Advanced Touch Interface Drivers Enable New User Applications - Embedded.com

Advanced Touch Interface Drivers Enable New User Applications

This “Product How-To” article focuses how to use a certain product in an embedded system and is written by a company representative.

Iconic designs are not just a matter of brand identity or even about the new or improved functions of the device compared to existing technology. For a design to be Iconic, it must truly change an aspect of your life through the way you interact with or use the device.

In the case of the Apple iPhone and related devices, the concept was to build the user interface first ” the capacitive touchscreen – and then to provide connectivity and application support through basic hardware and excellent software. Through this route the user is able to interact with the device in new and intuitive ways.

Resistive touchscreens are quite commonly used in consumer devices for basic touch button replacement functions or other simple virtual devices such as scroll bars. This allows a contextual approach to user interfaces, helping to reduce the size and outward complexity of the unit and opening up new industrial design options.

The poor optical properties, reliability issues, limited usefulness for gesture inputs, and marginal capability to interpret two or more concurrent touch points limit the usefulness of resistive screens and they are rapidly becoming the poorer cousin of capacitive touchscreens.

Capacitive touchscreen technology has matured rapidly over the past few years, bringing together advanced algorithms running on low cost hardware with sophisticated materials technology to generate highly reliable and robust user interfaces.

Early capacitive technology and some of the current lower end offerings on the market suffer from low resolution, problems with system level interference from the LCD or other sources of noise, leading to serious performance compromises.

Figure 1. Atmel's touch screen offering includes the touch screen controller IC and board reference designs as well as sensor reference.

A projective capacitive touchscreen works by measuring small changes in capacitance that arise when objects such as a finger approaches or touches the surface of a screen. There are many ways to measure and interpret a change in the capacitance on a touch surface as a finger or fingers come into contact.

The combination of the capacitive to digital conversion technique (CDC) and the spatial arrangement of the electrode structure (typically a transparent sensor film on top of the display) for the charge collection, both have a strong impact on the overall performance which can be achieved as well as the ease of implementation.

There are two fundamental ways of arranging and measuring the change in capacitance on a projected capacitance touchscreen: self capacitance and mutual capacitance.

The measurement of mutual capacitance where there are transmit and receive electrodes arranged as an orthogonal matrix is the only way to make a capacitive touchscreen which can reliably report and track multiple concurrent touch points.

For simplicity, this technique can be considered as consisting of an array of smaller touchscreens formed by the geometry of the electrode structure which is then interpreted as a complete touch surface ” this is achieved while maintaining the ability to resolve multiple touch points within each individual 'small' screen.

Because the capacitive coupling at each point in the matrix can be measured independently, it means that there is no ambiguity in the reported coordinates for multiple touches.

Contrast this arrangement with a self capacitance based touchscreen. In this arrangement an entire row or column is measured for capacitive change (as distinct from the intersection point of a row and a column for a mutual capacitance scheme). This leads to positional ambiguity when the user touches down in two places.

Some level of reconstruction of the touch positions is possible in software but there is always ambiguity which leads to 'ghost' positions for the interpreted touch points and which in turn leads to unintended actions being reported to the system host.

The other side effect with this method is that when two touches share the same row or column electrode, the reported coordinates tend to “snap” to that electrode causing a strong non-linearity. In practice, self capacitance is only useful for single touch or limited two-touch applications.

In a mutual capacitance based system, each touch is detected as a pair of X and Y coordinates whereas in a self capacitance system, the detection of X and Y coordinates of a touch is independent.

If two touches are present in a mutual capacitance system, this would be detected as (X1,Y1) and (X2,Y2), whereas in a self capacitance system it would be detected as (X1,X2,Y1,Y2). In the self capacitance system, it is then impossible to decide which of the touch points (X1,Y1), (X2,Y1), (X1,Y2) and (X2,Y2) are valid.

The underlying CDC measurement also has a significant impact on the way in which a capacitive touchscreen can be implemented. Many techniques can be used to acquire the signal, for example, relaxation oscillators, CSA, Sigma Delta converters, each with its own strengths and weaknesses.

All of these techniques have been described in detail by other sources, but it is interesting to note that from the point of view of making mutual capacitance measurements, they have a major drawback which seriously limits their usefulness.

During the measurement cycle, the lines remain sensitive to the touch (hot) ” this is something which is highly undesirable for a good measurement. This leads to positional inaccuracy in the measurements, it means that the sensor edge wiring contributes to the signals to calculate position and also means that it is almost impossible to route the connection from the sensor to the driver chip over more than a few centimetres. Some of these issues can be partially mitigated through careful design compromises but performance overall is heavily compromised.

Atmels's MaxTouch uses the Charge Transfer technique to perform CDC measurements. This technique effectively holds the receive lines at zero potential during the charge acquisition process and therefore only transfers charge between the transmitter X and receiver Y electrodes at the point of interest in the main sensor area.

The technique has the added advantage of minimizing the effect of local moisture or other potentially conductive materials in the proximity of or even on the surface of the touchscreen.

In combination, the Charge Transfer technique and Mutual capacitance “Matrix” style measurement is the only reliable way to build a genuinely multi-touch touchscreen.

