How to Implement Single-chip Touchscreen Interfaces on Home Appliances
Adopt the much-needed touchscreen interface at a low cost and with minimal design effort.
During the span of a mere decade, people of all ages worldwide have embraced touchscreen operation as their preferred and primary way of interacting with technology. The growth rate—and the resulting pervasiveness of this technology—is almost unparalleled in human history.
Still, many of the products that could benefit from a touchscreen interface, including home appliances, feature traditional design elements like mechanical or capacitive touch buttons and seven-segment displays. Despite advancements in technology, touchscreens are primarily available on a few high-end home appliance models. However, this is expected to change as the cost of implementation comes down. This article discusses an economical and fast approach for appliance manufacturers to capitalize on the appeal of touchscreens in products.
The History of Capacitive Touch
Touch interfaces have been around for a long time. This goes for both one-dimensional, which includes buttons and sliders, as well as two-dimensional, which includes touch pads and screens.
The ceramic cooktop was among the first adopters of touch buttons. In Europe, radiant, ceramic cooktops became popular and were integrated in the cooking range in the early 1990s, but soon after became available as standalone products. These cooktops required a user interface that could withstand liquid spills and the high heat from cooking, and capacitive touch emerged as the most viable solution. The first implementations were cumbersome as the immature technology only supported a few buttons within the strict noise tolerance requirements of the home appliance industry. To change the power of a cook zone, users needed to first press a button to select which cook zone to adjust, then press ‘+’ or ‘-‘ repeatedly to adjust the power. Eventually, noise performance of capacitive touch sensing improved. Today, induction cooktops—which are far noisier from an EMC perspective than radiant ones—provide an intuitive user experience through high resolution slider controls, one for each cook zone.
Figure 1: How a microcontroller moves image frames onto a Smart TFT Display.
As the capacitive user interfaces improved in quality, they provided a unique user experience compared to tactile mechanical switches. Soon the technology was adopted by other appliances—refrigerators, microwave ovens, ranges, washers, dryers, etc. This took place in the premium segment first and subsequently trickled down to lower range products. Some appliance manufacturers are now using touch on their entire product range.
The 2D surface capacitive touch was introduced with the PowerBook 500 series from Apple in 1994 as the first portable computer to feature a trackpad. Thirteen years later, the same company made the trackpad transparent and put it on a phone. We all know how that went.
The Rocky Road Forward
As home appliance products incorporate more functionality, they drive a growing need for interactivity beyond what can be conveyed through discrete controls and LEDs or vacuum fluorescent displays. Interacting with the product through a smart phone application is one way to manage this, but a washing machine that requires the user to bring their cellphone down to the basement to start a load of laundry is not likely to be embraced outside the techiest user groups. The solution is for home appliance products to fall in line with other consumer products and adopt touchscreens broadly.
Figure 2: Cross-sectional view of a touchscreen
It isn’t that straightforward, though. Although touchscreen modules and control electronics have come down in price mainly driven by the mobile phone, tablet and automotive infotainment markets, current technologies and business models are not always a perfect fit for the home appliance industry.
Today’s touchscreen vendors are mostly catering to markets requiring multi-touch operation, blistering fast response times and high accuracy in order to support mobile use cases such as gaming, stylus writing, image editing, and map navigation. Whereas a large display with this type of performance truly can make a luxury appliance model stand out in the market, it isn’t needed for a mid-range washing machine with a small display and a relatively simple user interface. A 4” or smaller display with single-touch menu icons and swipe-enabled menu screens already represents a huge leap in user experience compared to today’s discrete buttons and LED-displays.
Also, touchscreen controllers are sold as standalone products for integration with a microcontroller or microprocessor; thus, implementing the touchscreen requires another chip to be added to the bill of materials. However, the challenges extend beyond the touch control. Driving a thin-film-transistor (TFT) display further complicates the challenges. The menu systems required for a good user interface on an oven, for example, needs to support animations to provide a good user experience—such as swiping between menus the way a phone does. This bumps the display frame rate requirements to 50+ frames/second, and even for a four-inch display, designers will need a microcontroller with a built-in display interface. For the most part, these are only available in 100+ pin packages and can be quite costly. None of them incorporate a capacitive touch sensing unit, so it’s often assumed that a low cost, one-chip solution does not exist. Before we introduce how designers can get around these barriers, let’s cover the basic requirements for touchscreen interfaces.
Figure 3: In this example, the touch elevates the capacitance of four electrodes. While Rows 0 and 1 will exhibit an equal increase in capacitance, Column 0 will have a higher increase than Column 2 because a larger area is covered by the touch.
