FAQ

What is capacitive touch and how does it work?

Source: KONWHAI TOUCHAuthor: nick Date: 2018-12-24 Click:

  •      With the popularity of smart touch phones and touch tablets as well as touch computers and TVs, we often see or hear about touch screens as capacitive touches or computer touch screens.
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  •      But many friends in other fields don't understand what a capacitive touch screen is? And how does the capacitive screen achieve touch? Around these questions that everyone is more confused, Xiao Bian will come to explain with you in detail today.
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  • What is capacitive touch and how does it work?
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  •   What is a capacitive touch screen?  
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  •      Capacitive Screen Capacitor Technology Touch Panel Capacity Touch Panel (CTP) works with the body's current sensing. The capacitive touch screen is a four-layer composite glass screen. The inner surface and the interlayer of the glass screen are coated with a layer of ITO. The outermost layer is a thin layer of bauxite glass protective layer. The interlayer ITO coating is used as the working surface, and the four corners are taken out. Four electrodes, the inner layer of ITO is a shielding layer to ensure a good working environment.
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  •   Capacitive screen touch works :  
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  •      Chemically, ITO is an abbreviation for Indium Tin Oxides. The lateral electrode and the longitudinal electrode were fabricated using ITO. It is a kind of nano-indium tin metal oxide, which has good electrical conductivity and transparency, and can cut off electron radiation, ultraviolet rays and far infrared rays harmful to the human body. Therefore, indium tin oxide is usually sprayed on glass, plastic and electronic display screens, used as a transparent conductive film, while reducing the harmful electronic radiation and ultraviolet rays, infrared rays and the like.
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  • 1. Human body current sensing:
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  •      When the finger touches the metal layer, since the human body has its own electric field, the user and the surface of the touch screen form a coupling capacitor. For high-frequency current, the capacitor is a direct conductor, so the finger sucks a small current from the contact point. . This current is discharged from the electrodes on the four corners of the touch screen, and the current flowing through the four electrodes is proportional to the distance from the finger to the four corners. The controller calculates the position of the touch point by accurately calculating the ratio of the four currents. . This creates a capacitive touch effect.
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  •      When the human finger approaches, the local capacitance decreases, and according to the two-dimensional capacitance change data of the touch screen, the coordinates of each touch point can be calculated. As described above, even if there are multiple touch points on the screen, the true coordinates of each touch point can be calculated.
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  • 2. Multiple blocks support multi-touch:
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  •      Capacitive touch screens can support multi-touch (also single touch). According to the principle of the capacitive touch screen mentioned above, a single capacitive touch screen panel can not support multi-touch, but a large touch panel can be divided into multiple small blocks, each block is equivalent to a small independent capacitor. Touch screen panel.
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  •      Multiple blocks support multi-touch to make the capacitance touch screen coordinate calculation complicated, but this complexity is absorbed by the capacitive touch IC, or the touch information is reported through the digital interface and the host SoC communication (the number of touch points, the coordinates of each touch point, etc).
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  • 3. Provide external I2C access interface:
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  •      The entire capacitive touch screen consists of two parts: a touchpad sensor and a capacitive touch IC. The touchpad sensor is a physical device. The capacitive touch IC generally implements a flexible cable (FPC, such as the one on the flexible cable in the figure below) of the touch screen. The capacitive touch IC is responsible for manipulating the touchpad, and is obtained by AD conversion and analysis. Touch the number of points, touch coordinates, etc., and then communicate with the SoC with a specific digital interface. This digital interface is I2C.
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  •      For our host SoC, the capacitive touch screen is actually an I2C slave device. The host only needs to access the slave device through the I2C bus (the slave device has its own specific slave address). From here, in fact, there is no difference between a capacitive touch screen and other sensors (gsensor, etc.).
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