With the popularity of the Apple iPhone and the Nintendo Wii, accelerometers are widely used in consumer electronics. For example, drop protection in notebooks, automatic flipping of screens in MP4/mobile phones, tapping of mobile phones to hang up calls, and "flip mute" of mobile phones. These applications change the way that traditional devices interact with users, giving the product a better user experience. STMicroelectronics' micro-mechanical acceleration sensor not only outputs the value of acceleration, but also integrates many functional modules to achieve the above applications. We will use the functional modules inside the LIS35DE and LIS331DLH chips as a clue to introduce the performance characteristics of STMicroelectronics accelerometers and how to configure the functional modules of the sensors to achieve specific applications.
Double interrupt configuration
Drop protection is very important for portable devices equipped with hard drives. This feature has become very common in high-end notebook computers. The triaxial accelerometers LIS35DE, LIS33DE and LIS331DLH/M/F are capable of falling protection by interrupting the pin and setting the acceleration threshold register. Among them, the acceleration threshold register is a register for setting the threshold acceleration, and the interrupt setting register is a register for setting the condition of the interrupt output and the judgment of the interrupt source. The following describes the detection principle and the interrupt configuration method by taking the free fall detection as an example.
If the object is at rest, it is only affected by the acceleration of gravity, and the sum of the gravitational acceleration vectors on the three axes of the object is 1g.
If the object is in a free-fall state, the acceleration on the X-axis, Y-axis, and Z-axis of the object is theoretically zero. In the actual situation, due to the air resistance and the centrifugal force generated when the drop rotates, the sum of the acceleration vectors on the three axes should be a small value.
At rest, the acceleration of an object on three axes cannot be less than 577 mg at the same time. Of course, this is a theoretical upper limit reference. Considering the influence of the error and temperature change of the acceleration sensor at 0g on the measurement accuracy, to prevent false triggering, this threshold can be set to 360mg. If the acceleration on the three axes is less than 360mg at the same time, we can think that the object is in a free fall state and can trigger an interruption. It should be noted that this threshold value is an absolute value, and the threshold value is valid for the three axes of the X-axis, the Y-axis, and the Z-axis. Therefore, we can set the acceleration to generate an interruption when the threshold exceeds the threshold value on one axis. ("or" relationship) or acceleration exceeds the threshold on all three axes to generate an interrupt ("and" relationship). In this way, when doing fall protection, you should choose the relationship of "and" as the judgment condition of the fall.
The LIS35DE and LIS331DLH/M/F can provide two interrupt configuration registers and independent interrupt configuration registers and acceleration threshold registers. While one interrupt signal is used for drop detection, the other interrupt can be used to set wake-up, that is, when the acceleration is greater than the set threshold value, a wake-up interrupt is generated. In addition to the above two functions, the interrupt can be set to functions such as data update notification and tap detection.
High pass filter
The iPhone 3.0 integrates a new feature that simply cancels the phone and cancels the previous operation. If you use such simple actions instead of buttons or even make new applications, such as panning games, switching music, etc., you can make some common functions interesting. If you use the above interrupt pin register to achieve such a function, you will encounter a problem - gravity. Due to the presence of gravity, the user cannot simply set a uniform threshold value in the FF_WU_TSH1 register as a determination condition for a shaking or wake-up interrupt. To this end, the LIS35DE integrates a high-pass filter that filters out slowly varying or constant accelerations (such as gravitational acceleration) and selects the cutoff frequency, as shown in Table 1. Only accelerations whose changing frequency is higher than the cutoff frequency can pass.
With the high-pass filter, it is easier to switch the music played by the phone by shaking. The specific principle is to detect the change track of the acceleration when the mobile phone shakes to the left and right, thereby defining the order of switching songs. When shaking the phone to the left (switching to the previous song), a large acceleration is generated on the Y-axis, and the direction of the acceleration is negative and positive. Conversely, if you shake the phone to the right (switch to the next song), the acceleration on the Y-axis is positive and negative. In this application, the high pass filter removes the effects of gravitational acceleration. In the time range of effective sway (such as 200ms), both the positive and negative accelerations trigger the interruption, we can determine that it is an effective sway; then by judging the order of positive acceleration and negative acceleration, we You can determine the direction of the sway.
6D detection
In the three-dimensional space, the position of the object can be divided into six states according to the difference of the positive and negative directions on the three axes, which are up, down, left, and right. As shown in Figure 1. This spatial location information is useful for handheld devices. It not only allows users to understand the spatial status of the current device, but also develops new applications based on this, such as "flip mute" and so on.
The state of the object in three dimensions
The LIS331DLF/M/H series acceleration sensor integrates a 6D detection function module. If the spatial location of the device changes, it can notify the system in the form of an interrupt.
