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ADXL363BCCZ-RL Arkusz danych(PDF) 17 Page - Analog Devices |
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ADXL363BCCZ-RL Arkusz danych(HTML) 17 Page - Analog Devices |
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17 / 48 page  Data Sheet ADXL363 Rev. 0 | Page 17 of 48 POWER SAVINGS FEATURES Designed for the most power conscious applications, the ADXL363 includes several features (as described in this section) for enabling power savings at the system level, as well as at the device level. ULTRALOW POWER CONSUMPTION IN ALL MODES, OPERATING ALL SENSORS At the device level, the most obvious power saving feature of the ADXL363 is its ultralow current consumption in all configurations. The ADXL363 consumes between 1.1 µA (typical) and 5 µA (typical) across all data rates up to 400 Hz and all supply voltages up to 3.5 V (see Figure 29). An even lower power, 270 nA (typical) motion triggered wake-up mode is provided for simple motion detection applications that require a power consumption lower than 1 µA. At these current levels, the ADXL363 consumes less power in full operation than the standby currents of many other system components, and is, therefore, optimal for applications that require continuous acceleration monitoring and very long battery life. Because the accelerometer is always on, it can act as a motion- activated switch. The accelerometer signals to the rest of the system when to turn on, thereby managing power at the system level. As important as its low operating current, the 10 nA (typical) standby current of the ADXL363 contributes to a much longer battery life in applications that spend most of their time in a sleep state and wake up via an external trigger. MOTION DETECTION The ADXL363 features built-in logic that detects activity (presence of acceleration above a threshold) and inactivity (lack of acceleration above a threshold). Activity and inactivity events can be used as triggers to manage the accelerometer mode of operation, trigger an interrupt to a host processor, and/or autonomously drive a motion switch. Detection of an activity or inactivity event is indicated in the status register and can be configured to generate an interrupt. In addition, the activity status of the device, that is, whether it is moving or stationary, is indicated by the awake bit, described in the Using the Awake Bit section. Activity and inactivity detection can be used when the accel- erometer is in either measurement mode or wake-up mode. Activity Detection An activity event is detected when acceleration remains above a specified threshold for a specified time period. Referenced and Absolute Configurations Activity detection can be configured as referenced or absolute. When using absolute activity detection, acceleration samples are compared to a user set threshold to determine whether motion is present. For example, if a threshold of 0.5 g is set and the acceleration on the z-axis is 1 g for longer than the user defined activity time, the activity status asserts. In many applications, it is advantageous for activity detection to be based not on an absolute threshold, but on a deviation from a reference point or orientation. This is particularly useful because it removes the effect on activity detection of the static 1 g imposed by gravity. When an accelerometer is stationary, its output can reach 1 g, even when it is not moving. In absolute activity, when the threshold is set to less than 1 g, activity is immediately detected in this case. In the referenced configuration, activity is detected when acceleration samples are at least a user set amount above an internally defined reference for the user defined amount of time, as described in Equation 1. |Acceleration – Reference| > Threshold (1) Consequently, activity is detected only when the acceleration has deviated sufficiently from the initial orientation. The reference for activity detection is calculated when activity detection is engaged in one of the following scenarios: • When the activity function is turned on and measurement mode is engaged • If link mode is enabled: when inactivity is detected and activity detection begins • If link mode is not enabled: when activity is detected and activity detection repeats The referenced configuration results in a very sensitive activity detection that detects even the most subtle motion events. Fewer False Positives Ideally, the intent of activity detection is to wake up a system only when motion is intentional, ignoring noise or small, unintentional movements. In addition to being sensitive to subtle motion events, the ADXL363 activity detection algorithm is robust in filtering out undesired triggers. The ADXL363 activity detection functionality includes a timer to filter out unwanted motion and ensure that only sustained motion is recognized as activity. The duration of this timer, as well as the acceleration threshold, are user adjustable from one sample (that is, no timer) to up to 20 seconds of motion. Note that the activity timer is operational in measurement mode only. In wake-up mode, one-sample activity detection is used. Inactivity Detection An inactivity event is detected when acceleration remains below a specified threshold for a specified time. Inactivity detection is also configurable as referenced or absolute. When using absolute inactivity detection, acceleration samples are compared to a user set threshold for the user set time to determine the absence of motion. Inactivity is detected when a large enough number of consecutive samples are below the threshold. Use the absolute configuration of inactivity for implementing free fall detection. |
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