Atmel Touch Controller – CES
January, 2012 We talked with Sherif Hanna about the release of extensions and upgrades to their touch controller product line. The changes address the areas of noise, resolution, for the larger screen product and a reduced parts count.
The division grew from $140M in ’10 to over $300M last year. Now, they are working to address the changing functions in mobile devices. The biggest changes are in screen size and resolution, so the S series of products have been architected to improve overall performance. For phones, manufacturers are trying to make a more immersive experience using a thinner, lighter package, while keeping battery life as long as possible.
The mobile devices offer some capabilities for creation, but the touch controller affects overall performance. Some of the issues affecting touch screens include display refresh noise and charger noise both of which impact sensor accuracy and resolution. The increased number of I/Os on the screens exacerbates the problems. The signal/noise ratio is a function of drive voltage, so the new chips can handle up to 24 V for scanning, up from the 2.7 V currently in use.
The mXT224S is designed for phones with from 3.5- to 4.3-inch screens. The 336S is for phones with 4.3- to 5.5-inch screens and the 1664S is for tablets with screens up to 17 inches. The new parts offer higher noise immunity, faster performance, and an improved ease of design-in. The underlying algorithms have been improved and the number of drive-sense lines increased for higher node density and better positional accuracy. The firmware is embedded on the chips and modular functions are accessed through function calls from the system processor. The firmware is customizable through a GUI.
The analog circuitry has been redesigned to handle higher drive voltages and improve the common-mode rejection of display and charger noise. New DSP blocks and firmware allow for the elimination of a ITO layer for shielding, while accelerating the response functions. The driver voltage can be completely asynchronous to the display, further reducing display driver influence. The screen touch sampling is adaptive and uses a one time calibration process. Active noise cancellation is dynamically changing after turn-on of the device.
The new algorithms do in-place and on-going baseline calibration to modify the scan sequence for greatest signal with minimal noise. The DSP processes all the data coming from the sensors and subtracts out steady-state and sub-threshold motion artifacts for better noise immunity. The A/D converters and other analog circuitry clamp noise spikes to prevent saturation and signal the DSP to ignore the sample. The system firmware detects interference and dynamically changes the scan frequency to reduce synchronous interference.
Charger noise can be up to 240 Vp-p between 1 and 50kHz and 40 Vp-p form 50 kHz to 1MHz. This noise is generally common mode and is coupled to the sensors through the floating ground associated with the user. This function is only turned on when a charger is attached and supplying power. This allows simultaneous operation and charging even with noise after market chargers.
Coupled together, the new chips offer high node density, low power consumption, operation with Windows Phone and Android systems, and allow the use of an active or passive stylus. These improvements eliminate the separate noise analysis channel and permit touch on lens or sensor on lens touch stacks, which are thinner and less expensive to produce. The benefits for the user are brighter screens, lighter devices, and longer batter life.
