New technology makes it easier to develop touchscreen applications

Despite growing demand, creating touchscreens for industrial applications has remained a challenge. However, a recent development is looking to change that. John Carey, director of marketing for Cypress Semiconductor's True Touch division, claimed: "Until now, tuning sensor panels has been something akin to black magic. We're releasing technology that will make designing touchscreens a simple task." The recent development is the launch by Cypress of the True Touch generation 4 family of touchscreen controllers. Carey outlined the evolution. "The True Touch range started with basic buttons and sliders, but the world has changed. Customers are now interested in noise performance and, as we talk with potential customers, we find that noise related performance is important to them." The displays used in smartphones vary in quality. At the good end of the scale, the tfts might generate noise at 500mVp-p. But at the bad end, where customers might be buying on a price basis, rather than on quality, noise can reach 5Vp-p. "That's catastrophic for a touchscreen," Carey ventured. There are, of course, physical solutions to the noise problem. Two main routes are available. Carey said: "You can add an air gap between the sensor and the display. A gap of 0.3mm is usually enough to get rid of most of the noise. The other approach is to add a shielding layer, which may cost $1." If you're building a smartphone, size is everything and even a 0.1mm air gap is perceived as 'big'. "Adding 0.5mm to the depth of a phone, plus maybe adding $1 to the bill of materials is something regarded by phone developers as 'painful'," Carey continued. "The idea of True Touch gen 4 is to allow developers to work with low cost displays, but without having to add the shielding layer or an air gap." One of the ways in which True Touch gen 4 has been targeted at this particular application is by designing it to deliver what Cypress claims is the 'best signal to noise ratio (snr)' amongst such devices. "It's the first touchscreen chip that is capable of driving a display at 10V," said Carey. "The voltage at which you drive the panel is directly proportional to the signal you get back. By driving a panel at 10V, gen 4 has a performance nearly four times better than the closest competitor. "Touchscreens often require a 1.8V digital supply and a 2.7V analogue supply. By driving at 10V, there's a snr boost of almost four." Alongside the snr boost, Cypress says it has eliminated display noise completely through a patent pending feature which it calls Display Armor. According to the company, this offers 'unprecedented' immunity to noise from every type of display. It is Display Armor that supports thinner displays by removing the need for the air gap and shielding layer. The approach is also 'display agnostic', working with direct lamination, on call and in cell stack ups, regardless of the display chemistry. Carey explained more. "It's all done in hardware. We've designed a mechanism that can listen for display noise and which can then do two things. It can take the noise away, which is the approach used when noise is less than 1Vp-p, or it can detect the signal and sync it to horizontal and vertical noise. The latter approach allows gen 4 to work with displays that emit up to 5Vp-p." Essentially, Display Armor is an analogue front end (AFE) which Carey says has been completely redesigned since the last release. "It's a narrow band receiver," he continued, "into which only so much noise can be coupled. The AFE also has high gain." Cypress also claims to be competing on the power consumption front. Carey said most competitive devices drew around 4mW. "Gen 4 has an active power consumption of 2mW." Similarly, he pointed to better accuracy and linearity. "While other devices might support accuracy and linearity of 0.4 to 0.5mm, gen 4 offers a figure of 0.2mm for both. The combination of this with noise immunity will allow application developers to deliver better performing products which feature cheaper displays." Supporting a range of inputs Because capacitive touchscreen technology can be targeted at a range of applications, the controller needs to be able to work with a variety of inputs – from a stylus in a smartphone to a gloved finger in an industrial application. Gen 4 accommodates these varieties through the ability to switch dynamically between self capacitive and mutual capacitive sensing. "Self capacitive sensing measures the capacitance of lines," said Carey. "When you just measure lines, they project the field upwards and you get a lot of signal. But this approach is limited to sensing the touch of one finger, or two with ghosting. "Mutual capacitive scanning, however, measures the interaction between two parallel lines. While you get a smaller signal, this approach supports true multitouch." A further benefit comes with the use of differential signal analysis. With this, Carey claimed, capacitive touchscreens can be used in products sealed to IP67 standards. "This can also support the use of a 1mm stylus and detecting a 'hover', where the finger can be up to 1cm above the sensor." A further benefit for industrial applications is the ability for the chip to deal with ac noise of up to 95Vp-p, a feature designed to cope with low quality chargers for smartphones. True Touch gen 4 has been designed around an ARM Cortex-M0 core and supports screen refresh rates of 400Hz. The chip can also support a 'look for touch' mode which scans at 1kHz. Initially, five devices will be available in the family, handling from 31 to 40 sense I/Os with a 5mm pitch. These will support touchscreens ranging from 4.5 to 11.2in. Further devices will be introduced in the near future. Carey believes the launch of gen 4 will bring capacitive touchscreens to a wider range of applications. "Because of the noise issues, touchscreen design has required a lot of 'handholding'. This chip is robust enough to go into tough applications." And, to overcome problems with tuning the touchscreen to the application, Cypress has developed TrueTouch Host Emulator software. "It's a 'point and click' application. Just press the button and the software will tune the panel. This feature will make designing touchscreen applications simple," he concluded.