Mobile devices seemingly all have touch screen displays and often numerous other sensors, too. Understanding the underlying technologies can help you evaluate the inevitable trade-offs in these designs.
Modern gaming, medical, and mobile devices are dominated by centralized displays featuring touch sensing for the primary user interface. However, most of these products are systems that can benefit from multiple types of sensors for determining position, motion and other environmental conditions. The use model and response profile dictate the type of touch interface that can be used. The types of sensors available, their development and application support, product implementation issues, and graded availability will be addressed.
Touch sensors
There are a number of touch sensor interfaces and technologies which are available. Several are targeted to large-area applications or require line-cord-style power to execute. The mobile space is the new “volume” area for these interfaces. Guidelines for the mobile space include displays under 17 inches, battery operation, power-down modes, millisecond response times, and a single user at a time on the device. These requirements narrow the mainstream touch technologies to three types: resistive, projected capacitive and surface capacitive. An overview of the three technologies is shown in Table 1.
Resistive
Projected Cap
Surface Cap
Bare Finger
Yes
Yes
Yes
Glove or Stylus
Yes
Yes
No
Sealable Frame
Yes
Yes
No
Durability
Fair
Excellent
Good
Stability
Excellent
Excellent
Good
Light Transmission
75%-88%
<90%
88%-93%
Multi-Touch Capable
No
Yes
Yes
Table 1: Types of touch sensors.
Resistive touch is one of the oldest and simplest technologies. The touch screen is based on a flexible conductive layer (generally coated with Indium-Tin-Oxide [ITO]) that is spaced above another ITO conductive layer. When the top layer is flexed or depressed, it makes contact with the bottom layer and then creates a localized voltage change. The location of the voltage change is sensed, and that determines the position of the touch, as shown in Figure 1. This is a high accuracy technology, as the flex of the top layer can be from a large area object such as a gloved finger, or a very small area such as the end of a stylus. The technology also allows for weather and dust/foreign material resistance by allowing the edges of the top layer to be sealed. Digi-Key carries resistive touch control ICs from ten suppliers - Analog Devices, Inc., Atmel, Integrated Device Technology (IDT), Maxim, Microchip Technology, National Semiconductor, ROHM Semiconductor, Semtech, STMicroelectronics, and Texas Instruments (TI). The resistive touch ICs for the display systems, including those used by cell phones, are all supported by development systems featuring access to all stages of the data pipeline. Development systems such as the EVAL-AD7877EBZ-ND from Analog Devices, Inc. include software, cables, a touch sensor, control electronics, and a display. These systems allow for the application development and real-time test of the sensors. The high speed of resistive touch systems, in addition to the ability to have sealed displays, continued operation when moisture or fluid is on the screen, and a firm non-accidental touch, makes resistive touch the preferred technology for industrial control and medical electronics.
Figure 1: Resistive touch panel. (Courtesy of Planar Systems).
Projected capacitive touch is another technology that can deal with weather, dust, and a gloved touch. The technology works by using a sensor grid that is laminated between two layers of protective glass. The sensor grid is charged and projects an electric field beyond the surface of the top glass. The sensor circuitry detects changes in the field from touch objects on the front of the glass without physical touch of the sensor grid (see Figure 2). This technology can use vandal-proof materials and also supports the use of a stylus. Although this technology is robust like resistive, it is not as fast. It does support multi-touch and gesturing recognition. Since there is no power drop in the projected capacitive touch sensor grid as in resistive touch, this technology can operate in the 1.7 to 2.5 V range versus the 3.3 to 5 V range for resistive touch. The suppliers of application and development kits for these devices are highly overlapped with resistive touch and include Analog Devices, Inc., Atmel, Cypress Semiconductor, Freescale Semiconductor, Inc., Integrated Device Technology, Maxim, Parallax, Inc., ROHM Semiconductor, and STMicroelectronics. The architecture of a projected capacitive control IC is shown in Figure 3, along with an associated development kit (Part # DM160211) from Microchip Technology.
