News and New Products

Touch controller allows you to ignore extra touches

By Robert Cravotta, Technical Editor -- EDN, 10/2/2009

Atmel’s mXT224 maXTouch touchscreen controller combines the company’s mutual capacitive sensors, a charge-tansfer method of signal acquisition, and an Xmega microcontroller with bundled software to provide processing for handling an unlimited number of touches on a touchscreen. Mutual capacitance sensors avoid the ambiguity of multitouch positions that self-capacitance sensors experience by forming independent sensing nodes at each intersection of rows and columns over the display surface. At each intersection, the sensor emits a fixed number of mutually coupled voltage pulses along each set of row and column lines; this action causes a known current flow in the selected pair of lines. This sensing approach yields an SNR (signal-to-noise ratio) of 80-to-1, which enables the system to unambiguously manage multiple simultaneous touches and more accurately sense weak and adjacent signals with a refresh rate as high as 250 Hz.

The integrated processor and bundled software provide additional processing for advanced noise-suppression algorithms for even more immunity to coupled-noise issues. Altogether, this approach enables the system to better identify and ignore unintended touches, such as when a user’s hand overlaps the edge of the display while holding it or rests on the writing surface while using a stylus to provide a signature or when the user places his face next to the display as might happen with a mobile phone.

The mXT224 supports 224 X/Y sensing nodes and consumes less than 5 mW. It provides enough precision to support zoom, pinching, rotate, handwriting, and shape-recognition capability in screens as large as 10 in. You can use multiple mXT224 controllers together to provide smaller interspatial distances between touches on larger screens. The integrated single-cycle RISC AVR core includes two on-chip DSP engines that process the X and Y positions on the touchscreen. The system provides 16 X drive lines and 14 Y sense lines; you can reconfigure as many as four of the Y sense lines as X drive lines for a 20×10-line configuration. An event system and peripheral DMA controller offload all interperipheral communications and data-transfer operations from the CPU, freeing it for postprocessing of the sensor image.

The mXT224 is available now in a 5×5-mm BGA package and sells for $4.75 (1 million) quantity. The EVK-mXT224A evaluation kit is available now for $400. The kit includes a 4.3-in. touchscreen and a PCB (printed-circuit board) that can connect to a PC over USB. The bundled PC software demonstrates the multitouch and built-in gesture capabilities of the device.