Most smaller TFT displays are so-called “smart” displays. This means they contain an image buffer and a display controller. These offer several advantages because 8080-type parallel interfaces can provide performance close to a traditional RGB display, including:
- The interface is simpler and allows for use of a smaller MCU package.
- Data transfer rates are reduced because pixel information is transferred from the MCU to the display only when the image is updated. This reduces power consumption and EMI, as well as frees up CPU workload.
Driving animations on a smart TFT display follows a simple scheme: A sequence of images is fetched from an image bank in a data memory and pushed on to a display through a parallel data interface at a rate of 30 images per second or faster. This requires a data path with two interfaces and a buffer to hold two frames so that the current frame can be written to the display while the next is fetched from the image memory.
One frame of a 320 x 240 display contains 150 kilobytes (kBytes) of data at 16-bit color depth, and to obtain a frame rate of 30 frames per second (fps), the data must move at 36 megabits per second (Mbit/s) on average. The Direct Memory Accesses (DMAs) of most higher-end microcontrollers support these data rates, but they also need the correct interfaces in order to move the data in and out of the microcontroller.
↑ Figure 4: The 2D Touch Surface Library allows developers to easily implement touch pads using our 8-bit PIC and AVR, as well as 32-bit SAM, microcontrollers. ↓
For the image memory, a viable alternative is to use a Flash device with a Quad SPI (QSPI) interface to strike the optimal balance between pin usage and data throughput. On the display side, an 8080-series type parallel interface is quite common.
A touchscreen sensor can be described as a Flex Printed Circuit Board (FPCB) made from transparent film where the copper traces have been replaced with a transparent and conductive material such as indium tin oxide (ITO) or Poly(3,4-ethylenedioxythiophene) (PEDOT). This material is used to form sensor electrodes in a tight matrix covering the entire display surface. A glass surface covering the sensor provides protection and prevents the touching finger from getting too close to the sensor, as this will impair signal quality and interpolation of electrode signals.
The sensor in this example consists of six column electrodes and four row electrodes arranged in a pattern that makes it possible to determine where a touch occurs. Each electrode forms one plate of a capacitor. The other plate is made up by the surroundings, and when charged with a voltage, an electric field forms between the plates. When a finger touches the glass, the field from the electrodes directly under the finger is disturbed and the value of the affected capacitor increases. A hardware touch sensing unit scans all electrodes constantly and measures their capacitances. As seen in Figure 3, the touch will elevate the capacitance of four electrodes. Rows 0 and 1 will exhibit an equal increase in capacitance, whereas Column 0 will have a higher increase than Column 2 since a larger area is covered by the touch. The capacitances of all other electrodes are unchanged.
An interpolation algorithm then uses this information to determine with high accuracy where the touch occurred. If the finger moves, gesturing recognition algorithms are used to detect the direction and speed of a swipe, the difference between a touch, tap and double tap, and if required, measure the distance between two touch points in a pinch zoom gesture.
Figure 5: An overview of how to enable touch control using the SAM D51s.
Touch sensing is easily affected by common mode noise from the power supply and/or internal noise sources and moisture spills on the display, all of which can lead to unintended touches or erratic operation. These factors need to be addressed in home appliance products through filters and signal processing built into the hardware and software drivers of the microcontroller.
The laundry list of requirements—DMA, frame buffer, QSPI, 8080, a touch sensing engine, hardware and software filtering, plus gesture recognition—is not available in any single microcontroller on the market. However, there are microcontrollers that come with the flexibility of building what is missing.
We have made software available for configuring and tuning the touch operation. The only thing missing in this offering is the 8080-type serial interface between the microcontroller and the TFT display. However, this can be constructed using a timer, an I/O port and the SAM D51’s event system, which enables peripherals to interact without the CPU getting involved.
In summary, this approach yields a single-chip microcontroller design capable of controlling the entire touchscreen user interface with Flash program memory and CPU bandwidth resources left over for complex user interface code. Designers can benefit from the following by implementing capacitive touch using an application’s existing MCU and software:
- Short design time though the touch software library, which supports gesture recognition and configuration, as well as tuning software tools
- Rock solid, appliance-proven touch capable of handling the tough EMC and moisture requirements of modern home appliance designs
- Compact, low-cost design using a single 48-pin microcontroller
- Full integration with the user application code
This demonstrates a clever way for home appliances to adopt the much-needed touchscreen interface at a low cost and with minimal design effort. In addition, the robustness of this approach ensures that quality of operation matches what today’s consumers have come to expect.