When you think of touchscreen technology, the first thing to come to mind is probably a smartphone. Companies like Apple reinforce this association with new releases like Touch ID technology, which uses a layered structure of a light source and an optical sensor to scan a user’s fingerprint, generate a mathematical model unique to that user, and allow access control.
However, touchscreen technology is decades older than the smartphone. This technology has continued to evolve over the past ten years. New controllers enable larger, more complex screens, which has become popular in dashboard technology within the automotive sector.
Basic Technologies Behind Touchscreens
There are five standard technologies capable of producing touch-aware sensors: resistive, two capacitive types, infrared, and surface acoustic waves.
All but infrared use a transparent layer of electrically conductive material (mostly indium-tin-oxide) with sensor lines that can be disturbed by the presence of objects such as a finger or stylus.
A stack-up comparison between resistive (left) and capacitive (right) touchscreens. Image used courtesy of Cypress
The nodal elements are separated layers representing the flow of charge in the X-Y coordinate system. A touchscreen controller then detects a change in the X-Y charge field and relays that information to the central application processor.
Though there are multiple touch technologies, the most common option for touchscreens is capacitive touch sensors.
Capacitive Touchscreens Are King Among Options
Capacitive touch sensors come in two modes. The first mode is based upon mutual capacitance between two electrodes on a substrate (like a PCB) to generate a capacitance on the order of picofarads.
When an object enters the local field of the plates, there is a difference in capacitance because of the change in the dielectric constant.
An example of mutual capacitance being disturbed by a finger. Image used courtesy of Bare Conductive
The second mode is self-capacitance, which occurs when the electrodes are separately isolated and reference the dielectric of the space around the electrode. Mutual capacitance technologies produce more reliable detection and can be augmented by using self-capacitance to localize the touch to a specific X-Y coordinate.
Sensors are just one essential part of touchscreen technology; another critical component lies with controller chipsets.
Microchip’s Latest Touchscreen Controller
Recently, Microchip has released a new controller for touchscreen technology. This new touch controller offers both modes of capacitance sensing with the ability to optimize touch mode detection by using both types of disturbance.
Microchip claims that the MXT2912TD-UW is the first automotive-grade single-chip controller to support ultra-wide screens up to 45 inches.
Taking a look through the datasheet, engineers can better understand how the new controller achieves this level of control with a single chip. The screen configurations split the X-lines and share the Y-lines.
Configuration of X and Y lines for an ultra-wide touchscreen. Image used courtesy of Microchip
The screen configurations can share the Y-lines because of the scanning sequence, where the X-lines drive charge for the Y-line receivers. This sharing allows the controller to localize a change in Y-lines, which occurs with respect to the exclusive X-lines.
Additionally, taking an overview of the PCB layout recommendations reveals sound principles for routing each charge line. The X and Y lines are routed orthogonally to reduce the risk of parasitic capacitance from affecting the touchscreen sensor data.
PCB layout recommendation from an ultra-wide topology. Image used courtesy of Microchip
Microchip claims that its controller is designed for ultra-wide touchscreens—a now-commonplace feature within the automotive industry.
The Future of the Dashboard
There is a trend in the automotive industry to provide more data to drivers and passengers via digital dashboards, including GPS maps, rear-view cameras, and an entire host of system statuses.
Single-chip applications ease development and reduce cost while providing a deeper variety of display features to users.
A demo unit for an automotive dashboard. Video screenshot used courtesy of JOLED
Beyond the chip-level electronics, improvements in OLED technology offer lightweight panels with wide viewing angles of up to 4K resolutions. These developments could be critical to the mass deployment of data visualization human-machine interfaces (HMI) and touch capability in vehicles.
