Five functions of the hottest stmems acceleration

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ST: five functions of MEMS acceleration sensors simplify user design

with the popularity of Apple iPhone and Nintendo Wii game consoles, acceleration sensors have been widely used in consumer electronic devices. For example, drop protection in notebooks, automatic screen flip in mp4/, tap to hang up, flip mute, and so on. These applications have changed the interaction between traditional devices and users, and made the products have a better user experience. As the leader of this change, ST's micro mechanical acceleration sensor can not only output the value of acceleration, but also integrate many functional modules to realize the above applications. We will take the functional modules inside lis35de and lis331dlh chips as clues to introduce the performance characteristics of the Italian French semiconductor acceleration sensor, and introduce how to configure the functional modules of the sensor to achieve specific applications

dual interrupt configuration

drop protection is very important for portable devices equipped with hard disks. This function has been widely used in high-end notebook computers. The three-axis acceleration sensors lis35de, lis33de and lis331dlh/m/f can realize the fall protection function by interrupting the pin and setting the acceleration threshold register. Among them, the acceleration threshold register is a register used to set the threshold acceleration, and the interrupt setting register is a register used to set the conditions of interrupt output and the determination of interrupt source. Taking free fall detection as an example, the detection principle and interrupt configuration method are described below

if the object is at rest, it is only affected by the gravitational acceleration, and the sum of the gravitational acceleration vectors on the three axes of the object is 1g

if the object is in a free falling state, theoretically, the acceleration on the X axis, Y axis and Z axis of the object is zero. In practice, the sum of acceleration vectors on the three axes should be a smaller value due to air resistance and centrifugal force generated during falling and rotation

when at rest, the acceleration of the object on three axes cannot be less than 577mg at the same time. Of course, this is a theoretical upper limit reference value. Considering the influence of the error of the acceleration sensor at 0g and the temperature change on the measurement accuracy, in order 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 falling state, which can trigger the interruption. It should be noted that this threshold value is an absolute value, and the threshold value is sensitive to the operation of x-axis, Y-axis and static steel wire rope horizontal tensile testing machine product details, so that when the z-axis three axes are effective, therefore, We can set that when the acceleration exceeds the threshold value on a certain axis, there will be an interruption ("or" relationship) or when the acceleration exceeds the threshold value on all three axes, there will be an interruption ("and" relationship). In this way, when doing fall protection, we should choose the relationship between "and" as the judgment condition of fall

lis35de and lis331dlh/m/f can provide two interrupt configuration registers and mutually independent interrupt configuration registers and acceleration threshold registers. While one interrupt signal is used for drop detection, another interrupt can be used to set as wake-up, that is, when the acceleration is greater than the set threshold, wake-up interrupt will be generated. In addition to the above two functions, the interrupt can also be set as data update notification and knock detection

high pass filter

iphone3.0 integrates a new function, that is, you can cancel the operation just by shaking. If such simple actions can be used to replace buttons and even make some new applications, such as sending dangerous style dice games and switching music to avoid occasional power failure, some commonly used functions can become interesting. If the above interrupt pin register is used to realize such a function, there will be a problem - gravity. Due to the existence of gravity, users cannot simply_ WU_ A unified threshold value is set in the tsh1 register as the determination condition of sloshing or wake-up interrupt. To this end, lis35de integrates a high pass filter, which can filter out slowly changing or constant acceleration (such as gravity acceleration), and can select the cut-off frequency, as shown in Table 1. Only the acceleration with the change frequency higher than the cut-off frequency can pass

with the high pass filter, it becomes easier to switch the music played by shaking. The specific principle is to detect the change track of acceleration when shaking left and right, so as to define the order of switching songs. When shaking to the left (switching to the previous song), a large acceleration will be generated on the y-axis, and the direction of the acceleration is negative first and then positive. Conversely, if you shake to the right (switch to the next song), the acceleration on the Y axis is positive first and then negative. In this application, the high pass filter removes the influence of gravitational acceleration. Within the time range of an effective shaking (such as 200ms), both positive and negative accelerations trigger an interruption, and we can judge it as an effective shaking; By judging the sequence of positive acceleration and negative acceleration, we can determine the direction of shaking

in three-dimensional space, according to the difference of positive and negative directions on the three axes, the position of the object can be divided into six states, namely, up and down, left and right, front and back. As shown in Figure 1. This spatial location information is very useful for handheld devices. It can not only let users understand the current spatial state of the device, but also develop some new applications on this basis, such as "flip mute" and so on

lis331dlf/m/h series acceleration sensor integrates 6D detection function module. If the spatial position of the device changes, it can notify the system in the form of interruption

this function is realized by detecting whether the component of gravity acceleration on each axis exceeds a certain threshold value at rest. It allows users to set this threshold, giving users great flexibility. Xh/yh/zh respectively represent the positive direction of x/y/z axis. Xl/yl/zl respectively represent the negative direction of x/y/z axis. 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 status of the equipment at this time

users can define some states more accurately according to different thresholds. Take the x-axis/y-axis as an example. In order to define the position more accurately, the threshold can be set higher, which will leave some "dead zones" between the simple design of the fixture for the x-axis and y-axis. Within the "dead zone" range, the sensor will not trigger 6D interrupt. The interrupt is triggered only when the actual position of the object is closer to the user-defined area. This function can avoid user misoperation in some applications

next, take turning to mute as an example to illustrate the detection principle and the setting of 6D detection function. Since the gravitational acceleration always exists, the acceleration in the negative direction of the Z axis (pointing to the center of the earth) on the table is 1g, and there is no acceleration (zero or very small) on the X axis and Y axis

when mute is needed, turn it over and place it on the table with the back facing up. At this time, there is still no acceleration on the X and Y axes, but the gravitational acceleration is added to the positive direction of the Z axis. The current state can be known by detecting the gravitational acceleration in the positive and negative directions of the Z axis. This scheme needs to constantly detect the acceleration value, so it takes a lot of resources of the system

if you use the 6D detection function, you can identify whether to flip by waiting for the interrupt in the positive direction of the Z axis, which greatly saves system resources

single/double click recognition

in addition to the "flip mute" function, the Nokia 8800ca also has the function of double clicking to display the clock. The recognition of knocking can be realized by acceleration sensor. Lis35de integrates the function module of identifying single click or double click. Through register design, you can identify whether a single click and two clicks meet the conditions of double click. The single double click function of lis35de is controlled by registers with addresses of 38h ~ 3fh, as shown in Table 2

the process of click recognition needs to meet two conditions. The first is that the acceleration value generated by knocking is higher than the threshold value, and the second is that the acceleration value is higher than the threshold value and less than click_ Value requirements for timelimit. When the knocking meets these two conditions at the same time, it can be judged as a click. And greater than Click_ When the value of timelimit is required, there will be no interruption

on the basis of detecting the first click, the recognition of the second click must meet the click_ Latency and click_ Conditions set by windows. Only the second click falls on the click_ Within the windows time period, it is only a valid double click, otherwise it can only be judged as two clicks. C as the name suggests, lick_ Latency is used to set the function closing time after the first click is recognized. It is used to eliminate noise acceleration and prevent false triggering

with Ctrl_ The reg3 (22h) register is used to set the interrupt signal, which realizes the single double-click function by interrupting the output of pin 1 or 2 and the interrupt notification

sleep mode

portable electronic devices have relatively high requirements for the power consumption of electronic components. Therefore, the lis35de of STMicroelectronics can set two modes: normal working mode(

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