Eliminating Capacitive Touchscreen Design Barriers with Gen4

Designing a capacitive touchscreen into a product is not a simple task. This complex electromechanical system faces many tough integration challenges. End users want their products to be thin and light, and the market demands long battery life. The expectations for touch performance are quite staggering: 4mm finger, palm rejection, 1mm stylus, and hover. The system needs to work with low-cost universal USB battery chargers, reject noise from the growing number of wireless noise sources, and ignore the effects of water. Also note that it must be cheap. Solving these problems is not easy. Fortunately, Cypress has introduced Gen4 (Figure 1).

speed

Gen4 was designed from the ground up to address each of these design issues. The first challenge is performance. While some touchscreens on the market claim up to 250Hz, this is not possible in real-world applications where noise is present. This is because the 8-bit MCU cores in these devices are underpowered and do not have the math required to implement the advanced filtering algorithms required in today's environments. Once filtering is complete, 8-bit math needs to be performed like a DSP to achieve accurate position measurement without sacrificing system performance. Actual measurements have shown that in real-world environments, the touchscreen controller will slow down to 60Hz. This results in poor performance during fast taps, especially stylus tracking. Gen4 is designed to address these issues.

Figure 1: Gen4 touch screen system principle

The Gen4 platform features an on-chip ARM M0 core, which is the world's best processor in terms of size, speed and power consumption. The fully pipelined scan engine and true 32-bit processing power enable Gen4 to overcome the most demanding environments. It has built-in DSP functions and a touch screen subsystem that can scan the panel at a frequency of 1kHz, and it can also make the next panel scan independent of the previous data set processed by the CPU. All these capabilities add up to a refresh rate of up to 400Hz in mobile phone applications. This means that high-performance touch can cope with various noisy environments and keep up with future operating systems.

Power consumption

Another advantage of running fast is the ability to save power. By scanning and processing quickly, Gen4 can go to sleep between scans. In mobile device applications, saving battery life is critical. Active power consumption is very important and can be as low as 1.9mW. More important than active power consumption is standby power consumption because the display consumes more power. Gen4 has a unique ability to enter sleep mode and consume only 1.8uW. What is extremely unique is that in this mode, it can be awakened by an address match on the IIC or SPI port. This is critical because Gen4 can wake up faster than other devices (other devices require a pin change interrupt and must be started to a ready state to process touch data). This helps designers provide users with the most responsive user experience.

Solving the Charger Noise Problem

Charger noise is one of the most talked-about noise sources in capacitive touchscreen designs. This noise is physically coupled to the sensor through the battery charger when a touch occurs. It can manifest as a decrease in touch accuracy or linearity, false or phantom touches, or even the touchscreen becoming ineffective or incorrect. The culprit is typically a low-cost aftermarket charger (Figure 2).

Figure 2: Architecture of low-cost charger and its drawbacks

Some oscillating coil inverter chargers can be considered broadband noise generators, as they produce as much as 40Vpp of noise ranging from 1kHz to nearly 100kHz. Most also end up having a tendency to be more periodic noise with many harmonics.

OEM chargers designed to work with specific phones have tighter noise specifications, and the widespread adoption of USB connectors in charging circuits has created a huge opportunity for the accessory market. To compete, accessory manufacturers have substantially reduced the cost of chargers. A charger with low-cost electronics that charges a phone will inject so much noise into the touchscreen that it will most likely become unusable.

Therefore, OEMs are demanding higher levels of noise handling capabilities from touchscreen ICs. Many specifications require 40Vpp from 1kHz to 400kHz and 95Vpp rejection from 50 to 60Hz. Fortunately, there are specialized algorithms and methods on the market (e.g., Charger Armor from Cypress) that can meet stringent requirements and provide battery charger noise immunity of more than 400Vpp. This level is achieved by various means, whether through nonlinear filtering, frequency hopping, or other hardware methods. Gen4 can achieve it all.

Achieving noise immunity that meets the specifications required by today's mobile industry is not trivial. Touchscreens typically address this problem using processor filtering. The old saying "If the roots are not straight, the seedlings will be crooked, and the dyeing house cannot produce white cloth" is not 100% accurate, but it is still critical to use a clean signal at the beginning. One of the best ways to overcome charger noise, especially those with output broadband noise, is to overcome it with a clean signal. Because the unprocessed signal-to-noise ratio generated from the analog side of the touch controller is proportional to the voltage of the device driving the panel (SNR∝VTx), it is desirable to use a high voltage Tx.

A typical touch screen controller is driven by a 2.7V rail connected to the panel, but the Gen4 family is a little different. It also uses a 2.7V analog supply, but actually drives the panel at 10V. The charge pump and 10V transistors are integrated into the Gen4 device family, which enables them to achieve nearly 4 times the raw signal-to-noise ratio of any other chip.

Once the signal is acquired, typical techniques such as median filtering or other more advanced nonlinear filtering can be used to further improve the signal-to-noise ratio, but this will sacrifice the refresh rate. The 10V Tx has been shown to be able to handle up to 31Vpp of in-band noise from the charger output without the need for this advanced type of filtering.

