STMicroelectronics Unveils Microcontroller-Based Sensing Capability
STMicroelectronics has announced the availability of its open-source capacitive touch sensing software library for its 8-bit STM8 MCU platform. Touch controls improve the look and feel of end products such as MP3 players, televisions, domestic appliances, or printers, compared to mechanical switches or membrane keypads. Using touch controls also lowers bill-of-materials 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.STâ€
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 Bill-of-Materials 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 chipâ€™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 STM8â€™s low-power modes.
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 2009.
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