Knowledge

Data processing of capacitive touch screen



After receiving the touch signal, the capacitive touch screen converts the touch data into electrical pulses and transmits it to the touch screen control IC for processing.

The signal is amplified by a low-noise amplifier LNA, which is then converted and demodulated via analog-to-digital and finally sent to a DSP for data processing. Capacitive touch screens typically have a m+n (M-row N-line) physical capacitive touch sensor. This m+n a staggered sensor that makes up the m*n capacitance sensing point, which changes as the user's finger approaches the touch screen.  

The interval of the sensor (that is, the distance between adjacent rows or columns) is usually around a few millimeters, which determines the physical resolution of the touchscreen 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 with a resolution of 800*480, which means that there are 800 rows of 480 RGB pixels per line. As a result, a specific position can be determined by the pixel dots (x, y) in the y direction. The capacitive touchscreen module determines the coordinate system based on its original physical dimensions in the direction of X and Y. 

A reasonable mapping method must exist between two coordinate systems to ensure the correctness of the input and output operations.

Therefore, the DSP processor of the touchscreen control IC also has to convert the resulting data into a pixel map between the capacitive touch screen module and the LCD module to ensure that the user's touch point is sensed on the touch screen. 

In addition, in order to maintain the stability of touch coordinates, the touch screen control IC needs to further deal with the jitter of the touch point, including the jitter of the finger and the noise of the capacitance data, and change the filter coefficients of the low-pass filter according to the change of coordinates, and realize the smooth processing of the coordinates. Finally, before uploading the data to the mainframe, you have to use the software to analyze the data to determine what features are used for each touch. This process involves determining the size, shape, and position of the area that is touched on the screen. If necessary, the processor will group similar touch grooming. 

If the user moves the finger, the processor calculates the difference between the start and end points of the user's touch.