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ADXL375 Datasheet(Arkusz danych) 11 Page - Analog Devices

Numer części ADXL375
Szczegółowy opis  3-Axis, 200 g Digital Accelerometer
Pobierz  32 Pages
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Data Sheet
ADXL375
Rev. 0 | Page 11 of 32
POWER SAVING MODES
Low Power Mode
A low power mode is available for additional power savings. In
low power mode, the internal sampling rate is reduced, allowing
for power savings in the 12.5 Hz to 400 Hz data rate range at the
expense of slightly greater noise. To enter low power mode, set
the LOW_POWER bit (Bit D4) in the BW_RATE register
(Address 0x2C). Table 8 shows the current consumption in low
power mode for output data rates where there is an advantage to
using low power mode.
Table 8. Typical Current Consumption vs. Data Rate,
Low Power Mode (TA = 25°C, VS = 2.5 V, VDDI/O = 1.8 V)
Rate Bits
Output Data
Rate (Hz)
Bandwidth
(Hz)
IDD (µA)
1100
400
200
90
1011
200
100
60
1010
100
50
50
1001
50
25
45
1000
25
12.5
40
0111
12.5
6.25
35
For data rates not shown in Table 8, the use of low power mode
does not provide any advantage over normal power mode. There-
fore, it is recommended that low power mode be used only for
the data rates shown in Table 8.
Autosleep Mode
Additional power can be saved if the ADXL375 automatically
switches to sleep mode during periods of inactivity. To enable
the autosleep mode feature,
1. Set the THRESH_INACT register (Address 0x25) and
the TIME_INACT register (Address 0x26) to values that
signify inactivity. The appropriate values depend on the
application.
2. Set the AUTO_SLEEP bit (Bit D4) and the link bit (Bit D5)
in the POWER_CTL register (Address 0x2D).
Current consumption at the sub-12.5 Hz data rates that are used
in autosleep mode is typically 35 µA for VS = 2.5 V.
For information about the advantages of using low power
mode vs. autosleep mode, see the Sleep Mode vs. Low Power
Mode section.
Standby Mode
For even lower power operation, standby mode can be used. In
standby mode, current consumption is reduced to 0.1 µA (typical).
In this mode, no measurements are made, but the contents of the
FIFO buffer are preserved. To enter standby mode, clear the
measure bit (Bit D3) in the POWER_CTL register (Address 0x2D).
FIFO BUFFER
The ADXL375 contains patented technology for an embedded
memory management system with a 32-level FIFO buffer that
can be used to minimize host processor burden. This buffer has
four modes: bypass, FIFO, stream, and trigger. Each mode can
be selected by setting the FIFO_MODE bits (Bits[D7:D6]) in
the FIFO_CTL register (Address 0x38; see Table 9).
Table 9. FIFO Modes (FIFO_CTL Register, Address 0x38)
Setting
FIFO
Mode
D7
D6
Description
0
0
Bypass
FIFO buffer is bypassed.
0
1
FIFO
FIFO buffer collects up to 32 samples and
then stops collecting data, collecting new
data only when the buffer is not full.
1
0
Stream
FIFO buffer holds the last 32 samples.
When the buffer is full, the oldest data
is overwritten with newer data.
1
1
Trigger
FIFO buffer holds the last samples before
the trigger event and continues to collect
data until full. New data is collected only
when the buffer is not full.
For an in-depth description of the FIFO buffer and FIFO modes,
see the AN-1025 Application Note, Utilization of the First In, First
Out (FIFO) Buffer in Analog Devices, Inc., Digital Accelerometers.
Bypass Mode
In bypass mode, the FIFO buffer is not operational and, there-
fore, remains empty.
FIFO Mode
In FIFO mode, data from measurements of the x-, y-, and z-axes
is stored in the FIFO buffer. When the number of samples in the
FIFO buffer equals the level specified by the samples bits of the
FIFO_CTL register (Address 0x38), the watermark interrupt is
set (see the Watermark Bit section). The FIFO buffer continues
to accumulate samples until it is full (32 samples from measure-
ments of the x-, y-, and z-axes) and then stops collecting data.
After the FIFO buffer stops collecting data, the device continues
to operate; therefore, features such as shock detection can be used
after the FIFO buffer is full. The watermark interrupt bit remains
set until the number of samples in the FIFO buffer is less than
the value stored in the samples bits of the FIFO_CTL register.
Stream Mode
In stream mode, data from measurements of the x-, y-, and z-axes
is stored in the FIFO buffer. When the number of samples in the
FIFO buffer equals the level specified by the samples bits of the
FIFO_CTL register (Address 0x38), the watermark interrupt is set
(see the Watermark Bit section). The FIFO buffer continues to
accumulate samples; the buffer stores the latest 32 samples from
measurements of the x-, y-, and z-axes, discarding older data as
new data arrives. The watermark interrupt bit remains set until
the number of samples in the FIFO buffer is less than the value
stored in the samples bits of the FIFO_CTL register.




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