HMI Applications―Part 3-1: Highly Sensitive & Highly Noise-Resistant Touch Keys

Solutions: 13 of 20

Enabling Better “Touch Keys” for Embedded Devices

HMIs that utilize solid-screen electronic controls have improved radically in recent years and technology advances continue. Touch keys offer so many design and application advantages for new equipment that they have largely replaced the mechanical knobs and switches that were ubiquitous just a few years ago. This third installment in our series on Renesas' HMI solutions examines ways that system engineers can implement touch key controls in enhanced embedded systems.

The Many Advantages of Touch Keys

The continuing evolution of human-machine interfaces (HMIs) raises the expectations users have for both appearance and performance. Implemented on sleek touch panels, they look good, eliminate protruding knobs and switches, and provide better operational experiences. Such HMIs are increasingly versatile, too, because there are multiple types of touch controls—single keys, wheels and sliders, among others (see Figure 1)—and they can be created in an amazing variety of forms that aid product differentiation.

Figure 1: Examples of Touch-Panel Controls

Figure 1: Examples of Touch-Panel Controls

Electronics manufacturers have found that eliminating mechanical control components add to a product's value and also tends to lengthen its service life. Reliability increases because electronic controls don't wear and break. Additionally, since touch panels have no gaps and holes, they help prevent dust and liquids from entering the electronics and damaging the system. This boosts reliability in difficult application environments. Users are also very pleased to find that these panels can be easily wiped clean.

It is becoming commonplace today to encounter and use touch-key based HMIs because electronic panels are featured in an expanding range of consumer and business products: household appliances, copy machines, automotive dashboards and healthcare equipment, to name just a few. Touch panels are also appearing front-and-center in the latest production, test and process-control equipment, expanding environmental tolerance and resilience. User reactions to diverse applications of this HMI technology are very favorable.

To make it easier for customers to implement touch keys in embedded devices, Renesas gives engineering teams all the tools necessary for quickly creating products that utilize attractive, effective HMIs. We provide optimized MCUs (microcontrollers), tailored IP (intellectual property), proven software, effective HW/SW development aids and helpful application assistance, worldwide.

R8C and RL78 Microcontrollers Support Capacitive Touch Keys

Implementing the interface between an electronic control panel and an embedded system's circuitry becomes a straightforward task when embedded-device designers use Renesas MCUs. Our hugely popular R8C MCUs and the low-power MCUs in the RL78 Family have contributed greatly to the advance of touch-key operation systems. Chips in both product lines support capacitive touch-key designs and we are developing additional solutions.

  • R8C/3xT Series MCUs have built-in touch-key intellectual property (IP) and are best-in-class design solutions for control panels with up to thirty-six keys. Especially, devices in our R8C/3xT product group excel for such uses. R8C/3xT MCUs have an overwhelming presence in the electronics industry; over 10 million of them have been shipped around the globe, and incorporated in over 200 end products.
  • RL78 Family MCUs running touch-key interface software are optimum design solutions for smaller embedded systems with simpler control panels. These power-saving chips support up to five keys.

Thick Panel? Curved? Wet? No Problem!!
New RX113's Next-Generation IP Delivers Highly Sensitive, Highly Noise-Resistant Touch Keys

An electronic control panel's MCU must generate accurate, reliable system inputs from a large variety of touch keys applied in a wide range of environmental conditions. The touch key IP built into R8C/3xT MCUs, for example, meets these requirements. It offers a hardware-assisted touch control function that uses only 15% of CPU bandwidth, utilizes a low-power touch-detection technique, and has an API that supports self-diagnostics. Noise performance is IEC-compliant.

Because touch-key HMIs are such a dynamic application area, MCU suppliers must overcome new technical challenges on an ongoing basis to enable system engineers to take advantage of advances in user interfaces. Renesas application experts are doing just that, creating the next-generation capacitive touch-key IP for higher-performance MCUs. This new touch key will deliver exceptional capabilities, while making it easier for system design teams to develop robust, touch-key based HMIs with enhanced control functions.

Consider, for example, that conventional wisdom says that thick panels preclude touch keys; i.e., the prevailing belief is that touch keys can't be incorporated into embedded devices that require strong thick control panels. But our new touch-key IP renders that perception obsolete. It readily implements reliable touch keys on thick panels—and on curved ones, as well.

Until recently, the recommended maximum thickness for acrylic-plate touch panels was just 2mm; for glass plate panels, just 4mm. The new Renesas touch-key IP, however, achieves improved capacitive sensitivity and noise immunity. Tests show that it enables smooth operation through acrylic plates, even when a finger and the key's electrode are separated by a full 10mm—a gap five times wider than the previously recommended maximum.

