WMRK) asserts the F_WMRK_FLAG event flag.

5–0

F_WMRK[5:0](2)

FIFO Event Sample Count Watermark

These bits set the number of FIFO samples required to trigger a watermark interrupt.

A FIFO watermark event flag is raised when FIFO sample count F_CNT[5:0]

≥ F_WMRK[5:0] watermark.

• Setting the F_WMRK[5:0] to 00_0000 will disable the FIFO watermark event flag generation.

• Also used to set the number of pre-trigger samples in Trigger mode.

00_0000 (default)

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