Whether it's portable consumer electronics such as mobile phones and PDAs, automotive, kitchen appliances, medical devices, and industrial and commercial sensing applications, solutions based on intuitive capacitive touch technology are the preferred human-machine interface in these areas. Robust and reliable capacitive touch solutions are replacing traditional resistive sliders, mechanical buttons and rotary controls.
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Capacitive touch capture technology based on charge transfer
Capacitive touch capture based on "charge transfer" can be achieved in two ways.
The first solution is to charge a sensing electrode of unknown capacitance to a known potential. The electrode is typically a copper area on the PCB. The final charge is transferred to a measurement circuit. The capacitance of the sensing plate can be determined by measuring the charge after one or more "charge and transfer" cycles have been completed. Placing a finger on the touch surface creates an external capacitance that affects the charge flow at the contact point. This is a touch operation.
The second approach employs a pair of sensing electrodes: one is the emitter electrode, driven by the logic pulse charge in an intermittent mode; the other is the receiving electrode, which is coupled to the emitter via a blanket dielectric. When the finger touches the overlay, the field coupling is reduced and the touch is thus detected. This drive, receive, and processing logic is built into the microcontroller (MCU), so few external components are required.
Both options have their own unique advantages and are tailored to specific applications.
Touch function
Touch functions from capacitive touch capture solutions based on "charge transfer" can be divided into touch screens, touch buttons, sliders and pulleys.
The touch screen feature supports an unlimited number of touches, greatly improving the user experience and changing the way users interact with electronic products. Built-in gestures and the ability to ignore unintended operations make the user interface intuitive and reliable. The touch screen recognizes the touch of stylus, nails and gloves, providing a simple text input method for handheld devices.
Touch buttons, sliders, and pulleys are generally suitable for a single user touch and employ algorithms to determine the touch state and position, independent of signal strength, which makes touch detection accurate and reliable. Touch button, slider and pulley functions can be integrated into the product design in two ways: fixed function device solution; MCU touch library solution.
Developers select the touch functions and solutions they need based on the specific requirements of the different product designs.
The typical use environment for touch buttons, sliders and pulleys is shown in Table 1.
A typical capacitive touch sensor is connected to the MCU through various channels. Figure 3 shows a proximity button sensor using one channel, a touch button sensor using one channel, and a slider/pulley sensor using a set of three channels. The MCU port pins can be used as touch sensor channel pins.
Fixed function device solution
One of the solutions currently available on the market is a fixed-function touch device, and the so-called "fixed function" refers to a group of devices that are only used to process touch sensors. Therefore, a fixed function device is a dedicated touch microcontroller solution. Fixed-function touch devices provide single-channel or multi-channel support, and their touch sensor arrangement is typically pre-configured for use as a button, slider, pulley, or a fixed combination of these functions.
Fixed-function devices typically use a serial interface to update touch status information to the primary microcontroller, which is typically an I2C-compatible interface. Or use other interface technologies such as SPI, USART and bit banging. The touch status report of a fixed function device often includes a switch (ON/OFF) state close to a button or a touch button, a linear touch position value of the slider, and a touch angle position value of the pulley.
In addition, fixed-function touch devices have an additional feature that outputs strobos and pulses on the device pins to show a certain result without the need for a serial interface. This means that the switch state of the button or the position information of the slider and the pulley is not the only output information of the fixed function touch device. The secondary data output obtained by secondary processing of these data can meet the needs of the application. For example, in the power control of an appliance, the touch button status information can be converted into a programmed automatic shutdown delay, output to a pin of the device in the form of a trigger pulse, and finally the appliance is turned off.
The features of fixed-function devices are as follows: Capacitive touch solutions are available; fixed-function microcontrollers can be shipped with factory-programmed firmware; integrated fixed-function solutions into product designs compared to microcontroller touch-software solutions The amount of work required is relatively small; user configurability in output interface and sensor arrangement is limited; because the main application task is done by the main microcontroller, and fixed-function touch devices are added to increase capacitive touch support, so this The solution requires adding additional microcontrollers to the design.
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