17 Jun 2011

Touchscreen Controller - Key Points to Look For

Binay Bajaj, Sr. Product Marketing Manager, Touch Technologies, Atmel Corporation looks at the touchscreen market and the controllers needed for them.

With the arrival of the Apple® iPad and a host of competing tablets launching this year, the market for 5”-13” large-format touchscreen devices is set to explode.

The explosive growth in personal computing has already unleashed a flurry of activity among device manufacturers, who are actively porting touchscreen technologies to large-format hardware.

However, moving from small screens and simple touch-enabled applications to a new paradigm, where hands and fingers are the primary tools for interacting with full-scale computers, is not necessarily a straightforward transition.

Manufacturers need to rethink the way that touchscreens will be used by consumers and address a new and more demanding set of requirements. To cite just one example, “multi-touch” capabilities chiefly consist of a few finger strokes on today’s five-inch screens. What will they mean on a 12-inch or 40-inch device—or when multiple users are interacting simultaneously using both hands?

Basics of Touchscreen Technologies

Large or small, the success of any touchscreen device is a function of the technology choices made in designing it, the most important being projected capacitance technology, sensor design, and driver chip.

Today’s devices overwhelmingly use capacitive touchscreens, which operate by measuring small changes in capacitance—the ability to hold an electrical charge—when an object (such as a finger) approaches or touches the surface of the screen. However, all capacitive touchscreens are not created equal. Choices in the capacitive-to-digital conversion (CDC) technique and the spatial arrangement of the electrodes that collect the charge determine the overall performance and functionality the device can achieve.

Device manufacturers have two basic options for arranging and measuring capacitance changes in a touchscreen: self-capacitance and mutual-capacitance. Most early capacitive touchscreens relied on self-capacitance, which measures an entire row or column of electrodes for capacitive change. This approach is fine for one-touch or simple two-touch interactions. But it presents serious limitations for more advanced applications, because it introduces positional ambiguity when the user touches down in two places. Effectively, the system detects touches at two (x) coordinates and two (y) coordinates, but has no way to know which (x) goes with which (y). This leads to “ghost” positions when interpreting the touch points, reducing accuracy and performance.

Alternatively, mutual-capacitance touchscreens use transmit and receive electrodes arranged as an orthogonal matrix, allowing them to measure the point where a row and column of electrodes intersect. In this way, they detect each touch as a specific pair of (x,y) coordinates. For example, a mutual-capacitance system will detect two touches as (x1,y3) and (x2,y0), whereas a self-capacitance system will detect simply (x1,x2,y0,y3).

Self-Capacitance versus Mutual Capacitance

The underlying CDC technique also affects performance. The receive lines are held at zero potential during the charge acquisition process, and only the charge between the specific transmitter X and receiver Y electrodes touched by the user is transferred. Other techniques are available, but the key advantage of the CDC is its immunity to the noise and parasitic effects. This immunity allows for addition system design flexibility; for example the sensor IC can be place either on the FPC immediately adjacent to the sensor, or further away on the main circuit board.

Sensor design

Electrode pitch, a key parameter in sensor design refers to the density of electrodes—or more specifically, (x,y) “nodes”—on the touchscreen, and to a large extent determines the touchscreen resolution, accuracy, and finger separation. Naturally, different applications have different resolution requirements. But today’s multi-touch applications, which need to interpret fine-scale touch movements such as stretching and pinching fingertips, require high resolutions to uniquely identify several adjacent touches.

Typically, touchscreens need a row and column electrode pitch of approximately 5 millimetres or less (derived from measuring the tip-to-tip distance between the thumb and forefinger when pinched together). This allows the device to properly track fingertip movements, support stylus input, and with proper firmware algorithms, reject unintended touches. When the electrode pitch is in-between 3 to 5 millimetres, the touchscreen becomes capable of supporting input with a stylus with a finer tip—a boost in accuracy that will allow the device to support a broader range of applications.

Touchscreen Driver Chip

At the core of any successful touch sensor system is the underlying chip and software technology. As with any other chip design, the touchscreen driver chip should have 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. Any driver chip will be measured by the balance of speed, power, and flexibility it achieves.

Supersizing the Touchscreen

The considerations described above apply to any size touchscreen device. But what are the specific considerations for moving to large-format devices? Manufacturers will find that the key requirements for modern touchscreen technologies—multi-touch support, performance, flexibility, and efficiency—become even more critical when users adopt larger screens. . . . . . . .

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