Figure 2: Construction of a projected capacitive touch (PCT) panel. (Courtesy of Zytronic Corporation).
Figure 3: Architecture of a projected capacitive touch screen sensing technology, the mTouchTM from Microchip Technology, along with its associated development kit. (Courtesy of Microchip Technology).
Surface capacitive is the “lightest touch” technology, in that it requires the least amount of force to register a “touch.” This technology is the only one of the three that is on the exterior surface of the display, and as a result, has the highest level of transparency and brightness, with the trade-off being the most susceptible to mechanical damage. The technology works by having the outer glass of the display coated with a conductive layer (typically ITO), as shown in Figure 4. Electrodes are placed around the panel’s edges and a low voltage is applied across the conductive layer making a uniform electric field. A touch on the panel draws current from each corner of the display. The control IC then calculates the position of the touch by the current flow. This technology supports multi-touch and gesturing, but needs the capacitive differential of a bare finger touch, and does not work with objects like a stylus or a gloved hand. The technology, due to the edge conductors, does not allow for a sealed display, so it is not functional in the context of liquids and high contaminants. These last two constraints – gloves and liquids – are the primary reason that the technology is not applicable for medical applications. Development systems such as the STM8/128-EV/TS from STMicroelectronics include software, cables, power supplies, a touch sensor, control electronics, and a display. These systems allow for application development including resolution, gesturing, and multi-touch capabilities.
Figure 4: Surface capacitive touch panel. (Courtesy of Planar Systems).
Additional sensors
In the mobile space, new appliances have opened up the user interface (UI) to controls other than those that appear on the screen. For gaming, cell phones, and custom scanning devices, orientation and movement are key parts of the software control. Figure 5 shows a modern cell phone and the MEMS and sensors that are part of the system. Typical positioning sensors are gyroscopes, accelerometers, magnetometers, digital compasses, and dead reckoning modules. For medical, industrial control, and home automation applications, the touch display systems may include pressure sensors, temperature sensors, and color and optic sensors. These sensor categories encompass imaging sensors, dust and humidity sensors, gas sensors, shock and strain sensors, and vibration/energy harvesting sensors.
Figure 5: Cell phone system with MEMS and sensors. (Courtesy of Yole Development).
Traditionally, when designs are created using these types of sensors, the only interface is with the microcontroller and microprocessor in the system. The interface available for the sensor usually decides the interfaces that are available on the core processor. With a display in the system, there is a trade-off. The touch interface on the display is the effective “main console” for the system; it is interrupt-driven and uses the highest priority channels into the processor. The touch interfaces from the control chips are available as JTAG, SPI, and standard serial port interfaces and are usually the higher-speed data paths.
Figure 6: Cypress CY3280 touch sensor development kit. (Courtesy of Cypress Semiconductor).
When the additional sensors are included in the system, the interface that is compatible with the “console” interface must be used. Timing, data rate, and data transfer scheduling for the other devices must all be taken into account and verified for data bus compliance and for software application compatibility. Normally, the system firmware operates as the autonomous console and control, so the additional sensors are the only feedback in the system. With the inclusion of the touch interface, the interface path now has to be operational with multiple devices on it. Development systems, like those available from Cypress Semiconductor and shown in Figure 6, allow for application code development as well as integration with other sensors and systems. The development systems can support multiple microcontroller options.
System trade-offs
The additional sensors are available in several forms – bare sensors, sensors with controls, and full sensor subsystems. The lowest component cost is the bare sensor (see Figure 7). The bare sensor has an analog output that will have to be data converted, and then processed and interpreted by the central processor. The bare sensor component is a continuous time element and processor-based systems require sampled data discrete time interfaces.
