iPhone 5: In-Cell Touch

Anand Lal Shimpi, Brian Klug & Vivek Gowri, AnandTech:

Touch sensing has to be time multiplexed with display driving otherwise the touch signal might be entirely lost in noise. At the same time, touch sensing is often around double the frequency (120–175 Hz) of display drawing (60 Hz), so this has to be done carefully during quiet periods, and thus that required communication and integration. The iPhone 5 uses a combination of TI and Broadcom controllers to do display controller and touch sensing, where previous generations of iPhone simply just used a single chip TI solution. In future generations this will come back down to just being a single-chip solution.

A single-chip that acts as both display controller and touch sensing sounds good. Thanks to in-cell touch technology, the definition of mobile display will change soon in the future: A mobile display will mean a display capable of responding to touch.

[...] ITO traces are only laid down where they need to be on top of and below the glass substrate (for both transmit and receive layers of the digitizer), and the areas inbetween those traces are then filled with an index-matching space fill material to diminish their visibility. How well this space fill is done and how close the index is to ITO’s is one of the quality metrics of a digitizer to begin with, and often these rows and columns are visible under direct illumination either outdoors or with good eyes indoors.

This is an important observation. In-cell touch completely eliminates these visible traces of ITO in the touch layer, because the touch layer is no more. Now the only thing to get rid of is the cover glass.

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