What Makes Touchscreen Technology Tick?June 30, 2020 by Robin Mitchell
In the past 20 years, touchscreen technology—be it resistive or capacitive—has dominated interface markets. What makes them tick? One major factor is touchscreen controllers.
While the touchscreen has only been around for the past two decades, it has transformed how users interact with hardware. One of its main advantages is that it allows designers to create totally custom GUIs that would otherwise cost a fortune in custom hardware. A touchscreen, however, only needs to update firmware and can display new image options.
While a touchscreen itself can be expensive, it is a desirable technology because it can be integrated into almost any project, and once installed, it can accept changes to its operation. As a result, touchscreens have become a dominant interface, being implemented into smartphones, tablets, interfaces, cars, and even point-of-sale systems.
One example we can use to understand touchscreen technologies is Microchip's newly-released touchscreen controllers, the maXTouch portfolio.
MXT288UD touch controller family. Image (modified) used courtesy of Microchip
This portfolio is said to bring engineers the industry’s smallest automotive-grade touch screen controllers. How exactly does touchscreen technology work? And how does a touchscreen controller like the one from Microchip come into play?
Touchscreen technologies come in a wide variety of types, but the two most common fall in either the resistive or capacitive categories, according to Cypress Perform.
Resistive touchscreens use two layers separated by an air gap with each layer having rows of electrodes.
Visual of resistive touchscreen structure. Image used courtesy of Texas Instruments
The two rows are set perpendicular to each other, and when the screen is pushed in a particular location, an electrode from the top layer is pushed and makes contact with an electrode on the second layer. From there, a simple decoder can determine the position of the push.
The second technology type is capacitive touchscreens, which itself falls into two main types: surface and projection.
Surface capacitive touchscreens use a single insulative layer with a conductive material coated on its backside. Each of the corners of the screen is connected to sensors, and when a finger makes contact with the top-side of the insulative layer, it triggers a capacitance change on the screen. Each of the four sides takes capacitance measurements, and the combination of these four sensors provide a position reading.
The layers of resistive screens (left) and capacitive screens (right). Image used courtesy of Cypress Perform
In projection capacitive touchscreens, the insulative layer either has a conductive grid pattern etched into the backside or two separate layers, each having electrode rows placed on top of each other. When a conductive object (such as a finger), makes contact with the top insulating layer, the capacitance between the two conductive layers is altered, which can be used to determine the position of the touch.
However, projection capacitance has advantages over surface capacitance touchscreens since it is able to record multiple touches simultaneously. This allows for complex gestures such as zooming, panning, and swiping. For more information on capacitive touchscreen technology, check out Robert Keim's technical article on circuits and techniques for implementing capacitive touch sensing.
Resistive technologies are cheaper than capacitive systems, but because of their low resolution and requirement for hard on-screen pressure, capacitive systems are now the dominant technology.
The touchscreen itself is only half the solution. The importance of a touchscreen controller cannot be understated.
While users can more easily interact with resistive screens using a simple digital output, capacitive screens require a special charge/discharge cycle to determine the capacitance at each location on the screen. This is further complicated when the touchscreen incorporates multi-touch capabilities, and the controller needs to be able to identify these different touches.
Handling touch events can be a complex task for a processor, which is why many touchscreen controllers also implement keys. A key is a specific area on the screen that when pressed, the controller will send a message to the main system indicating that a “virtual button” has been pushed.
From there, instead of needing to map the current touch locations with button locations, a controller simply needs to ask if a specific area (i.e. a button) was pushed.
A Recent Example: the "Industry's Smallest" Automotive Touchscreen Controller
To continue the development of touchscreen systems, Microchip has announced its latest range of touchscreen controllers—the maXTouch range—to provide designers with the "industry’s smallest touchscreen controller for automotive applications."
The two devices, the MXT288UD-AM and the MXT144UD-AM, offer low power modes, weatherproof operation, and multi-touch detection, even with gloves on. This makes using touchscreens in applications such as vehicles, motorcycles, and e-bikes more practical since most of these applications often involve users wearing gloves.
The small package size of the range, 7 mm by 7 mm, means that less PCB space is needed (up to 75% less), which can either result in a reduction of the overall cost by reducing the PCB size, or an increase in complexity with the ability to integrate more components onto the same size board.
The mXT144UD-AMT/mXT144UD-AMB is 7 mm by 7 mm. Image used courtesy of Texas Instruments
The low power features of the maXTouch provide a current consumption of less than 50 uA while still remaining responsive for user input. The maXTouch also provides the ability to detect and track multi-finger interactions through a wide variety of materials, including leather, wood, uneven surfaces, and surfaces in the presence of moisture.
“Automotive OEMs are looking to enhance the user experience through smart surfaces and multi-function displays, while still providing a convenient and distraction-free environment," explains Fanie Duvenhage, VP of Microchip’s human-machine interface and touch function group.
"Addressing these needs in the market, Microchip is building on its already leading portfolio of automotive touchscreen solutions with the new MXT288UD touch controller family—bringing increased performance and cost savings to the industry’s smallest package of automotive-grade touch controllers.”
The maXTouch range has both hardware and software support available to aid in the development of maXTouch-based products. The maXTouch studio and maXTouch analyzer suites give designers access to maXTouch software while the evaluation kits provide either a 5” touchscreen or a 2.9” screen.
The Future of Touchscreen Technology
Being able to implement a touchscreen interface not only provides designers with plenty of design opportunities; it also makes interfacing with systems simpler.
Touchscreens provide users the ability to interact with menus that would otherwise be too complex. Additionally, they can interact with these menus using buttons that can read through thick gloves, opening doors for touchscreen functionality in harsh environments where removing gloves is not possible.