Go ahead and touch!
25 March 2010
Product designers and developers have various choices to make when considering controls for their appliances. A new Touch Sensor Controller greatly simplifies the development of such control panels by offering a component with simple flexible functions integrated into hardware. In this article Christian Harders, takes a brief look at some of its potential applications and the processes used.
Probably everyone has come in contact with them, knowingly or not, whether using a coffee maker, an MP3 player or a ticket machine. Capacitive touch sensors are rapidly replacing the mechanical buttons that have been so familiar over many years.
The advantages of touch sensors are obvious. Eliminating moving parts reduces wear and permits simpler designs for dustproof and waterproof control panels. Touch sensors also give designers greater freedom in appliance construction, which is a significant benefit for developing new white goods and portable consumer products, such as MP3 players and mobile phones. In addition, the substrate carrying the sensor electrode, the cover surrounding it and the front panel can be made of various materials. This means the sensor electrodes can be integrated flexibly in many different types of housing.
Capacitive sensors are the most commonly used. These employ a number of different processes to detect when the sensor surface/front panel is touched. The sensor field can be integrated in almost any form directly on to the circuit board or on to flexible foils. This makes unusual arrangements possible – a key factor in the growing popularity of these types of sensors. Because of the simple layout, this applies especially when the process involved does not require a reference electrode. Sensor sensitivity can be influenced by the size of the sensor electrode. So, for example, the different thicknesses and dielectric constants of control panel materials (sometimes several millimetres thick) can be evened out; while in ideal circumstances, fine adjustments can be achieved through software alone.
Many ways to achieve a goal
As the name implies, capacitive processes measure the capacity of each sensor surface. More precisely, they measure the change effected by a finger nearing the sensor.
There are many approaches to this. Some methods measure the capacitive coupling between two ‘send and receive’ electrodes per touch pad. When idle, this coupling depends mainly on sensor geometry and is increased by a finger nearing both electrodes. Unfortunately, fluids that cover both send and receive electrodes have a similar effect – a factor that must be recognised and accounted for by designers.
Other methods operate without a second electrode by measuring the electrode’s capacity directly; this can be achieved by incorporating it in an RC oscillator. If touched or approached by a finger, the capacity of the ‘imaginary plate capacitor’ formed by the electrode and finger increases as the distance decreases. This increasing capacity changes the frequency of the oscillator, which in turn can be detected. This method has the advantage that, under certain conditions, it can be used with standard micro-controllers; although a disadvantage is the often relatively slow response arising from the frequency measurement.
Other capacitive processes are based on the charge retention principle. They use a (known) reference capacity to measure the charge saved on the sensor pad. For this reason, the sensor surface is initially charged to a defined voltage. The charge, and therefore the sensor surface capacity, is then determined by means of controlled charge transfer processes between the two capacitances.
Digital capacity measurement
The FMA1127 Touch Sensor Controller (TSC) from Fujitsu employs technology patented by ATLab, Korea, which converts the impedance of the individual touch pads directly into a corresponding digital parameter. This is done by measuring the clock signal delay caused by the RC behaviour of the pad and comparing it with an integrated reference. This purely digital process avoids the many disadvantages of analogue-based methods.
Thanks to this, the FMA1127 not only detects the slightest capacity changes in the double-digit femtofarad range (and, at 0.2ms, react to it very quickly) but it can also be adapted to a wide range of applications by software configuration.
This, together with the reference impedances integrated on the chip, ensures a minimum of external components are required. For a great many electrode arrangements, only the TSC-IC itself and the 'infrastructure' for supply and communication are needed on the touch board; the adaptation to the sensor field is done in software. Only where the capacity difference of the sensor surfaces to each other (or in relation to the integrated reference) is too great are additional capacities necessary for adaptation. For example, this may be the case with large sensor surfaces and/or long pcb tracks between the TSC-IC and the sensor electrodes.
The extended ESD protection of the ICs (8kV HBM) and the integrated LDO (low drop-out) voltage regulator also helps to reduce the number of external parts required.
Additional features of the FMA1127 also reduce the software effort needed for stable operation - including automatic impedance calibration (AIC for short) of the individual touchpad. This algorithm, completely integrated in hardware, tracks the sensitivity of every touchpad to possible modifications in environmental conditions, such as temperature or humidity. It does this by monitoring the impedance at regular, configurable intervals and adjusting the respective reference when the pads are inactive. This guarantees a consistent sensitivity under many different conditions.
A further potential problem for designers arises from the difference between intentional and unintentional touching of the touch sensor panel. A special hardware function is available to deal with this. Called APIS (Adjacent Pattern Interference Suppression), this filters the raw data coming from the touch pad and, depending on the configuration, signals only the strongest, the two strongest or all signals above an adjustable threshold value to the host MCU. In this way, the application receives a clean, de-bounced signal so that, in conjunction with the FMA1127’s touch interrupt signal, the operation is as simple as using conventional mechanical keys.
If necessary, the strength of the capacity change, (and therefore touch), can be read out with a resolution of up to 256 steps from special registers using software. This is particularly useful to increase the resolution of scroll bars or wheels by interpolating the touch position between adjacent sensors.
Simple integration
Additional flexibility in system development is achieved by integrated IO pins. Depending on the chip housing, up to twelve pins can be configured individually as input or output - or allocated directly to the (maximum of 12) sensor inputs as visual feedback.
The pins used as input can signal a status change via the GINT signal to the host MCU, in order to integrate switches or additional conventional keys, for example. Another interesting feature is the potential to connect a conventional parallel LCD module via these GPIO pins. So, for example, the complete control unit of a vending machine, including an LC display, could be connected with only an I2C bus - helping to save costs by using fewer cables.
All of these features can be configured and used flexibly by over 120 registers from the host MCU via I2C. This enables the performance and sensitivity of the sensors to be adapted across a wide range using only software.
In order to simplify application design, Fujitsu offers various tools with the FMA1127 to support developers. A tuning kit enables them to calculate the optimum parameters for arranging sensors and so achieve results more quickly. Using a USB box on a PC, several parameters from the TSC’s internal registers can be read out and changed. Example software for various microcontrollers - notably the 16FX series from Fujitsu - is also available, as are application notes for designing circuit boards, and much more besides.
Conclusion
With the FMA1127 Touch Sensor Controller, Fujitsu offers the developer a simple and quick introduction to the world of capacitive touch sensors. The functions combined in the IC allow reliable touch sensor fields to be constructed with a minimum of hardware and software effort. The I2C interface also enables implementation with many different microcontrollers, so that existing applications can be converted easily. The wide supply voltage range of 2.4-5.5V and the very low power consumption of only 120µA (active) and 0.1µA (sleep) also facilitates mobile applications. Various development tools can also be used to facilitate speedy design implementation.
Christian Harders is Application Engineer at Fujitsu Microelectronics Europe
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