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Touch sensing software for MCUs cuts BOM cost

Posted: 09 Mar 2009 ?? ?Print Version ?Bookmark and Share

Keywords:touch sense software? MCU? touch control?

STMicroelectronics has released an open-source capacitive touch sensing software library for its 8bit STM8 MCU platform to help engineers implement touch-sensitive controls easily, with minimum additional cost and size.

Touch controls improve the look and feel of end products such as MP3 players, TVs, domestic appliances or printers, compared to mechanical switches or membrane keypads. Using touch controls also lowers BOM costs while enhancing reliability because they have no moving parts. Typical controls include single or multiple buttons, slider controls, or rotary touch sensors, and colored decals or screen-printed graphics can be applied to show their positions on the control panel.

ST's touch-sensing software library transforms any 8bit STM8 MCU into a capacitive touch-key controller. Designers can combine touch sensing functions through multiple configurations, such as touch keys, wheels or sliders, while preserving the traditional STM8 MCU features for the application firmware, including communication, LED control, beeper, and LCD control. The software is provided as royalty-free C source code and offers a maximum of flexibility to software engineers to configure, adapt or accommodate the library according to application needs.

Human touch is detected by monitoring the charge and the discharge timing cycle of a RC network formed by a single resistor and the touch electrode capacitance. The BOM is low as only one resistor is required by the touch channel. Depending on which STM8 variant is being used, as many as 24 individual touch pads can be supported. The touch sensing library only uses a small amount of the chip's memory and processing capability allowing designers to combine other application tasks in a single device. To optimize power consumption, the software also takes advantage of the STM8's low-power modes.

In addition, the touch sensing library embeds advanced features to help engineers overcome key challenges when implementing touch controls, such as configuring a combination of sensors, adjusting the sensing system, and ensuring robust operation throughout the lifetime of the end product. These include tools to help define keys, slider controls or rotary sensors, and to optimize touch sensitivity by adjusting parameters such as the touch-detection threshold and the resolution of sliders or rotary controls. There are also innovative programmable features to optimize measurement accuracy and make the final application immune from noise.

An embedded automatic calibration software layer saves final adjustment during manufacturing and enables faster time to market. To maximize long-term reliability the library contains an Environmental Change System (ECS). The ECS compensates for natural fluctuations in operating voltage and capacitance as well as environmental effects such as temperature and humidity.

As well as source code for the touch-sensing software library, full supporting documentation includes the API description, a user manual for the software library, application notes, and board-layout recommendations. Code examples for the STM8 are also included, and ST plans to introduce a touch-sensing evaluation board in Q2 09.





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