Design Con 2015

SENSORS: Atmel maXTouch brings mutual capacitance to touchscreens

September 09, 2009

Bernard Cole-September 09, 2009

San Jose, Ca. - Atmel Corporation has just released to volume production its new maXTouch family of capacitive touchscreen controllers, capable of supporting an unlimited number of unique simultaneous touches with a video-quality screen refresh rate of 250 Hz.

Atmel's new maXTouch technology platform supports the development of touchscreens surpassing 10 inches with full zoom, rotate, handwriting and shape recognition functionality.

According to Peter Jones, Managing Director of Atmel's Microcontroller Business Unit, the devices have been developed to provide uncompromising capacitive touch capability combined with the industry's lowest power consumption and unsurpassed configurability to exceed today's user interface requirements.

The first device in the family, the mXT224, has 224 nodes that allow it to accurately report the positions of unlimited, simultaneous touches and it can completely redraw the screen every 4/1000 of a second (4 mS).

He said the mXT224's large number of nodes and fast performance makes it the world's first touchscreen solution suitable for advanced touch screen functionality, such as rejection of unintended touches, stretch/pinch and rotate gestures, handwriting and shape recognition such as face detection on mobile phones, mobile Internet devices (MID) and netbook screens surpassing 10 inches.

By integrating the entire capacitive sensing circuitry on-chip, said Jones, a fully integrated solution is possible without the need for external components to support the capacitive sensing, minimizing the cost and PCB footprint requirements.

Multiple mXT224 touchscreen solutions can be used to provide smaller interspatial distances between touches on larger screens. Products utilizing the advanced mXT224 touchscreen solution are currently progressing in design at several leading handset, netbook and other consumer product OEMs.

According to Jones, the mXT224 is the first capacitive touchscreen solution able to support the use of a stylus for drawing or signature capture and character recognition, thanks to its 80:1 signal-to-noise ratio (SNR) and extremely fast refresh rate.

A high SNR is critical for the accurate reporting of adjacent or weak signals, and allows for precise reporting in noisy environments such as products with noise coupled from radio transceivers, LCD displays and battery chargers. Solutions without a suitably high SNR consume more power and decrease their response time with extra filtering and processing in an attempt to extract a weak signal from a high noise environment.

In contrast, the nearest competing off-the-shelf touchscreen solution has half as many nodes as the mXT224, a screen refresh rate of only 83 Hz (66% slower) and an SNR of only 25:1 (66% less). In addition to offering the best SNR rates in the market, maXTouch offers advanced noise suppression algorithms to provide end products with the ultimate immunity against coupled noise issues.

Mutual Capacitance and Charge Transfer. By combining Atmel's patented Charge Transfer and XMEGA technologies, he said, Atmel's engineering teams have produced an optimal and scalable architecture which enables smart processing of a capacitive touch image to accurately regenerate and report the user's interaction with the touchscreen.

Unlike self capacitance technologies that sense individual rows or columns leaving ambiguity in reported multi-touch positions, mutual capacitance sensors create a matrix of independent capacitive sensing nodes at the intersections of these rows and columns.

Using Atmel's maXTouch technology, each of these nodes are independently scanned within the matrix accurately sensing the position of an unlimited number of touches and delivering smooth movement in any location of the screen.

Processor Considerations. The processing efficiency provided by the XMEGA microcontroller, said Jones, allows the chip to ignore unintentional activity such as facial touch when on a call with a mobile phone.

The mXT224 integrates Atmel's single-cycle RISC AVR core with 32 registers and two on-chip DSP engines that process the X and Y positions on the touchscreen. An event system and peripheral DMA controller off-load all inter-peripheral communications and data transfer operations from the CPU, freeing it up for post-processing of the sensor image. This architecture enables the simultaneous processing of 224 nodes at 250 Hz, while consuming less than 5mW.

The mXT224 product is the first in a family of capacitive sensing controller solutions that eliminate the need for external components to support the capacitive sensing thereby minimizing cost and reducing PCB footprint requirements. It is available now in a 5x5mm BGA package and is priced at $4.75 in 1M quantity. Additional maXTouch touchscreen solutions will be introduced in the fourth quarter of 2009 and 2010.

To learn more, go to www.atmel.com

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