In Part 1 of this series on touch sensing, we examined the various types of touch sensor technology. Here in Part 2, we will explore the elements that go into the creation of a touch sensing system and consider some controllers and development kits that empower engineers to create streamlined, attractive, and reliable touch-based product designs.
When creating touch displays a variety of components have to mesh including sensors, tin-oxide film, and other transparent conductor materials, touch controller ICs, cover lens glass, LCD and OLED display modules, and more.
Technical advances can be seen in overlay thickness, interconnects, projected capacitance, and form factor. Today’s capacitive sensor designs require an overlay of 3 mm or less, adding heightened accuracy compared with previously thicker solutions.
Touch has been integrated into LCD modules, too. Indium-tin oxide (ITO), the conductor used in touch displays, allows the capacitive sensor to be up to 90 percent transparent for one-layer designs. Projected capacitive touch (PCT) is now more attractive based on the accuracy and flexibility gained by etching the conductive layer, replacing earlier sensor-in-pixel solutions.
In touch and LCD integration the “cell” is the area between the color filter glass and TFT glass. When the touch transmitter and receiver are located below the top of the color filter glass, the integrated LCD/touch system is referred to as an “in cell solution.” This integration can eliminate redundant structures creating a slimmer display. "Glass-on-glass" touchscreen technology is also known as "on cell," meaning both touch transmitter and receiver are on top of the color filter glass.
Controllers and demo boards
Engineers can find a number of good evaluation and demo boards and kits available to help implement touch technology into their designs, including the Analog Devices Evaluation Board for the AD7147 CapTouch Controller, which allows users to evaluate all features of the AD7147 or AD7147-1. It contains the board, USB cable, evaluation software, EVAL-AD7147 datasheet, and utility program for software serial downloads. The applications are the evaluation of the AD7147 and evaluation of capacitance sensors.
The ADI7147 capacitance-to-digital converter increases performance for touch interfaces and dramatically improves sensor response. It provides the ability to locate the sensor remotely from an IC, shielding noise in consumer device applications. It is also appropriate for industrial-grade harsh environments.
The company’s AD7148 (Figure 1) is a programmable touch controller for single electrode capacitance sensors. It is used with capacitance sensors implementing such functions as buttons, scroll bars and wheels. The sensors in this case need only one PCB layer and the result is achieving an ultrathin form factor. Applications are cell phones, personal music and multimedia players, smart handheld devices, TV, A/V and remote controls, digital still cameras, and gaming consoles.
Figure 1: The AD7148 by Analog Devices is a programmable touch controller for single electrode capacitance sensors.
The device is designed for single electrode capacitance sensors. An active shield output minimizes noise pickup in the sensor. It has on-chip calibration logic that compensates for changes in the ambient environment. The calibration performs automatically at intervals as long as the sensors remain untouched. This ensures that there are no false or non-registering touches on the external sensors.
Freescale’s proximity capacitive touch sensor controller, the MPR03X is an integrated Circuit Communication (I2C) driven capacitive touch sensor controller that is optimized to manage two electrodes with interrupt functionality, or three with the interrupt disabled. Typical applications include PC peripherals, MP3 players, remote controls, mobile phones and lighting controls.
Features of the touch sensor controller include:
- 6 μA supply current with two electrodes monitored with 32 ms response time and IRQ enabled
- Compact 2 x 2 x 0.65 mm, 8-lead μDFN package
- Supports up to three touch pads
- One external component required
- Intelligent touch detection
- 4 μA maximum shutdown current
- 1.71 to 2.75 V operation
- Threshold based detection with hysteresis
Figure 2: IDT LDS6120 block diagram.
Applications include mobile handsets and smartphones, MP3/MP4, gaming devices, remote controls, TV and audio/video, set-top boxes, multi-function printers, enterprise telephony, white goods, industrial and medical devices. On-chip calibration logic monitors the environment and adjusts touch sensitivity. Noise-filtering algorithms may be activated to prevent sensor activation in noisy environments.
The Audio Capacitive Touch BoosterPack (ACTBP) plug-in board for the LaunchPad Development Kit (Figure 3) offers a complete reference design for capacitive touch solutions using the capacitive touch I/O ports of select Texas Instruments MSP430 microcontrollers. This MCU series features built-in 16-bit timers, up to 24 I/O touch-sense enabled pins, a versatile analog comparator, and built-in communication capability.
Figure 3: The MSP430 microcontroller large and small version chips feature up to 24 I/0 touch-sense-enabled pins.
Typical applications for the MSP430 include low-cost sensor systems that capture analog signals and convert them to digital values, processing the data for display, or transmission to a host system.
In summary, no longer are touch technologies too expensive to consider for a variety of applications. Resistive touch displays and their durability will be designed into increasing numbers of appliances, and the availability of capacitive receptors will drive down cost as well. As we have also shown, a variety of dev kits and reference designs are available to help simplify integrating touch sensing into your next design.
For more information on the parts and kits mentioned here (and in Part 1) use the links provided to access product information pages on the Digi-Key website.