Data processing process of capacitive touch screen

After the capacitive touch screen receives the touch signal, it converts the touch data into an electric pulse and transmits it to the touch screen control IC for processing. The signal is first amplified by a low-noise amplifier (LNA), then amplified by analog-to-digital conversion and demodulation, and finally sent to a DSP for data processing.

Capacitive touch screens generally have an M + N (M column, N row) physical capacitor touch sensor. The M + N interlaced sensors form the M * N capacitance induction points, which will change as the user; fingers approach the touch screen. The interval of the sensor (that is, the distance between adjacent rows or columns) is usually a few millimeters. Left and right, this interval distance determines the physical resolution of the touch screen, M * N.

The coordinate system between the capacitive touch screen module and the LCD module is completely different. The pixel coordinates of the LCD module are generally determined by its resolution. For example, a WVGA screen has a resolution of 800 by 480, which means that if there are 800 lines of 480 RGB pixels, Thus, a specific location can be determined by the pixels (x, y) in the X and Y directions. The capacitive touch screen module determines the coordinate system according to the original physical size in the direction of its X and Y. There must be one between the two coordinate systems. A reasonable mapping method can only ensure the correctness of the input and output operations.

Therefore, the DSP processor of the touch screen control IC also has to convert the capacitive pixel mapping between the touch screen module and the LCD module on the obtained data so as to ensure that the touch point of the user is detected on the touch screen at the point indicated by the user.

In addition, in order to maintain the stability of the touch coordinates, the touch screen control IC needs to further handle the jitter of the touch point, including the jitter of the finger and the noise of the capacitance data, and change the filtering coefficient of the low-pass filter according to the change of the coordinates so as to realize smooth processing of the coordinates.

Finally, before the data is sent to the host, software analyzes the data to determine what function is used for each touch. This process involves determining the size, shape, and location of the area being touched on the screen. If necessary, the processor groups similar touches. If the user moves the finger, the processor calculates the difference between the beginning and the end of the user's touch.