MMA8491Q
Sensors
Freescale Semiconductor, Inc. 11
3 Modes of Operation
Figure 7. MMA8491Q operating modes
3.1 ACTIVE mode
The accelerometer subsystem is turned on at the rising edge of the EN pin, and acquires one sample for each of the three axes.
Note that EN should not be asserted before V
DD
reaches 1.95V. Samples are acquired, converted, and compensated for zero-g
offset and gain errors, and then compared to an internal threshold value of 0.688g and stored.
• If any of the X, Y, Z axes sample’s absolute value > this threshold, then the corresponding outputs on these axes drive
logic highs.
• If any of the X, Y, Z axes sample’s absolute value ≤ this threshold, then the corresponding outputs on these axes drive
logic lows.
Read register 0x00 in this stage to determine whether the sample data is ready to be read.
3.2 STANDBY mode
The device enter STANDBY mode automatically after the previously described function (powers into SHUTDOWN mode,
ACTIVE mode) is accomplished. The digital output system outputs valid data, which can also be read via the I
2
C communication
bus. This is the appropriate phase to read the measured data, either from the 3 push-pull logic outputs or through the I
2
C
transaction. All other subsystems are turned off.
These outputs are held until the MMA8491Q operation mode changes. For lower power consumption, deassert the EN pin as
soon as data is read (to enter SHUTDOWN mode).
3.3 Next sample acquisition
The MMA8491Q needs to be brought back to the ACTIVE mode again by pulling EN pin up to a Logic 1. Another option is to
power down the device and start from OFF mode as illustrated in Figure 7.
For applications where sampling intervals are greater than 30 seconds, the host can shut off the tilt sensor power after acquisition
of tilt sensor output data to conserve energy and prolong battery life.
Table 7. Operating modes
Mode Conditions Function Description Digital Output State
OFF
V
DD
= OFF
EN =
Don’t Care
Device is powered off.
Hi-Z
SHUTDOWN
V
DD
= ON
EN = Low
All blocks are shut down.
Hi-Z
ACTIVE
V
DD
= ON
EN = High
All blocks are enabled.
Device enters Standby mode automatically
after data conversion.
Deasserted, Xout = 0, Yout= 0, Zout = 0
STANDBY
V
DD
= ON
EN = High
Only digital output subsystem is enabled.
Data is valid and available only in this stage.
Active, I
2
C outputs become valid
EN = Low
V
DD
= Off
ACTIVE STANDBY
OFF*
Mode
SHUTDOWN
Mode
Mode Mode
EN =
Don’t Care
V
DD
= On
EN = Low
One sample
EN = High
V
DD
= On
EN = High
V
DD
= On EN = High
is acquired
V
DD
= On
*OFF mode can be entered from any state by removing the power.
