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Design capacitive touch sensors the easy way (Part 2)

18 Dec 2014  | Shruti Hanumanthaiah, Subbarao Lanka

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The tool-box also uses the system properties as inputs and recommends the maximum trace length and sensor size a design can have, based on the maximum sensor parasitic capacitance supported by the system.

 Button diameter and trace length estimator

Table 1: Button diameter and trace length estimator in Design Tool Box.

Considerations before moving to PCB manufacturing
Sensor characteristics: Once the layout is completed with the recommended values of sensor size and trace length, verify your design with the aid fo the tool-box using the actual values before moving to PCB design. Use the design tools to calculate the approximate parasitic capacitance based on inputs that you provide, such as the trace length and sensor size. The tool will then provide a warning if the parasitic capacitance of a sensor is out of the acceptable operating range.

 Parasitic Capacitance calculator

Table 2: Parasitic Capacitance calculator in Design Tool Box.

Power consumption optimisation is another critical requirement for most designs. The design tools can also be designated to estimate the power consumption of the device based on the device parameters. This helps designers to adjust each parameter to reduce overall power consumption as needed.

 Power calculator

Table 3: Power calculator.

Response time, the length of time it takes a sensor to recognise a touch, is another critical parameter that can be estimated. Developers can verify the response time of sensors based on the device parameters and adjust the parameters to bring the response time within the set requirements when necessary.

There are other general guidelines to be followed while designing layout, including ground fill in different layers, via size, trace thickness, etc. These are straightforward recommendations that are not as involved as those outlined in this section. Your vendor will provide design guidelines and best practices. All of these should be followed for the most robust capacitive sensing design.

Step 3: Create the configuration for your design
Once the hardware aspects of the design (schematics and layout) are ready, the next step is to create the configuration for your design. MBR devices are plug-and-play so, instead of requiring complex programming, they give developers flexibility through the ability to configure the features of the device specific to each application.

MBR devices expose the configuration of their features to the developers through a set of registers that can be accessed via communication protocols like I2C. Developers write the configuration into these registers and send appropriate commands to the device to save the configuration to a non-volatile

memory and operates based on the configuration.

There are some advanced and complex features tha are often a part of the user interface with capacitive sensing buttons. These features, however, are not easy to design and develop. With an MBR device, these features are already implemented in the device and can be customised through configuration for each application.

Now let us look at some example applications and explore how to choose features on an MBR device for a particular application. We will also see how to configure these features.

Touch buttons in mobile phones
Touch buttons in mobile hones are used for high-level functions such as quick access of various applications and accessing various key screens such as Menu or Home. A typical mobile phone has up to three touch buttons and requires LED backlighting. In addition to buttons and backlighting, current mobile phone designs use proximity sensors to detect when a user lifts the phone near his/her face. Once the face is detected, the touch screen is turned off so that there are no false touches of the icons on the screen by the face.

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