Datasheet

MMA8451Q
Sensors
Freescale Semiconductor, Inc. 23
The F_OVF and F_WMRK_FLAG flags remain asserted while the event source is still active, but the user can clear the FIFO
interrupt bit flag in the interrupt source register (INT_SOURCE) by reading the F_STATUS register. In this case, the SRC_FIFO
bit in the INT_SOURCE register will be set again when the next data sample enters the FIFO. Therefore the F_OVF bit flag will
remain asserted while the FIFO has overflowed and the F_WMRK_FLAG bit flag will remain asserted while the F_CNT value is
equal to or greater than then F_WMRK value. If the FIFO overflow flag is cleared and if F_MODE = 11 then the FIFO overflow
flag will remain 0 before the trigger event even if the FIFO is full and overflows. If the FIFO overflow flag is set and if F_MODE
is = 11, the FIFO has stopped accepting samples.
F_CNT[5:0] bits indicate the number of acceleration samples currently stored in the FIFO buffer. Count 000000 indicates that the
FIFO is empty.
0x09: F_SETUP FIFO Setup Register
The FIFO mode can be changed while in the active state. The mode must first be disabled F_MODE = 00 then 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 in the CTRL_REG1 register. When Auto-
SLEEP is active the ODR is set by the ASLP_RATE field in the 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 shall use the I
2
C
multi-byte read transaction to empty the FIFO.
Table 14. FIFO Flag Event Description
F_OVF F_WMRK_FLAG Event Description
0 — No FIFO overflow events detected.
1 — FIFO event detected; FIFO has overflowed.
— 0 No FIFO watermark events detected.
— 1
FIFO Watermark event detected. FIFO sample count is greater than watermark value.
If F_MODE = 11, Trigger Event detected.
Table 15. FIFO Sample Count Description
F_CNT[5:0]
FIFO sample counter. Default value: 00_0000.
(00_0001 to 10_0000 indicates 1 to 32 samples stored in FIFO
0x09 F_SETUP: FIFO Setup Register (Read/Write)
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
Table 16. F_SETUP Description
BITS Description
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. Default value: 0.
00: FIFO is disabled.
01: FIFO contains the most recent samples when overflowed (circular buffer). Oldest sample is discarded to
be replaced by 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 that 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.
F_WMRK[5:0]
(2)
FIFO Event Sample Count Watermark. Default value: 00_0000.
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.