Design and development of capacitive touch screen

Response Time: Response time is defined as the time between a finger touch on the touch screen and the interrupt signal generated by the touch screen controller. It is measured by electronically stimulating a finger touch screen environment or moving a simulated finger across the panel. Response time is particularly important because it directly affects how fast a user can move a finger across the screen; pan or flick; or write on the screen with a finger or pen. Touch panels with slow response times will experience brief pauses and no movement detected. The response time of a touch screen is part of the system response time, which includes:

X/Y Axis Scan: The time it takes for the touch controller to scan and measure the change in capacitance on the sensor.

Finger detection: Compare the panel capacitance change with the pre-defined finger default value. If the change exceeds the finger default value, a finger touch is detected.

Finger position: The actual position of the finger is determined based on the result data obtained from multiple sensors.

Finger Tracking: When multiple fingers are placed on the sensor, each finger must be correctly identified and assigned a unique identification symbol.

Interrupt latency: refers to the delay between the interrupt indication and the service on the host. In most systems, this delay will not exceed 100 microseconds.

Communication: Generally, the system uses I2C at 400kHz or SPI at 1MHZ to communicate with the host.

市面上有许多工具能用来缩短响应时间，关键在于触控芯片的智能，比如较有创意的方法仅需扫描部分屏幕，即可侦测到手指位置，当侦测到手指后，就能快速扫描，计算出手指实际的定位，藉此节省耗电与时间。另一个重要工具是并行处理，使用不同的硬件组件进行扫描、手指处理及通信，使这些工作同步进行。采用高度优化的算法进行手指侦测、手指定位及手指识别码(ID)，能够缩短处理与响应时间。

Frame refresh rate: The time between two adjacent frames of touchscreen data in a data buffer when a finger appears on the touchscreen. Low frame refresh rates can cause the system to pause in detecting movement, and the detected movement path will become discontinuous line segments instead of smooth curves. In other words, if the touch panel has a high frame refresh rate, it can provide more data points that can be translated into smooth or complete shapes or movement trajectories. In addition, a high frame refresh rate can also improve the interpretation of gestures. Intelligent touchscreen controllers such as TrueTouch can adjust their frame refresh rate to match system requirements. Hand-drawn or handwritten applications require a very high frame refresh rate, but a mobile phone dial keypad only needs to intercept the host terminal when the user presses or releases a button.

Average power consumption: refers to the average power consumption of the touch system, including the time scanning, processing, communication, sleep, etc. when the controller IC is working, as well as the time the main processor receives and interprets touch data.

Power consumption is a common performance parameter: the current consumed by the measuring device is multiplied by the voltage, and the power consumption can be inferred. When it comes to power consumption of touch panels, more sophisticated calculation formulas are required, because different usage modes will produce different power consumption. The standby time of a mobile phone depends on the current consumed by the touch screen in standby or sleep mode.

When the touch screen is working, it is also divided into many modes, such as wake-up on touch (WOT) and cheek detection (Cheek Detect). For example, when answering a 5-minute call, while checking or entering a phone number, the phone may switch to touch mode for 10 seconds, and then switch to WOT or cheek detection mode when reminding the call. Even when sending text messages (SMS), it is still a mixture of WOT mode and actual finger contact. When typing or thinking, the controller IC will switch between various sleep modes.

If you don't consider these power modes, it's easy to be misled by the system's power consumption. In most cases, the touch screen switches to cheek detection mode and touch wake-up mode 90-99% of the time. Some systems allow users to set the ratio of processing time to sleep mode, even when the finger is still on the panel. If the system only detects the finger in the same position, it doesn't need a 200MHz screen refresh rate. To develop a high-performance touch screen, you must use a low-power system in sleep mode and work with innovative sleep and wake-up modes.

There are many other important factors that system developers need to consider when designing a capacitive touchscreen system:

Finger capacitance: refers to the capacitance measured between a finger and a single sensor component. When measuring finger capacitance, a real finger is used instead of a metal mechanical finger to ensure that the measured data is consistent with the actual situation. Factors that affect the feedback capacitance (CF) include the thickness of the lens covering the upper layer and the dielectric constant of the outer covering material.

System Noise Floor: System noise floor is the noise measured at the output of the capacitor-to-digital converter and is the input (capacitance) value of the data converter.

Signal-to-Noise Ratio: The signal-to-noise ratio (SNR) is the ratio of the finger signal measured by the sensor to the measurement noise. This is an important parameter that designers must understand in order to develop an efficient touch panel. The system must be able to adjust, adapt, and filter out parasitic noise in the mobile system. To achieve high signal numbers and very low noise numbers, consider using accurate analog front-end components for touch functions.

诸如TrueTouch系列可编程解决方案这类产品，可在滤除噪声方面提供许多绝佳的机制。PSoC可编程模拟组件能重新组态，以整合持续一段时间的信号，藉此滤除噪声。不同的信号频率，包括扩频与虚拟随机频率，亦可用来避免电磁干扰。标准的数字滤波器能移除1～2位的信号抖动或提供类似IIR的低通滤波器。智能数字滤波器能比对附近区域侦测到的样本，滤除不正常的样本，智能滤波器仅受限于系统设计人员的创意。图3显示一个组件的噪声水平范例，及侦测到的触控行为。在这个例子中，撷取到的SNR为5。

Figure 3. Signal-to-noise ratio (SNR) example.

Understanding and mastering the key touch screen performance parameters can significantly improve touch screen design. Understanding these criteria also helps in choosing the ideal design partner who has the right technology to properly address the noise and electrical issues of mobile consumer products.

The appeal of touch screens lies in their deceptively simple design. Replacing bulky buttons, trackballs, or traditional screens, touch screens offer a whole new mode of operation, creating a user experience that is enjoyable. The difficulty in designing a touch screen is that in order to provide a beautiful and simple design, sophisticated hardware, firmware, and manufacturing techniques must be used. Understanding the design essentials, key performance parameters, and trade-offs of touch screen design is the first step to developing a first-class touch screen product.