Tablets and smart phones have a number of common elements, but the most prominent is the touchscreen display. Apple’s iPhone and iPad made multitouch the first must-have feature and revolutionized the market. No self-respecting tablet, smart phone, or (soon) e-reader will lack multitouch support. The most responsive and flexible multitouch interface is capacitive, but the feature-rich approach is more costly than alternatives like resistive touch interfaces.
Cypress Semiconductor’s TrueTouch Gen 4 significantly improves multitouch interface performance while reducing the cost and complexity of overall multitouch display (see the figure). It allows direct lamination of the interface layers above the display (see “Direct Lamination Multitouch Technology Delivers Top Notch SNR” at www.electronicdesign.com).
Combine TrueTouch Gen 4 with the single-layer support that Cypress Semiconductor also developed (see “Single-Layer Cap Touch Targets Low-Cost Resistive Competition” at www.electronicdesign.com), and the result is a less expensive, more responsive, multitouch interface than previously available.
Flattening The Touch Interface
The TrueTouch Gen 4 family of multitouch controller chips is built around a 32-bit ARM Cortex-M0 processing core. The core not only provides better touch sensing support, it also enables the implementation of more sophisticated gesture algorithms in firmware.
The chips employ self and mutual capacitance that enables them to deliver 0.2-mm accuracy and linearity as well as 1-mm stylus support. They additionally handle hover mode detection in addition to direct contact sensing.
TrueTouch Gen 4 also includes Cypress Semiconductor’s Display Armor technology, which eliminates display noise using two different techniques. The first method tests for noise in the analog front end and then subtracts it from the input. The approach delivers 1-V p-p directly coupled noise immunity.
The second method targets noise above those levels. In this case, the chips check for the horizontal and vertical synch signals that are considerably noisier. The system then gets a lock on the signals so it can avoid them when taking measurements effectively. This allows the processor to get its information during the quiet times. It also permits the system to operate more quickly with a 400-Hz refresh rate and 1-kHz panel scan rate.
Also, TrueTouch Gen 4 incorporates a built-in 10-V Tx driver so designers can eliminate air gaps and shield layers. Combined with the other technologies, this allows direct lamination and on-cell and also use with in-cell applications. Direct lamination significantly reduces touchscreen display costs. It makes the overall system thinner while being significantly more accurate as well.
Putting layers closer together does have advantages. But there are disadvantages as well, because it brings the noise sources closer together. This is why Display Armor is so important. It is all part of the package.
Furthermore, TrueTouch Gen 4 supports Cypress Semiconductor’s Charger Armor, which eliminates noise related to the battery charger when the device is plugged into a charger. Charger Armor provides 95-V p-p noise immunity. The chips handle Cypress Semiconductor’s earlier CapSense button support too.
Cypress Semiconductor’s TrueTouch Host emulator allows designers to configure and tune a system in minutes. It uses the chip’s I2C/SPI interface for real-time control and configuration. The software can enable individual features so designers can experiment with a system’s design. Built-in manufacturing test support works with the Cypress Manufacturing Test Kit (MTK). It supports low-cost sensor patterns.
Right now, the family addresses displays up to 5 in. (diagonal). Larger screens will be addressed in the future. The approach can even meet IP-67 waterproofing support. The first chips consume only 2 mW and only 1.8 µW in deep-sleep mode. They will be available in chip-scale packaging (CSP) as small as 3.8 by 3.8 mm.
Even small cost changes for tablets and smart phones can impact profitability. Likewise, the performance of the user multitouch interface is critical to a good user experience. TrueTouch Gen 4 delivers across the board with faster response time, lower power, better signal-to-noise (SNR) ratios, and lower costs. The technology will have a major impact in the mobile arena