CN-0274 Datasheet, PDF (4/6 Pages) Analog Devices – Ultralow Power, 3-Axis, Motion Activated Switch

CN-0274
Interrupts
Several of the built-in functions of the ADXL362 can trigger
interrupts to alert the host processor of certain status conditions.
Interrupts may be mapped to either (or both) of two designated
output pins, INT1 and INT2, by setting the appropriate bits in
the INTMAP1 and INTMAP2 registers. All functions can be
used simultaneously. If multiple interrupts are mapped to one
pin, the OR combination of the interrupts determines the status
of the pin.
If no functions are mapped to an interrupt pin, that pin is
automatically configured to a high impedance (high-Z) state.
The pins are placed in this state upon a reset as well.
When a certain status condition is detected, the pin that condition
is mapped to is activated. The configuration of the pin is active
high by default, so that when it is activated, the pin goes high.
However, this configuration can be switched to active low by
setting the INT_LOW pin in the appropriate INTMAP register.
The INT pins may be connected to the interrupt input of a host
processor and interrupts responded to with an interrupt routine.
Because multiple functions can be mapped to the same pin, the
STATUS register can be used to determine which condition
caused the interrupt to trigger.
The CN0274 evaluation software configures the ADXL362 such
that when activity is detected, the INT1 pin is high, and when
inactivity is detected, the INT1 pin is low.
Test Results
All testing was performed using the EVAL-CN0274-SDPZ and
the EVAL-SDP-CS1Z. Functionality of the part is demonstrated
by setting the activity threshold at 0.5 g, the inactivity threshold
at 0.75 g, and the number of inactivity samples at 20. When
looking for activity, only one acceleration sample on any axis is
required to cross the threshold.
Starting with the circuit oriented so that the battery pack is flat
against the table, the printed circuit board (PCB) can be slowly
rotated 90° in any direction causing the acceleration to cross the
threshold as it approaches perpendicular to the initial orientation.
Figure 2 shows a screen shot of the CN0274 evaluation software
showing the ADXL362 first asleep, looking for activity. Then,
when Sample 11 crosses the threshold, the ADXL362 enters the
awake state and begins looking for inactivity. The thresholds
adjust to show the device is now looking for inactivity.
Circuit Note
Figure 2. Screen Shot of Evaluation Software Output
For better visibility, the X-axis and Z-axis plots are disabled
using the radio buttons above the chart.
The output of the ADP195, or the interrupt pin itself, was
measured using a digital multimeter. When the ADXL362 is awake,
the interrupt goes high and drives the EN pin of the ADP195
high, which in turn drives the gate of the MOSFET low, causing the
switch to close, connecting any downstream circuitry to the power
supply. Conversely, when the ADXL362 is asleep, the interrupt
drives the EN pin of the ADP195 low, which in turn drives the
gate of the MOSFET high, causing the switch to open.
PCB Layout Considerations
In any circuit where accuracy is crucial, it is important to
consider the power supply and ground return layout on the
board. The PCB should isolate the digital and analog sections as
much as possible. The PCB for this system was constructed in a
4-layer stack up with large area ground plane layers and power
plane polygons. See the MT-031 Tutorial for more discussion on
layout and grounding, and the MT-101 Tutorial for information
on decoupling techniques.
Decouple the power supply to the ADXL362 with 1 µF and
0.1 µF capacitors to properly suppress noise and reduce ripple.
Place the capacitors as close to the device as possible. Ceramic
capacitors are advised for all high frequency decoupling.
Power supply lines should have as large a trace width as possible
to provide low impedance paths and reduce glitch effects on the
supply line. Shield clocks and other fast switching digital signals
from other parts of the board by digital ground. A photo of the
PCB is shown in Figure 3.
A complete design support package for this circuit note can be
found at www.analog.com/CN0274-DesignSupport.
Rev. A | Page 4 of 6