In recent years, capacitive touch interfaces have become commonplace on products that have traditionally used mechanical buttons. Products as diverse as remote controls, wall-mounted panels, white goods appliances, and of course smart phones all now routinely use capacitive input surfaces. Despite the popularity of capacitive sensing surfaces, designers should weigh both the advantages and disadvantages of capacitive sensing technology against older mechanical button technology before deciding on the best interface for their products.
In many cases, mechanical buttons require less current to monitor than capacitive sensing inputs because mechanical buttons can be tied to port match wake-ups that allow the system to be in an extremely low power state without waking for any monitoring or maintenance. Mechanical buttons are also generally less susceptible to generating false positive touch events due to coupled interference on the MCU’s input pin. This is because mechanical buttons are monitored using digital input pin logic, whereas capacitive sensing solutions require more sensitive analog hardware. Finally, mechanical buttons tend to do a better job of sensing touches from gloved hands, because many fabrics electrically insulate fingers and prevent capacitive sensing solutions from detecting them.
Capacitive sensing inputs became a popular choice for interface design because of the flexibility offered by capacitive sensing input surfaces compared to mechanical inputs. With capacitive sensing inputs, buttons can be shaped in a wide variety of ways, and slider and control wheel designs can be implemented in a way that can be visually attractive while not adding additional manufacturing burden. A capacitive sensing input surface enables designers to create attractive and distinct products.
Capacitive sensing inputs also mean fewer mechanical parts on a bill of materials relative to a design with mechanical buttons. Many capacitive sensing interfaces consist only of a PCB with electrodes, an acrylic overlay, and a decal adhered to the overlay. This reduced BOM count can lower manufacturing cost and time. Fewer mechanical parts also means lower risk of failure in the field, as capacitive sensing buttons don’t wear out and break like mechanical buttons do.
Mechanical and capacitive solutions both have susceptibility to moisture, however. Mechanical button designs that are not waterproof can be permanently damaged when water slips into the product’s circuitry, often through gaps in the mechanical button components of the interface. Capacitive sensing solutions are often more robust against permanent damage because the acrylic overlays are often a solid acrylic material with no gaps or seams where moisture can enter. However, capacitive sensing solutions can detect false positive touch events due to moisture on the sensing surface without sophisticated firmware-implemented countermeasures.
Silicon Laboratories’ capacitive sensing technology minimizes some of the potential disadvantages of capacitive sensing interfaces. Low power functionality on capacitive sensing MCUs provides an average current draw of less than 1 uA in most applications, which matches the lowest power state of many MCUs monitoring mechanical buttons in sleep mode. The capacitive sensing firmware library, aided by a high frequency on-chip system clock and large Flash/RAM footprint, provides the processing and buffering horsepower to guard against an array of interference sources that could lead to false positive touch events in other MCUs.
Developers decide to design with mechanical buttons or capacitive sensing buttons based on a set of criteria ranging from use cases and environments to ‘look and feel’ design choices. Silicon Labs’ capacitive sensing MCUs provide a robust capacitive sensing option that mitigates potential use case disadvantages and enables developers to concentrate on look and feel design choices.
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