But what happens when the in-band noise is too high? This is where other advanced technologies come in handy. If the in-band noise that a device needs to deal with becomes too high, the unique processing capabilities of Gen4 come in handy: it dynamically adjusts its transmit frequency and switches channels to avoid the noise. Adaptive frequency hopping technology is another key technology in touch screens to solve the charger noise problem.

Solving monitor noise issues

Displays present many challenges for projected capacitive touchscreen systems. This is because they generate considerable noise, which can be conducted directly to the capacitive touchscreen sensor. Making matters even more difficult is that OEMs are demanding thinner phone models, which means moving the actual touchscreen sensor closer to the display, or even inside it.

For years, the industry has used shielding layers to protect sensors from display noise. This adds cost and thickness to any phone, but is effective. The industry has also used a small air gap, typically about 0.3 mm thick, between the display and the sensor, using the natural air to eliminate conducted noise from the display. However, as phones get thinner, neither option is ideal for today's designs.

In traditional TFT LCDs, the common electrode (VCOM) is driven by a DC or AC voltage. The ACVCOM layer is often used to reduce the operating voltage of the display driver while keeping the liquid crystal voltage constant. This is a relatively low-cost display, but this type of display consumes more power and has more noise than a DCVCOM display. Let's take a quick look at the typical waveform of an ACVCOM display (Figure 3).

Figure 3: Typical noise waveform of ACVCOM display

A typical ACVCOM type display will have noise of anywhere from 500mVpp to 3mVpp centered at 10 to 30kHz (as shown in Figure 3), while the DCVCOM noise will be smaller. It is very simple to measure the noise of the display. You can add a small copper tape on the top of the display, connect the oscilloscope to the small copper tape, connect the oscilloscope ground to the circuit ground of the display, and then run the display to capture the waveform. This level of noise is catastrophic for capacitive touch screen controllers, but it can be improved by air gaps or shielding. Air gaps make the phone thicker and cause mechanical design problems. Shielding layers also increase thickness and cause cost issues.

Fortunately, display noise can be easily mitigated using Gen4. Cypress' Display Armor technology is the industry's most advanced method for combating display noise. By integrating a built-in monitoring channel into the touchscreen device, Gen4 can eliminate display noise in two different ways. One method uses advanced algorithms to distinguish between noise and data; the other detects the source of noise and obtains the waveform so that the capacitance measurement is made when there is no noise. Both methods are completed in hardware and do not affect the normal execution of the CPU or touchscreen subsystem. The result is a low-cost, thin, advanced capacitive touchscreen stackup.

Accuracy

Touchscreen controllers must provide a precise user experience, and both accuracy and linearity are key. Accuracy is defined as how close the position reported by the touchscreen controller is to the actual center of the target; linearity measures how close the reported position is to the path of the target across the screen. Both are critical parameters and are very important around the edges. Gen4 can help designers improve accuracy and linearity to less than 0.2mm. It also supports a wide range of finger sizes: from small fingers as small as 4mm to large fingers as large as 30mm, they can all be accurately recognized.

Cost savings

The noise immunity of the Gen4 family allows designers to not only withstand harsh environmental requirements, but also save costs. How? When using Gen4, manufacturers can use low-cost battery chargers and low-cost displays. There is no need to use shielding layers in sensor designs, nor other expensive components to protect the touch screen IC from noise sources. Gen4 can even save costs on FPCs because it can use single-layer routing. This is thanks to its flexible IO design and the lack of external components. A typical design may have the Tx pins on either side of the device, and all the Rx pins are located on the top, as shown in the FPC schematic (Figure 4).

Figure 4: Gen4 single-layer wiring FPC.

Easy to use

Another challenge for designers is to constantly learn new tools and chips. This is because there are different controllers for different projects. Whether it is a project for feature phones, another project for smartphones, or the next project for their handwriting tablet design, trying to complete all these projects in a timetable and learning a lot of new knowledge will be extremely challenging. Gen4 can solve this problem because it is a platform with a single chip solution that can support the needs of all these markets. With 31, 35, 36, 40, 56, and 60 sensor IO options (screen sizes from 1.5" to 12"), designers no longer need to worry about new structures proposed for new products. In addition, designers can use the same development environment in all projects: TrueTouch Host Emulator (TTHE). It does not require any coding, and can fully configure and adjust the touch design with simple clicks, which can save designers design time.

Conclusion

Projected capacitive touchscreen controllers will continue to evolve. Performance will increase, and the market will continue to put pressure on system prices. Fortunately, we have found a way to solve this problem. Gen4 brings unparalleled noise immunity to the market, combined with revolutionary refresh rates and power consumption. Due to its level of integration, designers can achieve advanced performance and reduce the cost of system design at the same time. Adopting a Gen4 platform approach allows designers to save design time and cost, and provide an excellent user experience for the end customer.