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.
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| 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.
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