Sensor design options
The sensor in a Touchscreen consists of one or more layers of a patterned transparent film placed on top of the screen. In order to build a sensor that is capable of resolving one or more finger touches through a glass or plastic front panel, the orthogonal grid of electrodes needs thoughtful implementation The electrode grid is formed as an array of patterned conductors on a substrate material, typically made of PET or Glass.

Figure 2. Ateml's Qfield Technology enables single layer sensor implementations including single touch screens and with a stack thickness as low as 100 micrometers requires no shield layer in most designs.

Typically the patterned conductors (electrodes) are made from an etched pattern of a highly transparent material called ITO (Indium Tin Oxide) that has good optical clarity yet retains a moderately low ohmic resistivity.

Low resistivity is important because it makes it possible to do fast measurements of tiny capacitive changes, in the order of 10's of femtofarads (10-15 Farads), in the presence of background capacitances of 10's of picofarads (10-12 Farads).

Atmel's QMatrix using Charge Transfer has an underlying property that allows commonly available ITO with good optical properties to be used to fabricate a true matrix sensor where the only region that is touch sensitive is the immediate vicinity where a row and column electrode couple to each other.

This localised coupling means that all other parts of the row and column are, largely not touch sensitive. Without this property, it is not possible to make a true multi-touch touchscreen, rather, it is only possible to partially address the requirements with significant compromises.

Other CDC techniques attempt to emulate a true matrix but in order to do so they require a more restrictive range of ITO material with much lower resistivity and inferior optical properties.

This lower resistivity reduces voltage drops across the row and column which reduces their intrinsic touch sensitivity. However, without QMatrix, they are still touch sensitive to some extent and so this is always a compromise that leads to poor multi-touch behaviour and significant side effects around the edge of the sensor.

Of the two most common substrate materials, PET offers some cost advantage over Glass but typically requires two separate layers to realize the orthogonal grid. Glass on the other hand is a little more expensive but does allow a single layer design, using micro cross-overs to bridge the pattern intersections.

Glass sensors also offer much better mechanical stability than PET and so are suitable for deposition of very thin metalized tracks, as narrow as a few 10's of microns wide. PET on the other hand normally uses screen printed tracks of the order of a few100's of microns wide, although the technology in this area is improving rapidly.

The overall effect is to define the minimum dimensions of the edge-wiring to the sensor where space is typically at a premium, particularly in small portable devices.

Important features in touch sensors
Making a great multi-touch sensor requires high electrode density and a true matrix CDC method, like QMatrix based on Charge Transfer.

High electrode density in this context means that the row and column pitch should approximate 5mm 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.

Extensive trials with user interfaces have demonstrated that a pinch separation of 10 to 11mm constitutes the best compromise between spatial resolution and increasing sensor complexity.

Increasing the pitch above 5mm can in some cases be accepted for single touch applications, but in order to make a truly excellent single touch capacitive touchscreen there is a strong argument that says it needs to be fully multi-touch capable at its core. This is to allow tracking and rejection of unintended touch points.

It is also worth noting that sensor resolution is directly linked to the number of electrodes in each axis so adding more rows or columns for a given screen size can have a beneficial effect even though the sensor is more complex to manufacture.

>b>More channels means higher Signal-to-Noise ratio (SNR)
A high electrode density enables another important feature; permitting the use of a passive conductive stylus. With the right sensor design, combined with the best CDC method and a very advanced touch tracking algorithm, it is possible to use a simple passive conductive stylus with a tip size of 3-5mm.

This same feature also enables people to drive a capacitive touch user interface with a short fingernail and provides a more accurate pointing device than a typical prod of a fingertip. This increases the range of applications that can be suitably addressed with a device featuring capacitive touchscreen as the main input source.

Chip spec
The ITO sensor design plays a vital role, and a true Matrix CDC provides the basis of a proper multi touch device, however the underlying chip and software technology which makes all of this possible is the core of any successful touch sensor system.

The touchscreen driver chip must approximate all the usual characteristics of a chip as with any other design; high integration, minimal footprint, and close to zero power consumption along with the flexibility to support a broad range of sensor designs and implementation scenarios.

Providing an optimal mix of speed/power and flexibility is something to take very seriously. Can the controller chip run at typical low system Vdd power supplies? Higher Vdd means better Signal-to-Noise ratio but also leads to higher power consumption. Are level shifters needed to connect to the host? Does the communication protocol allow for future expansion without completely rewriting the drivers?

By integrating the entire capacitive sensing circuitry on-chip, the MaxTouch devices provide a solution without the need for external components to support the capacitive sensing, minimizing the cost and PCB footprint requirements.

The devices front-end is a customized Capacitive Touch Engine (CTE), highly capable of various Digital Signal Processing operations on the raw data from the sensors, thus only waking the main AVR' CPU when a touch is confirmed and more advanced algorithms must be executed. This ensures minimal power consumption, where most of the system can stay in a very low power mode of operation for the majority of the time.

