Controllers open new markets for touchscreens28 Apr 2012 | Margery Conner
Touchscreens have been around for decades, and they typically employ resistive-touchscreen technology. With resistive touchscreens, a user's finger physically deforms the top layer of the screen, causing the resistive sensor to make contact below the finger. The resistive sensors are in a grid of X and Y traces, separated by a thin, transparent insulator.
Note the use of the word "press." A press is a different action from a touch or a swipe. Resistive touchscreens have limited capabilities in their response to multitouch gestures, such as pinches, zooms, swipes, and scrolls. Users who have become accustomed to navigating their smartphones and tablets with these gestures become frustrated with simple touchscreens that lack these features. Touchscreens that can respond to complex multitouch gestures generally rely on capacitive sensing.
Capacitive-sense-touchscreen technology generally comes in self-capacitance and mutual-capacitance flavors, although other types, such as projected capacitance, exist. Self-capacitance sensors comprise a series of thin lines of indium-titanium oxide, a transparent, conductive material in an XY grid with an insulating layer between the X and the Y traces. Touching an area in the grid changes the parasitic capacitance of the sensors to ground. However, this approach can't handle multiple-finger touches because the sensor can't distinguish between multiple fingers along the same grid line. Mutual capacitance senses the change in the capacitor at the small intersection of the X and the Y lines. Because the area of the intersection is small, the capacitance is also small, but it is precise and can measure multiple-finger placements.
There are pros and cons to each approach. Although self-capacitance sensors generally cannot distinguish between multiple simultaneous finger actions, they also generate a stronger electromagnetic field that can detect objects even if the objects don't actually touch the screen. Mutual-capacitance touchscreens can detect and track the touch of multiple fingers, but the fingers must touch the screen because the electromagnetic field from the tiny capacitors formed by the intersection of the two sensors is so small.
The need for close contact between the finger and the touchscreen can be a problem when the user is wearing gloves. This restriction on the part of capacitive touchscreens causes a shift in favor of resistive screens. Resistive technology also has an advantage in liquid applications or in humid climates in which moisture affects the behavior of the electromagnetic field. Cypress' TrueTouch controller technology seeks to overcome these hurdles by combining both self- and mutual-capacitance techniques.
Both self-capacitance and mutual capacitance require the same XY sensor grid. In self-capacitance, the controller must drive both the X and the Y lines. In mutual capacitance, the controller transmits into the X lines and receives on the Y lines. Because the TrueTouch controller IC uses Cypress' PSoC (programmable-system-on-chip) core, the controller can dynamically configure its I/O pins and turn the transmitters into receivers on the fly. Thus, the controller can sense in both modes—self- and mutual capacitance—whenever the controller scans the sensor's grid panel. Combining self-capacitance sensing with mutual capacitance allows for multitouch capability even with hands wearing thick ski gloves. This ability raises the question of how safe are the touchscreens in automobiles.
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