The sensor with control system allows for control electronics that produce a digital output for the system and handle the calibration of the sensor. This setup eliminates the continuous time aspect of the sensor, and allows the control system to operate in either asynchronous or synchronous mode. Data size and resolution of these sensor electronics should be on the same order as the touch electronics. Typically, these standalone sensors have signal processing ICs that produce from 8-bit to 24-bit results. If these results are available at a millisecond data rate, similar to the touch control, then scheduling software and local buffers are needed to cache the multiple types of data so they get to the core processor in an orderly manner. The sensors with control system tend to have simple output register banks on the data converter logic, and hold calibration data for the device. This means that successive cycles of the sensor require a clearing of the local data at the sensor and a shift to the digital core for processing.
Figure 7: An example of a pre-housed uncompensated pressure chip. (Courtesy of TDK EPC Sensor Business Group).
The final form of these sensors is the full module. At the module level, you get the sensor, the control electronics, and local buffer memory. This format is the easiest to integrate with the touch sensor systems as there is non-core memory available to resolve timing issues. The full modules, however have the highest component cost. This cost may be offset by improved system performance and reduced development time for the project.
Timing concerns
Some of the trade-offs for these multi-sensor systems, like a cell phone or tablet, are related to the timing of the sensor data with respect to the application program being run. The device operating system has a lot to do with how the data gets there, and how many of those details the application has to deal with. The operating systems in communication and computing devices tend to avoid any asynchronous signal paths from the sensors. Rather, they are generated on a continuous time basis, quantized, and then sent to a memory/register for transfer in a synchronous fashion on the master data bus. Some of these signals also generate a priority interrupt, indicating position or activity change by the system. Temperature and other environmental sensors may also create interrupts in addition to data. These signals then are processed by the system to determine if the device should be shut down for protection. For systems with many sensors, the current synchronous solution is to use a polled basis for all the systems, and in the polling sequence, the touch UI may be called on multiple times.
Incompatible placement of some of the other sensors when touch systems are used can be a major issue. Magnetometers and electronic compasses have some conflicts with the generated electric field for the surface and projected capacitive touch systems, as well as with placement near power drivers for LCD backlights. Other sensors that have placement concerns are temperature sensors that need to be placed away from the drive electronics for surface capacitive and resistive displays. Those driver electronics tend to be hotter than other parts of the display, and create a false profile for the temperature sensors.
The whole point of the touch interface is to allow for a simplified GUI that facilitates rapid selection of new modes or for embedded modes for functions. Project systems can be fully prototyped allowing for the application and OS software to be run on the multi-developer kit environment. One of the key functions for these development tools is to help reduce the latency of the touch system in the application.
For cost reduction purposes, most of the systems are developed using touch screens and module level MEMS or sensors. After the initial configurations are developed, the module systems can be backed off to the lower-cost sensor/control logic design if there is timing margin, when the data is loaded to central memory, and if the applications still operate. This makes the availability of individual unit devices and development kits a key portion of the design flow.
Prototype creation
Touch and multi-sensor systems have a challenge for the final form, fit, and function of prototypes over a standard system. Things like the selection of a standard project box cannot be used. Either the plastic is too thick and it disrupts the edge drive of the active capacitive touch system, or it is a metal box which blocks the signal for a magnetometer, compass, or dead reckoning system.
Air flow is critical for these systems as the display function is generally “on” and creating a constant electric field and heat. As a result, placement of pressure sensors where they will not pick up venting airflow is vital. The final prototypes, as shown in Figure 8, tend to be multiple stacked PCBs with the display on top and the set of connectors on the edges of boards found below the touch screen.
Figure 8: Prototype touch system. (Courtesy of the University of Electro-Communications Tokyo).
Summary
Touch screen systems are now a dominant form factor for mobile devices in communication, computing, data visualization, industrial control, and medical applications. The selection of the touch technology is dependent on the environmental and use functions for the end product. These systems are usually found with other sensors. The touch technology is a large active surface area device, and requires the product undergo design trade-offs at the architectural level to allow for high-reliability operation compatible with other sensors. The use of multi-vendor development and application kits for the displays and sensors is critical to the realization of a functional design with functional application code.
