As smartphones have become more ubiquitous, the capacitive touchscreen technology they use has become common in other devices as well. This allows for easy-to-use interfaces that accept input and also provide dynamic, graphical feedback. Renesas has recently announced the RX113 product line, that provides second-generation touchscreen technology. Specially designed for usage in the emerging market of the Internet of Things and the corresponding increase in Human Machine Interfaces, this touchscreen technology is a fusion of two technologies that uses the best aspects of older technologies.
Previously there have been two types of capacitive touch-sensor screens: self-capacitance and mutual-capacitance. A self-capacitance screen measures the increased capacitance between the ground and a user’s finger. With this type of screen, the x and y locations of the point on the screen are determined separately. This allows for fast and low-power operation, but this method can be confused by multi-touch gestures, such as the pinch and rotate gestures used on a smartphone, when there are multiple x and y points that need to be matched together. (See figure 1.)
Mutual-capacitance screens work, in some ways, like a CMOS imager: each position on the screen can be detected individually, but this requires that one axis be scanned while the other axis is measured. (See figure 2.) As you would expect, this approach works better in a multi-touch scenario, but is slower and uses more power. Up until this point, designers have had to weigh the benefits and drawbacks and decide if their particular application needed more responsiveness or needed full multi-touch sensing.
What the Renesas engineers have done with the RX113 is combine these two approaches. The simpler self-capacitance method is used until a multi-touch gesture is detected, and then the RX113 switches to a mutual-capacitance mode. They also managed to increase sensitivity and noise immunity, with better resilience in the presence of water droplets.
As well as advanced touchscreen technology, the RX133 also has a standby mode that allows the device to operate just the real-time clock (RTC) and a liquid-crystal display (LCD). They can be used together to provide a blinking, attention-getting display while minimizing power usage.
If you would like to learn about these advances, then I recommend this article: Capacitive Touch Sensor System (Second generation touch key technology). If you would like to experiment with some hardware, then I recommend the Renesas Starter Kit for RX113. It comes with the e2studio, documentation, and a tutorial to get you started quickly.