Touch screens or touch panels are part of our daily routine. Besides the countless consumer and infotainment devices, POS terminals, ticket machines or industrial control units, the iPod success story may be the latest and most impressive proof that touch sense has made its breakthrough to the market.
Compared to the traditional approach based on mechanical parts attractive applications like sliding or zooming enable new experiences on panel/display designs. On the other hand displays with touch control are more reliable and robust against wear out and dirt. Therefore there is an increasing demand to improve the human machine interface (HMI) design with innovative touch control to make it more intuitive, robust and cost efficient. With inTouch Infineon provides cost-effective and optimized microcontroller solutions for fast time-to-market HMI designs.
The majority of touch screens is using either resistive or capacitive touch sensing technologies. The resistive approach is based on two thin, metallic and electrically conductive layers separated by a narrow gap. When an object, such as a finger, presses down on a point on the panel's surface the two metallic layers become connected. This causes a change in the voltage/current, which is registered as a touch event and sent to the controller for processing.
A capacitive panel consists of an insulator such as glass, coated with a transparent conductor. Capacitive sensing is a technology for detecting proximity and position based on capacitive coupling effects. More and more engineers choose capacitive sensors for their flexibility, unique human-device interface and cost reduction over mechanical switches. In addition capacitive touch screens are faster and more responsive than resistive products, while offering higher design flexibility
Capacitive touch sensing
A capacitive sensing system is a set of conductors which interact with electric fields, based on the physical effect that the human body (i.e. finger) varies the capacitance of this system. The fundamental components of such a system are a programmable current source, a precision comparator, and an array of capacitive sensors. In order to make a product out of this concept, a microcontroller is needed to process the sensor data. The microcontroller based control circuit senses and decides whether a touch incident or not and what action has to be triggered. There are different ways to realize such a solution with a microcontroller.
Touching the sensor array creates a change in capacitance, which is detectable using a variety of techniques. The Infineon capacitive touch sense solution uses the switch capacitor as part of a relaxation oscillator configuration. The hardware peripherals necessary to add touch-sensitive switches are available on the new XC800 microcontrollers. The capacitive touch sense solution with relaxation oscillator has many advantages, including low BOM cost, high reliability towards noise, simple calibration and adjustment of sensitivity.
A relaxation oscillator is a circuit that repeatedly alternates between two states with a period that depends on the charging of a capacitor. Based on figure 1 the working principle of the relaxation oscillator can be explained. Initially, the discharge switch is open, and the pull-up resistor charges the sensor pad. The voltage on the sensor pad ramps positively until it exceeds the comparator's threshold. The comparator's output transitions from low to high, causing the discharge switch S to close. The sensor pad quickly discharges through this low impedance path to ground. The process causes the comparator's output to transition from high to low, and the cycle repeats. The output frequency (fout) is dependent on the charging current and capacitive sensor value.
An innovative way to advanced touch control
Fig 1: Capacitive touch sensing using relaxation oscillator technology.
Infineon's technology based on the capacitive touch sense principle was optimized for an easy use in highly integrated industrial and automotive applications. The relaxation oscillator topology used by the approach results in increased robustness against electrical noise and higher reliability with reduced errors.
Infineon provides an optimized software library with the basic algorithms for this solution as part of the ROM code, reducing the calibration and adjusting efforts. Therefore flexible designs supporting different surfaces are possible. The innovative approach also leaves Flash memory and CPU resources for customer specific solutions and designers don’t waste time dealing with the basic functionality. The special peripheral unit (LED/TS controller) in XC82x and XC83x series (figure 2) features a combined solution for touch and LED-Matrix control through time multiplexed operation.
Fig 2: Block diagram XC822: With inTouch Infineon provides innovative, easy to use and cost effective solutions for touch control. The integrated LED/TS controller enables combined touch sense and LED matrix control.
Operating with high frequency the touch sensing can be combined for example with the control of an LED matrix used in displays. This enables a low-pin-count and cheap single layer PCB solution. For example a standard 12 buttons key pad can be realized with 5 I/O pins only.