Additional Advantages of Our New Touch-Key IP

The heightened sensitivity delivered by our new touch key will let customers build embedded devices with electronic panels that users can operate while wearing gloves. Device operators also will be able to make system adjustments simply by bringing a finger close to the panel, without touching it. Such system control possibilities significantly broaden the opportunities for touch-key implementations and HMI innovations.

Renesas' advanced touch-key interface technology even allows touch keys to be implemented on curved surfaces. This capability gives device manufacturers a significant benefit by eliminating the need for corrective touch-sensitivity adjustments if production processes can't produce panels that are perfectly flat.

Additionally, the excellent noise immunity of this new touch key IP gives system engineers important problem-solving advantages. Thicker panels mandate higher touch-key sensitivities, raising concerns about susceptibility to electrical noise. With our cutting-edge IP, though, EMI isn't an issue. Test panels with widths exceeding 10mm have successfully passed the rigorous IEC 61000-4-3 and IEC 61000-4-6 noise tests. The noise-resistant touch panels that our state-of-the-art IP makes possible will facilitate customers' efforts to develop robust, high-quality products.

New Touch-Key IP Supports Self-Capacitance and Mutual-Capacitance Method

To obtain maximum application potential, Renesas technologists have designed the new touch key IP to handle either self-capacitance or mutual-capacitance touch key designs.

Self-capacitance method works by electronically detecting increases in the parasitic capacitance between a user's finger and the nearest electrode (see Figure 2). The amount of parasitic capacitance involved is quite small, which imposes application limitations.

Figure 2: How Self-Capacitance Method Works

Figure 2: How Self-Capacitance Method Works

Figure 3: How Mutual-Capacitive Method Works

Figure 3: How Mutual-Capacitive Method Works

In a mutual-capacitance design, each touch key has transmitter and receiver electrodes, and generates its own electric field between them. When an operator's finger approaches the key, some of the electric field flows through the person into the ground?resulting in a detectable change in the strength of that electric field.

Mutual-capacitance method performs well even if the wiring from the electrodes to the MCU is relatively long. Importantly, the electric field that the keys generate isn't significantly disrupted by the presence of water. Simply put, the embedded system can tell the difference between a finger touch and a water drop. This capability is essential for products that must operate reliably when water falls on, or is sprayed on, the control panel.

Some electronic control panels arrange touch keys and their associated electrodes in a matrix *1. In such designs the mutual-capacitance approach offers higher sensitivity than the self-capacitance method.

Matrix-type mutual-capacitance implementations also allow true multi-touch operations. They eliminate the ghosting problem that occurs in self-capacitance panels when two or more spots are touched simultaneously. Additionally, matrix-type panels built with MCUs that have relatively few pins can create more touch-key channels than would otherwise be possible.

Figure 4 shows the main characteristics of self-capacitance and mutual-capacitance method implementations. The optimum design choice depends on the requirements of the particular application.

  Advantages Disadvantages
Self Capacitance
  • Fewer restrictions on electrode shape
  • Can be implemented with low-cost (single-layer) circuit boards
  • Susceptible to water damage
  • In matrix arrangements, simultaneous touches in multiple places generate ghost signals
Mutual Capacitance
  • Withstands water (Can remove water effects)
  • Matrix arrangements allow true multi-touch operations
  • Greater restrictions on electrode shape
  • Fairly high board cost (requires multiple layers)

Video Highlights Performance of Our New Touch Key IP

The demo video showcases the capabilities of our new touch key designed specifically to support highly versatile matrix-type mutual-capacitance touch key implementations. We expect these implementations to open the way to a wide variety of new HMIs.

  • 5mm-thick panel and gloves
  • Water on the touch-key area
  • Curved panels

Video : Click to play video

Wide Variety of Solutions to Precisely Meet User Needs

Renesas touch-key solutions built into the R8C and RL78 MCUs are widely respected throughout the industry. Our new touch key IP follows the same approach, incorporated directly into our new RX113. This new one-chip solution builds on the high performance and low-operation of our RX100 Series by adding not only the new touch key IP, but also LCD drivers and other essential peripheral capabilities. It opens the way to a new touch-key system development. Renesas stand ready with a wide variety of solutions to meet the diverse and evolving needs of our customers.

*1 In matrix mutual-capacitance implementations, transmit and receive electrodes are positioned at the intersections of rows and columns. Transmit electrodes can be placed along the rows, for example, with receive electrodes connected to the columns. In a 5-by-5 matrix, such designs provide five transmit channels and five receive channels, allowing 25 distinct keys. This mutual-capacitance control panel approach enables full multi-touch functionality.