Remember when the stylus was the common tool for inputting text and controlling navigation on early PDA-type devices? Back then, resistive screen technologies dominated most initial touchscreen designs. With the advent of smooth, shiny capacitive touchscreens with the sensing layer behind the front surface, stylus usage has dropped—but not for long.

Consumers now see their tablets and smart phones as devices for content creation, not just for browsing or consuming content. The stylus is a natural choice, providing a familiar and precise pen-like experience for tasks such as entering text, taking notes, and drawing.

Under The Screen

With resistive technology, a mechanical sensor is mounted on top of a display and an embedded controller (Fig. 1). The sensor comprises a flexible polyester top layer and a rigid glass bottom layer separated by air and/or insulating dots.

The inside surface of each of the two layers is coated with a transparent metal oxide coating (indium tin oxide, or ITO) that facilitates a gradient across each layer when voltage is applied. When a stylus compresses the flexible membrane, it touches the lower resistive layer, activating the signal.

The control electronics alternate voltage between the layers and pass the resulting X and Y touch coordinates to the touchscreen controller. The touchscreen controller data is then passed on to the CPU for processing.

Implementing the stylus capability for a resistive touch system is relatively simple and straightforward. The resistive touch sensor is designed to offer optimized performance for both stylus and finger use. Now, however, capacitive touch is the technology of choice for most mobile devices, from cell phones to e-readers, tablets, and notebooks.

Capacitive touch technology offers a rich user experience, delivering a clearer, crisper display due to its superior optical properties, along with rock-solid reliability that’s not found in resistive touch. Stylus implementation for capacitive touch technology is not as simple or straightforward, though, as there are many factors to consider.

Inductive Technology: Good Performance, Higher Cost

When reviewing the potential stylus technologies available, design engineers have three possible choices: inductive technology, passive capacitive stylus, and active stylus. For many years now, the inductive technology approach has been popular, particularly with graphics tablets and tablet PCs.

Inductive technology comprises a printed-circuit board (PCB) sensor, a mixed-signal IC controller, driver software, and a stylus pen. Sitting underneath the LCD and backlight, the sensor is made up of copper tracks providing a multitude of over-lapping antenna coils in both the X and Y directions. The coils emit electromagnetic signals that can be detected by a special electromagnetic pen with active or passive circuitry.

The magnetic field’s energy maintains the circuitry, taking energy from the sensor into the pen. The pen’s own circuitry receives the energy, and an inductor/capacitor resonates to the frequency to determine its value. The energy is then reflected back toward the sensor, where it is received as an analog signal and then passed to the controller IC to provide position coordinates.

The inductive approach exhibits good performance, but it tends to be more expensive to implement (Fig. 2). The extra stack-up layer required for inductive stylus operation increases the device thickness, requires additional circuitry, and increases the associated costs.

Passive Capacitive Stylus: Moderate Performance, Low Cost

Based on a projected capacitive field charge transfer sensing technology, the passive capacitive stylus provides a low-cost solution with medium levels of performance. Widely adopted for cell phones and newer tablet devices, projected capacitive touchscreens operate by measuring small changes in capacitance that arise when objects such as a finger approaches or touches the surface of a screen.

The combination of the capacitive-to-digital conversion (CDC) technique and the spatial arrangement of the electrode structure (typically a transparent sensor film on top of the display) for the charge collection strongly impacts overall performance. This mix also facilitates implementation.

There are two fundamental ways of arranging and measuring the change in capacitance: self capacitance and mutual capacitance. The only way to make a capacitive touchscreen that can reliably report and track multiple concurrent touch points is to measure mutual capacitance where there are transmit and receive electrodes arranged as an orthogonal mix.

With a self-capacitance arrangement, an entire row or column is measured for capacitive change. This leads to positional ambiguity when the user touches down in two places. In practice, self capacitance is only useful for single-touch or very limited two-touch applications.

The sensor in a touchscreen consists of one or more layers of a patterned transparent conductor on a transparent substrate material—typically PET or glass (Fig. 3). This sensor is located over the display. To build a sensor that can resolve one or more finger touches through a glass or plastic front panel, the orthogonal grid of electrodes needs thoughtful implementation.

Typically, the patterned conductors (electrodes) are made from an etched pattern of ITO, a highly transparent material that has good optical clarity yet retains a moderately low ohmic resistivity. ITO can be used to fabricate a true matrix sensor where the only touch-sensitive region is the immediate vicinity where a row and column electrode couple to each other.

Using interpolation, the resolution can be determined fairly accurately where the center of a single touch is located. The difficulty occurs when several adjacent touches need to be uniquely identified, as this requires a high electrode density.

This means the row and column pitch should approximate 5 mm or less, a requirement that is simply derived from measuring the tip-to-tip distance between thumb and forefinger when pinched together and then dividing by two to separate them. Extensive trials have demonstrated that a pinch separation of 10 mm to 12 mm constitutes the best compromise between spatial resolution and increasing sensor complexity.

High electrode density enables another important feature: the use of a passive conductive stylus. With the right sensor design and a very advanced touch-tracking algorithm, it is possible to use a simple passive conductive stylus with a tip size of 3 to 5 mm.

Active Stylus: Excellent Performance, Lower Total Cost

The third approach to stylus implementation involves using an active stylus. This technique embraces the excellent performance characteristics of a projected capacitive field touchscreen and incorporates a stylus that can detect the presence of the field and communicate with the touchscreen controller.

For example, Atmel’s maXStylus mXTS100 active stylus supports the company’s maXTouch touchscreen controllers. The configuration of these technologies simplifies the hardware and lowers the total solution cost because it requires only a single ITO sensor that interfaces with the maXTouch controller to detect both finger touches and stylus proximity.

Through the system driver and serial interface, the system host controller interfaces with the maXTouch chipset for touch and stylus data. The simultaneous touch and stylus capability is called multiSense functionality.

The mXTS100 device capacitive sensing to detect an active maXTouch sensor presence and responds with its own signals to indicate location, pressure, button-click timing, and other information. The maXTouch controller receives stylus information through the sensor while also detecting finger touches.

After the maXTouch controller detects a stylus presence, special algorithms activate to process the stylus data to provide high linearity and resolution. Further processing provides excellent palm rejection, resulting in a smooth and comfortable pen-like stylus writing experience.

In addition, with a tip diameter of 1 mm and a fast frame rate of 140 Hz, the maXStylus active stylus can provide fast, accurate capture of gestures such as taps on the touchscreen.

Conclusion

To deliver a user experience that is closest to using a pen or pencil, an active stylus approach fits the bill. With an active stylus on a touchscreen, users can write or draw naturally and also interact with the screen through a variety of advanced gestures such as zooming, scrolling, erasing, and turning pages.

The combination of active stylus technologies available today, along with the advanced capacitive touchscreen solutions, makes it easier than ever for design engineers to create touch experiences that truly delight their users.