Publication date: 26 July 2010
Capacitive touch has found a spectrum of application domains. From portable consumer products including mobile phones and PDAs, automotive, kitchen appliances, medical to industrial and commercial sensing, the intuitive capacitive touch based solution has made its mark as a preferred human-machine interface. The robust and reliable capacitive touch solution replaces conventional resistive sliders, mechanical buttons and rotary controls.
‘Charge transfer’ -based capacitive touch acquisition overview
‘Charge transfer’ based capacitive touch acquisition can be done in the following two methods.
The first approach involves charging a sense electrode of unknown capacitance to a known potential. The electrode is typically a copper area on a printed circuit board. The resulting charge is transferred into a measurement circuit. By measuring the charge after one or more charge-and-transfer cycles, the capacitance of the sense plate can be determined. Placing a finger on the touch surface introduces external capacitance that affects the flow of charge at that point. This registers as a touch.
The second approach uses a pair of sensing electrodes. One is an emitting electrode into which a charge consisting of logic pulses is driven in burst mode. The other is a receive electrode that couples to the emitter via the overlying panel dielectric. When a finger touches the panel the field coupling is reduced, and touch is detected. The drive, receive and processing logic is built into the MCU and so very few external components are required.
Both approaches offer a unique set of advantages and can be suited to a specific application.
Touch functions derived from a ‘Charge transfer’ based capacitive touch acquisition method can be classified as touch screens, touch buttons, sliders and wheels.
The touch screen function supports an unlimited number of touches that greatly enhances the user experience and changes the way users interact with electronic products. Built in gestures and the ability to ignore unintentional touches result in a user interface that is intuitive and reliable. Touch screens that can recognize input from a stylus, fingernails and gloves, provides easy text entry on a handheld device.
Touch buttons, sliders and wheels typically cater to a single user touch and use algorithms to determine touch status and position, independent of the signal strength that results in accurate and reliable touch detection. The touch buttons, sliders and wheels function can be incorporated in product design in two forms –
• Fixed function device solution• Microcontroller Touch library solutionDepending on specific product design requirements the desired touch function and solution can be chosen.
The typical use case scenario for touch buttons, sliders and wheels are captured in the table below.
Product design need
Capacitive
Proximity keys
Capacitive touch buttons
Capacitive touch sliders
Capacitive touch wheels
Use case
Proximity keys are typically used for user presence detection. A Bluetooth headset or a TV remote control turns ON automatically when picked up.
Capacitive Touch buttons are discrete touch buttons. A simple use case here can be a Play/Pause button in a media player.
Capacitive touch sliders replace the variable resistor sliders. Common use can be as a volume control in a media player.
Capacitive Touch wheels replace rotary switches. Common use case is for song selection scrolling in a media player.
Typically capacitive touch sensors are connected to a microcontroller by means of channels. Figure 3 indicates a proximity key sensor using one channel, a touch button sensor using one channel and a slider/wheel sensor from a group of three channels. The microcontroller port pins can be used as a touch sensor channel pin.
One solution available today is a fixed-function touch device. ‘Fixed-function’ here indicates that a set of devices can be used only to process the touch sensors. Hence, a fixed-function device is a dedicated touch microcontroller solution. The fixed-function touch device can provide single-channel or multi-channel support. The touch sensor arrangement in a fixed-function device is usually pre-configured to work as a button, slider, wheel or a fixed combination of these.
Fixed function devices typically update the touch status information to a Host microcontroller. This is usually achieved using a serial interface, typically an I2C compliant interface. Various other interfaces such as SPI, USART and bit banging are also possible. Touch status reporting in fixed-function devices commonly includes proximity or touch button ON/OFF status, a linear touch positional value of a slider and an angular touch positional value of a wheel.
The fixed-function touch device can also come with an additional feature, whereby, instead of using the serial interface, strobes and pulses can be output on the device pins to indicate a certain result. This means that the ON/OFF status of a button or the positional information of a slider or wheel need not be the only output information from a fixed-function touch device. A secondary level of processing can be done on this data and the resulting secondary output can be suited to the application need. For example, in power control of appliances, the touch button status information can translate into a programmed auto-off delay, output as a trigger pulse in one of the device pins to switch off the appliance.
Some of the characteristics of fixed-function devices are captured below.o Ready to use capacitive touch solution o The fixed-function microcontroller can come with factory programmed firmwareo Less effort required to include this solution into the product design in comparison with the microcontroller touch library solutiono Limited user configurability on the output interface and sensor arrangemento Since a host microcontroller does the main application task and the fixed function touch device is brought in to add capacitive touch support, this approach involves additional microcontroller count to product design
The touch library offers users a desired set of touch buttons, sliders and wheels function that can be linked into the firmware in order to provide touch sensing capabilities to a project. The ‘Charge transfer’ based touch acquisition facilitates support of capacitive touch in general purpose microcontrollers by using commonly available on-chip hardware to process the touch sensors. Microcontroller vendors are able to support this solution as part of their standard microcontroller product line. This gives product designers the opportunity to use an off-the-shelf touch solution.
In comparison to the fixed-function device solution, the touch library microcontroller solution can provide added flexibility to the hardware and firmware design. Using the touch library API, the user can arrive at any desired custom configuration of sensors. The touch library also provides the user the facility to choose a desired set of port pins to be used as touch sensor channels on a standard microcontroller of choice. Also, processing touch can now be a part of a main user application task.
This user configurability provided by a touch library microcontroller solution can play an important role in upgrading an existing design and adding capacitive touch functionality. The general purpose microcontroller in an existing design can be re-used. The port pins used for mechanical buttons, sliders or wheels in the current design can instead be configured to support capacitive touch using the touch library. The familiarity with the usage of the MCU and the awareness in the usage of associated microcontroller tools can come as added benefits here. The touch library can therefore be used to develop a single chip solution for many control applications, or to reduce chip count in more complex applications.
The application-specific microcontroller case is an interesting extended benefit of the touch library solution. Automotive, wireless, LCD and USB MCU for example have a dedicated microcontroller product line. The ‘Charge transfer’ acquisition based touch library enables the possibility to accommodate capacitive touch onto this set of microcontrollers opening up new product design avenues.
Finally, many products use a host microcontroller and a dedicated controller for the touch functionality. By integrating the touch functionality into the host microcontroller, both board space and product cost can be reduced. The on-chip touch hardware in this host microcontroller case allows for minimal software overhead by the touch library.
Some of the points to consider in the case of the touch library based microcontroller solution are captured below.
• Royalty free software library for developing touch applications.• Key-count to chip pin-count efficiency -- the number of microcontroller pins required to support a desired number of touch sensors varies depending on the ‘Charge transfer’ approach chosen to measure capacitance• Touch library code memory and data memory consumption.• Processor MIPS consumption of the touch library.• On-chip peripheral resource requirements to process touch sensors.
The fixed function devices and touch library solution provide debug information that allows users to monitor real time data related to a touch sensor. Information such as signal, reference and delta values corresponding to capacitive measurements corresponding to a touch sensor as well as ON/OFF status and positional information can be relayed using a serial debug interface. The PC software tool allows users to capture this debug information that aids in monitoring sensor functionality as well as tuning certain sensor parameters for optimized touch performance.
In essence, product designers have a wide range of microcontroller-based capacitive touch solutions to choose from. Fixed-function devices provide users with a ready to use capacitive touch solution. The touch library provides user customizable touch functions on general purpose, application-specific and host microcontrollers catering to product needs across different application domains. PC software tools aid in the integration of touch technology making it very easy to implement.
