The touch screen display can detect whether or not the touched and touched positions are on the display area. Therefore, mechanical buttons on various devices are being replaced by such displays, including smart phones, MP3 players, GPS navigation systems, digital cameras, notebooks, and games. Machines and laboratory instruments. The first generation of such devices was not very accurate, with the problems of high false detection rate and excessive power consumption. New touch screen controllers, such as the AD7879, provide higher accuracy, lower power consumption and result filtering. These devices can also detect temperature, supply voltage, and touch pressure, helping modern touchscreen displays to achieve robust detection.
How touch screens work First, let's see how resistive touch screens work. Figure 1 shows the basic structure and working principle diagram of the touch screen.
The touch screen consists of two layers of plastic film. Each film layer is coated with a layer of conductive metal (usually indium tin oxide). The air gap in the middle separates the two. The power supply voltage excites one of the boards. The figure above shows the X board. When the screen is touched, the two conductive plates come together and form a resistor divider on the X board. The voltage at the contact point is detected by the Y+ electrode and represents the position on the X-plate, as shown in Figure 2. Then the above process is repeated, and the power supply voltage activates the Y-plate and detects the Y position through the X+ electrode.
Next, place the supply voltage on Y+ and X– and perform two screen measurements: measure the voltage on X+ to get Z1 and measure the voltage on Y– to get Z2. These measurements can be used to estimate touch pressure in two ways. If the resistance of the X board is known, the touch resistance is calculated as:
If the resistances of the X and Y plates are known, the touch resistance is calculated as:
The larger the touch resistance value, the smaller the touch pressure.
AD7879 Touch Screen Controller The AD7879 Touch Screen Controller is designed to interface with a four-wire resistive touch screen. In addition to detecting touch actions, it also measures temperature and auxiliary input voltage. All four touch measurements plus temperature, battery, and auxiliary voltage measurements can be programmed into their on-chip sequencers. Wide supply voltage range (1.6V to 3.6V), small size (12-pin, 1.6mm × 2mm WLCSP package or 16-pin, 4mm × 4mm LFCSP package) and low power consumption (480μA during conversion, shutdown mode 0.5? A), this controller can be used flexibly for various products.
Wake-on-touch allows the AD7879 to be configured to start and transition when a touch screen event occurs, and enter power-saving mode after release. This configuration is ideal for battery-powered devices that focus on saving power. After each conversion sequence is completed, the AD7879 sends an interrupt to the host microcontroller, waking it from the low power mode to process the data. In this way, the power consumption of the microcontroller before the screen is touched is also extremely low. Figure 3 shows the settings for the Touch Wake function.
When touching the screen, the X board comes in contact with the Y board, pulls down the limiter input, wakes up the AD7879, and then starts the conversion. At the end of the conversion, the AD7879 sends an interrupt to the host.
Results Filtering In a typical display, the resistor board is located above the liquid crystal display (LCD), which generates a lot of noise and affects position measurement. This noise is a combination of impulse noise and Gaussian noise. The median filter and averaging filter provided by the AD7879 reduce this noise. The sequencer can be configured to take position measurements with 2, 4, 8, or 16 samples instead of using a single sample for measurement. These samples are sorted, median-filtered, and average-filtered to produce lower-noise, higher-accuracy results. Figure 4 clearly shows the principle. Get 16 position measurement results, then sort them from low to high. Four maximum measurement results and four minimum measurement results are eliminated to eliminate impulse noise; the remaining eight samples are averaged to reduce Gaussian noise. This method has the additional benefit of reducing the amount of host processing required and the host's communication with the touch screen controller.
How touch screens work First, let's see how resistive touch screens work. Figure 1 shows the basic structure and working principle diagram of the touch screen.
Figure 1. Structure of a resistive touch screen
The touch screen consists of two layers of plastic film. Each film layer is coated with a layer of conductive metal (usually indium tin oxide). The air gap in the middle separates the two. The power supply voltage excites one of the boards. The figure above shows the X board. When the screen is touched, the two conductive plates come together and form a resistor divider on the X board. The voltage at the contact point is detected by the Y+ electrode and represents the position on the X-plate, as shown in Figure 2. Then the above process is repeated, and the power supply voltage activates the Y-plate and detects the Y position through the X+ electrode.
Figure 2. X position measurement
Next, place the supply voltage on Y+ and X– and perform two screen measurements: measure the voltage on X+ to get Z1 and measure the voltage on Y– to get Z2. These measurements can be used to estimate touch pressure in two ways. If the resistance of the X board is known, the touch resistance is calculated as:
If the resistances of the X and Y plates are known, the touch resistance is calculated as:
The larger the touch resistance value, the smaller the touch pressure.
AD7879 Touch Screen Controller The AD7879 Touch Screen Controller is designed to interface with a four-wire resistive touch screen. In addition to detecting touch actions, it also measures temperature and auxiliary input voltage. All four touch measurements plus temperature, battery, and auxiliary voltage measurements can be programmed into their on-chip sequencers. Wide supply voltage range (1.6V to 3.6V), small size (12-pin, 1.6mm × 2mm WLCSP package or 16-pin, 4mm × 4mm LFCSP package) and low power consumption (480μA during conversion, shutdown mode 0.5? A), this controller can be used flexibly for various products.
Wake-on-touch allows the AD7879 to be configured to start and transition when a touch screen event occurs, and enter power-saving mode after release. This configuration is ideal for battery-powered devices that focus on saving power. After each conversion sequence is completed, the AD7879 sends an interrupt to the host microcontroller, waking it from the low power mode to process the data. In this way, the power consumption of the microcontroller before the screen is touched is also extremely low. Figure 3 shows the settings for the Touch Wake function.
Figure 3. Touch wake up setting
When touching the screen, the X board comes in contact with the Y board, pulls down the limiter input, wakes up the AD7879, and then starts the conversion. At the end of the conversion, the AD7879 sends an interrupt to the host.
Results Filtering In a typical display, the resistor board is located above the liquid crystal display (LCD), which generates a lot of noise and affects position measurement. This noise is a combination of impulse noise and Gaussian noise. The median filter and averaging filter provided by the AD7879 reduce this noise. The sequencer can be configured to take position measurements with 2, 4, 8, or 16 samples instead of using a single sample for measurement. These samples are sorted, median-filtered, and average-filtered to produce lower-noise, higher-accuracy results. Figure 4 clearly shows the principle. Get 16 position measurement results, then sort them from low to high. Four maximum measurement results and four minimum measurement results are eliminated to eliminate impulse noise; the remaining eight samples are averaged to reduce Gaussian noise. This method has the additional benefit of reducing the amount of host processing required and the host's communication with the touch screen controller.
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