The MaxTouch devices all contain an in-system self-programmable flash, providing maximum flexibility. The MaxTouch devices are capable of being upgraded in-system through the regular communication port over the entire operating voltage range, providing possibilities to upgrade without requiring additional pins or circuitry.

Flexibility on the placement of the device is an important design parameter. A true Matrix CDC does not suffer from touch sensitive connections to the ITO (aka hot tracks).

This is a huge advantage from a flexibility point of view. It means that the chip can be either placed close to the sensor, perhaps as a chip-on-flex, or it can be much further away, on an entirely separate board. The latter option allows a passive flex to connect the ITO to the chip, which can be up to 100mm or more away.

Another critical factor in creating the optimum touch screen is the response time. Handwriting recognition requires XY update rates of 70Hz to 120Hz. Other scenarios, such as the use of a virtual keypad for two concurrent finger/thumb typing requires positive feedback to the user in sub 100ms for accurate input.

At first consideration, this seems straightforward, but by the time various system latencies are factored in, it typically means that the touch screen needs to report a first touch position in less than 15ms. Unless the sensing circuitry is carefully architected, this could lead to excessive power consumption with reduced battery lifetime as consequence.

Another important point of note is that parasitic capacitance build up on the ITO connections, due to the connecting flex, is only of secondary importance for optimal CDC methods. Chose the wrong CDC method and the chip dilutes the effort by measuring useless background parasitic capacitance, compromising the effect of touches on the touchscreen and so reducing SNR and resolution.

Many-touch
In the discussion of multi-touch so far, it is not apparent that use-cases beyond 2-touch are of any importance. We have all become familiar with the iPhone-popularised pinch and stretch gestures. In fact there are examples of such gestures used with much larger multi-touch interfaces that date back to long before the iPhone.

But what happens with 3, 4, or even more touches? How can those touches be measured and characterised? The question is perhaps not only what gestures can be imagined to use this capability, but rather, what can the controller chip do with this rich information.

One example of such use is to be able to track multiple touches around the edge of a touchscreen and classify them as suppressed. This allows a user to comfortably hold a small product with some amount of finger/screen overlap, yet the touchscreen continues to operate normally.

However, there is hidden subtlety here. The suppressed touches must all be tracked and stay suppressed even if they stray into the active region. This means the controller must be able to uniquely and unambiguously resolve, classify and track many touches at once.

Another potential use of the underlying many-touch data is to recognise shapes on the touch surface. This allows all kinds of potentially useful interface enhancements.

Basic shape recognition for a nose, cheek, or even an ear allows further suppression of real-world situations that would otherwise falsely trigger the touchscreen. Nevertheless, as more touches can be uniquely identified and reported to the host, the applications will start making use of multiple touch data.

Noise and System Issues
As noted earlier, capacitive touchscreen controllers measure very small changes in the row to column coupling capacitance. The way the controller performs the measurement has a strong influence on the susceptibility of the controller to external noise.

One common noise generator encountered with touchscreens is the LCD itself. It often has voltage transients measured as several volts with rise/fall times measured in micro seconds ” this is a very challenging environment. There are clever ways to reject this noise in the controller chip, but with the right type of CDC it is also possible to reject most of it at source.

A second approach is to add a shielding layer to the sensor to keep the noise away from the electrodes. This can lead to excessively thick sensors. A third and better approach is to use a novel sensor electrode pattern that uses two ITO layers but is self shielding from behind.

This is also highly beneficial because it renders the sensor immune to capacitive changes if the front panel bends down towards a ground plane such as that on the front surface of an LCD due to touch pressure.

As display technology moves forward devices such as OLED displays offer a much cleaner noise environment and are a very good match for capacitive touchscreens. They are suitable for single or dual layer sensor designs. LCD technology is also being evolved to improve compatibility.

The second most problematic noise source is found with 'floating' power supplies. These often capacitively couple several hundred volts of distorted 50/60Hz waveform relative to earth, into the entire touchscreen device.

When a user touches the device, the sensor effectively becomes part of a capacitive voltage divider which contaminates the measurements with huge amounts of low frequency noise. Again, with clever chip design it is possible almost eliminate this effect at source, and remove the last traces with DSP functions on-chip.

Conclusion
In summary, state of the art DSP and microprocessor based technology allows the realisation of high performance capacitive touch sensor arrays which are capable of producing an 'image' of the change in the charge on a surface as it is touched by a user

By use of an appropriate combination of CDC (Charge Transfer) and mutual capacitance (QMatrix) based sensor construction and signal acquisition, the system can be made highly immune to unwanted sources of system interference and background loading.

Once a charge image has been acquired, highly efficient microprocessor technology (AVR) can be used to process the data to provide multiples of position data for touch points or higher level processing to rejected unintentional touch points or to interpret movements of one or more fingers on the touch surface as 'gestures' which can be used to simplify the user interface in many applications.

With suitably efficient processing of the acquisition, processing and reporting of the data, all of the above can be achieved in a very low power budget, compatible with the most demanding battery powered applications.

Christopher Ard is Product Marketing Director, Touch Technology and Dr. Gaute Myklebust is Strategic Marketing Director at Atmel Corp.

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