Design Automation


Open-source touch sensing library

1 April 2009 Design Automation

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

The software library transforms any 8-bit 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 engineers the flexibility 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 an RC network formed by a single resistor and the touch electrode capacitance. Depending on which STM8 variant is 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 optimise power consumption, the software also takes advantage of the STM8’s low-power modes.

The touch sensing library embeds advanced tools to help define keys, slider controls or rotary sensors, and to optimise 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 optimise measurement accuracy and make the final application immune to noise.

An embedded automatic calibration software layer saves final adjustment during manufacturing and enables faster time to market. To maximise long-term reliability the library contains an environmental change system (ECS), which 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, as well as code examples.



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