Low power consumption can use MEMS Accelerometer sensors to increase battery life. Sensors become more and more power efficient, and the embedded features also help reduce overall system energy consumption. For example, when the user does not use the device, the motion-sensing wake-up function keeps the entire system asleep. However, there are many other possibilities to use MEMS accelerometers to reduce overall power consumption.
Starting from the MEMS accelerometer sensor itself, the operating mode should be flexible. As shown in Figure 1, we know the resolution of the sensor and the output data rate. There must be some trade-off between the current consumption and the current consumption. The higher the resolution or data rate, the greater the current consumption. ,vice versa. Fortunately, some sensors on the market can operate with only a few micro-amps and consume only a few nano-amps of power when they are off or in standby mode.
Figure 1: Sensor parameters affect battery life
For some demanding applications, the sensor's operating mode can be quickly replaced, and the resolution and data transfer rate will be improved only when it is really needed. Some sensors can even switch modes automatically. The customer can configure the required resolution and data transmission rate in the active state and customize the conditions for starting it. At this time, the sensor will enter a stationary state, but it will continue to measure the data and perform the data transmission at a very low rate and resolution. Only when the set conditions (action events) occur will the sensor switch back to the startup state.
Another good design principle is to use low power levels because lower power levels also mean lower current consumption. This is why 1.8V power is the first choice for low power applications.
In some designs, power cycling of the sensor can be used. The sensor's power supply will only be activated if it needs to be measured, otherwise the sensor will be turned off. We can do this by providing power to the sensor from the pins of the microcontroller. as shown in picture 2. When applying this technique, it is necessary to correctly calculate the power budget, because each sensor's start needs to be configured and waited until the output is determined and the correct data is provided.
Figure 2: Transmitting Control Sensor Power Supply for Controlling Microcontroller Pins
Most MEMS accelerometers are digital sensors, which means they can convert the measured analog signal to digital data. Because there are integrated analog signal converters, coupled with low sensitivity to signal distortion, BOM items are reduced, but this is not the only advantage. The embedded interrupt generator MEMS accelerometer can generate a trigger signal when a user-set parameter condition is satisfied. This is a way of using the motion sensing wake-up function. The microcontroller (MCU) configures the sensor to generate a wake-up trigger and enters an extremely low-power sleep mode. When an action is detected, the sensor will generate an interrupt signal. After the MCU receives the signal, it will switch to a suitable operation mode and then deal with the situation just happened.
Digital sensors can also take over tasks related to motion processing performed by the microcontroller. The MCU can of course perform the same work, but the power efficiency is much lower - the power consumption of the MCU is one milliampere, and the sensor is microampere. Detection of free fall, single point, double-click (user actions like mouse clicks), portrait/landscape direction detection, etc. is achieved through the internal logic of the sensor. The MCU does not need to perform any calculations, it just needs to wait for an interrupt trigger and react to the action only when it happens.
Digital sensors often integrate configurable filters that are used to measure acceleration data. Low-pass, high-pass, or even alias filters can be used to preprocess data for the MCU and increase offload shunts. .
The data buffer embedded in the sensor is mostly of the first-in-first-out (FIFO) type because it allows the MCU to reduce the frequency of reading data, thus reducing the current consumption. This allows the microcontroller to perform other tasks, extend the sleep time, and save the time required to communicate with the sensor serial port.
Serial communication between the sensor and the microcontroller also increases overall power consumption. For ultra-low-power applications, serial communications can have a significant impact every time a microampere-ampere is processed. Most MEMS accelerometers communicate via the Serial Peripheral Interface (SPI) and the I2C interface. The SPI interface is more efficient in terms of power consumption for three reasons: one is that there are no leads on the communication line that will cause extra current consumption; the second is to support higher data rates; and third, the overhead of the serial protocol is reduced.
Regardless of which interface is used, we can drastically reduce serial communications by letting applications take advantage of data ready interrupts without sensor polling, that is, continuously requesting new data availability status. After the sensor completes the data measurement and conversion, the data preparation interrupt is automatically generated and the new data set will be read by the MCU. When this interrupt is activated, the MCU will immediately read the output data from the sensor through a single reading action.
As mentioned earlier, the low data rate output by the sensor means that the current power consumption is low. Therefore, the so-called single data conversion mechanism can be a perfect match between the sensor and the data required by the application, as shown in Figure 3. Using this mechanism, either an external trigger signal on the sensor pin or a register written from the MCU using serial instructions. The data thus obtained is stored in the sensor. The sensor can also initiate a data preparation interrupt signal to inform the MCU that the data conversion has been completed and the data can now be read by the application program. With this feature, data rates other than 1 Hz or any other predefined range can be achieved.
Figure 3: Single data conversion mechanism
This article discusses MEMS accelerometer sensor functions that are important for low-power applications and how they can be used in system design. ST's latest LIS2DW12 ultra-low-power 3-axis MEMS accelerometer uses an accelerometer sensor to provide flexibility for designing new applications because it consumes up to 1Ua of current, plus multiple run modules, outputs Extremely wide range of data rates, rich embeddedness, high temperature stability, and various enhancements such as digital filters and first-in, first-out buffers. We believe that many low-power applications can enjoy the advantages of the LIS2DW12. This sensor will provide users with advantages, especially in the areas of motion sensing function and user interface, smart energy-saving function of handheld equipment, electrical-related motion monitoring, and impact recogniTIon logging of wireless sensor nodes.
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