This function is achieved by detecting whether the component of the gravitational acceleration on each axis exceeds a certain threshold value at rest. It allows the user to set this threshold value, giving the user a great deal of flexibility. XH/YH/ZH represent the positive direction of the X/Y/Z axis, respectively. XL/YL/ZL represent the negative direction of the X/Y/Z axis, respectively. When the detected acceleration is greater than the threshold value in the positive direction, XH/YH/ZH is 1; when the detected acceleration is less than the threshold value in the negative direction, XL/YL/ZL is 1. By reading the interrupt status register INTx_SRC, we can determine the state of the device at this time.
Depending on the threshold value, the user can define certain states more precisely. Taking the X-axis/Y-axis as an example, in order to define the position more accurately, the threshold value can be set higher, so that some "dead zone" is left between the X and Y axes. The sensor does not trigger a 6D interrupt within the "dead zone" range. An interrupt is triggered only if the actual position of the object is closer to the user-defined area. This feature can avoid user misuse in some applications.
The following is an example of turning the mobile phone to mute, illustrating the detection principle and the setting of the 6D detection function. Since the gravitational acceleration is always present, the mobile phone lying flat on the table has an acceleration of 1 g in the negative direction of the Z-axis (pointing to the center of the earth), and there is no acceleration (zero or small) on the X-axis and the Y-axis.
When you need to mute, turn your phone over and place it on the back with the back facing up. There is still no acceleration on the X and Y axes, but the gravitational acceleration is added in the positive direction of the Z axis. By detecting the gravitational acceleration in the positive and negative directions of the Z axis, you can know the current state of the phone. Such a scheme requires constant detection of acceleration values, thus consuming a large amount of resources of the system.
If you use the 6D detection function, you only need to wait for an interruption from the positive direction of the Z axis to recognize whether the phone is flipped, which greatly saves system resources.
Single/double click recognition
In addition to the "flip mute" function, the Nokia 8800CA phone also has the function of double-clicking the phone to display the clock. The identification of the tap can be realized by an acceleration sensor. The LIS35DE integrates a function module that recognizes clicks or double-clicks, and the register design identifies the click and whether the two clicks meet the double-click condition. The single-double-click function of the LIS35DE is controlled by registers with addresses 38h to 3Fh, as shown in Table 2.
Clicking on the identified process requires two conditions to be met. First, the acceleration value generated by the tap is higher than the threshold value, and the second is that the acceleration value is higher than the threshold value and less than the value of CLICK_TImeLimit. When the tap meets both of these conditions, it can be determined as a single click. When the value greater than CLICK_TImeLimit is required, no interruption will occur.
On the basis of detecting the first click, the recognition of the second click must satisfy the conditions set by CLICK_Latency and CLICK_Windows. Only a second click falls within the CLICK_Windows time period, which is a valid double click, otherwise it can only be determined as two clicks. CLICK_Latency is used to set the function off time after the first click is recognized. It is used to eliminate noise acceleration and prevent false triggering.
With the CTRL_REG3 (22h) register to set the interrupt signal, the single-double-click function is implemented by interrupting the output of the interrupt pin 1 or 2.
Sleep mode
Portable electronic devices have high power consumption requirements for electronic components. Therefore, ST's LIS35DE can be set to two modes: normal operating mode ("0.5mA" and power-down mode ("1μA"). Although the LIS35DE consumes less than 0.5mA in normal operation, in order to achieve longer standby times, the LIS35DE can be put into power-down mode to reduce power consumption.
These two modes of operation are sufficient for most applications, but for the ability to wake up with an accelerometer, these two modes of operation are slightly insufficient. Because the wake-up function requires the main processor to configure the corresponding registers, the LIS35DE wakes up from the power-down mode and requires tens of milliseconds of latency; at the same time, the 0.5mA current is still somewhat large during standby. For example, when using an acceleration sensor to wake up GPS (Global Positioning System), if the GPS is at rest and there is no acceleration change, the GPS main processor enters the standby mode; once motion occurs, the acceleration sensor senses the change in acceleration. An interrupt signal is issued to wake up the main processor and the LNA. When the GPS is in standby, the acceleration sensor is always in working state and cannot enter the power-down mode. Otherwise, the acceleration sensor needs other devices to wake it up.
In the LIS331DLH/M/F accelerometer, not only the above two modes but also a low power mode are available. In this mode, the output update frequency of the data can be set to 0.5Hz/1Hz/2Hz/5Hz, and the current consumption is only 10μA ~ 60μA, which is much smaller than the current of 0.5mA during the normal operation of the LIS35DE.
In low power mode, the accelerometer is still active but consumes very low power. It does not need to be woken up by other devices. Instead, it is possible to detect the motion state of the device and wake up the main processor under certain conditions. This greatly reduces the system standby power consumption. The device can be used not only for intelligent motion detection of PNS (portable navigation system), but also for dynamic lighting applications of mobile phone screens and some mobile pet games.
In general, ST has a broad portfolio of accelerometers that integrate easy-to-use functional modules for different applications. Each functional module not only simplifies the software design for a specific application, but also coordinates the work between functional modules to complete more complex applications.
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