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Mobile handset taps capacitive-touch sensing

Posted: 09 Jul 2007 ?? ?Print Version ?Bookmark and Share

Keywords:capacitive-sensing user interface? mobile handsets? capacitive sensor?

By Mark Lee
Cypress Semiconductor Corp.

The capacitive-sensing user interface is beginning to emerge as a practical and innovative upgrade for mechanical buttons in mobile handsets. While the capacitive sensor can be viewed as a drop-in replacement for those buttons, this technology has more to offer than just being a stand-in for dome switches. When a handset is enabled with touch sensors, handset makers obtain exciting new look-and-feel options for their designs.

Using capacitive sensors, the handset buttons, known as the key mat, can be made with no moving parts, resulting in a smooth and sleek touch surface. Alternatively, a designer could opt for capacitive sensing on top of mechanical buttons, where a light touch would trigger the capacitive sensor and a heavy touch would actuate the mechanical switch.

The handset incorporating such technology would sense both the location of a finger and how hard it is pressing the button. A light touch could be associated with paging through a menu of phone numbers and a more forceful key press could initiate a call to the selected number.

One of the most interesting trends to emerge in handset design in years is the result of a combination of capacitive sensors and transparent conductors. The transparent key mat offers the handset designer many creative options.

SNR for capacitive sensing
The key to a robust capacitive-sensing design in a mobile handset is a high SNR. In electronic communications and other engineering fields, SNR is commonly measured in decibels. In finger-sensing applications, the decibel unit is not recommended for SNR measurements because of uncertainty in its calculation. The equation for decibel based on power is 10 log(P2/P1), and based on voltage magnitude is 20 log(V2/V1)). It is not clear which equation is more appropriate for touch applications. There is also confusion in the interpretation of "decibels of touch." To avoid those problems, Cypress Semiconductor has adopted a simple ratio as the preferred metric for capacitive-sensing SNR. The best-practice guideline offered by Cypress is to achieve a signal that is at least five times larger than the noise. Stating that in engineering terms, this is a minimum SNR of 5:1.

How to measure SNR
SNR in a touch-sensor application is measured in terms of counts at the sensor output. In a typical example, a finger not on the sensor results in peak-to-peak noise of eight counts. When a finger is placed on the sensor, the result is a signal of 118 counts. SNR is 118:8, which reduces to a ratio of 15:1.

SNR should be measured with best-case and worst-case fingers. A best-case finger would be a large finger centered on the sensor pad. A worst-case finger would be a small finger placed off-center. Using an actual finger is an acceptable method for early development of a handset system. A metal disk or rod can be substituted for a real finger if the developer wants to make the testing more operator-independent and repeatable.

Figure 1: Capacitive sensor technology has more to offer than just being a stand-in for dome switches in mobile handsets.

The overlay thickness has an attenuating effect on signal strength, so a conservative approach is to develop the system using an overlay that is slightly thicker than planned. To avoid masking effects of higher-level firmware, measure SNR using raw, unprocessed sensor counts. Turn off any self-compensating or autocalibration features that force the sensor output to zero when a finger is not present.

Create a noise budget
One way to manage the performance of the capacitive sensor is to create a noise budget, which involves making a list of noise sources that can decrease the SNR of the system. For mobile handsets, these noise sources include internal IC noise, RF noise and AC line noise. Estimate the effect on the sensor counts for each noise source. The sum of all these count values plus some extra counts thrown in for design margin should result in SNR greater than 5:1.

A mobile handset by nature creates an environment that is high in RF energy and this may have a bigger effect on the system than adding a few counts of noise to the system. The problem with operating capacitive sensors next to an RF transmitter is that a sensor trace can act as an efficient antenna. Coupling large amounts of RF energy into a controller IC can cause unexpected results in the sensor system, making touch sensing impractical. A simple solution to this potential problem is to damp the resonance using series resistors. A few hundred ohms in series with the sensor inputs, placed as close as possible to the pins of the control IC, are enough to prevent this problem from occurring.

Mobile implies low power
The power consumption in a capacitive-sensing solution for mobile handsets must be low. As with any battery-powered mobile device, the low-power goal dictates that the controller should report to the host no faster than required, should scan the sensors no longer than required and should sleep if no other events are pending.

The key to long battery life is to minimize the levels of average current that flow when the sensor is actively scanning and processing data. Average current is computed using a simple time-weighted average of the active current and sleep current, so the longer the controller is sleeping between scans, the longer the battery life.

A practical limitation to long sleep intervals is system latency, which is the delay between the touch event and the response of the system to the touch. A nontechnical user would describe a high latency as slow or sluggish buttons. In the extreme, very long sleep intervals would result in buttons that did not make contact some of the time.

Figure 2: SNR in a touch-sensor application is measured in terms of counts at the sensor output.

The challenge in handset design is to find a good balance between fast sensor response and low power consumption. Latency of 30-50ms is a good goal for mobile-handset design. To lower the power consumption even further, it is common for the developer to have the sensor enter a longer latency mode if there has been no user input for an extended period. This slower scanning mode, called standby, has a latency of 100ms or more. As soon as user input resumes, the system enters the active-scanning mode that has more-responsive buttons.

The following example calculation shows how to reach an average current in standby mode of just 33?A in a 12-sensor handset design. The equation for average current, IAVE, is shown in equation 1. Scan time is set to 0.5ms per sensor (t1 = 12(0.5) = 6ms). The report rate in standby mode is 100ms, so the sleep interval is set to 94ms (t2 = 100ms - 6ms). Sleep current and active current are read from the controller IC data sheet (ISleep = 3?A, IActive = 1,500?A). Evaluating equation 1 with these parameters yields an average current of 93?A.

If only a subset of sensors is scanned in standby mode, then the average current can be reduced even further. Arranging the 12 sensors in groups of three reduces the scan time (t1 = 12/3(0.5) = 2ms). The average current in this case drops to 33?A.

Mechanical considerations
Mechanical stackup is an important consideration in the system's design since the packaging trend in mobile handsets is toward thinner packages. In fact, poor layout of the sensor traces and excessive overlay material thickness are the leading causes for low SNR in mobile handsets.

The circuit board is typically a flex circuit or, in some cases, a thin rigid board. The circuit board is mounted on the overlay using a thin layer of nonconductive adhesive film, which improves the coupling of the electric field from the sensors to the overlay. The adhesive layer also creates a stable mechanical system that has consistent response for both light and heavy finger pressure. A target overlay thickness of 1-3mm gives the handset package mechanical strength without excessively attenuating the capacitive-sensing signal.

Programmable solution
When it comes to programming, the system controller has a number of options. At the application-specific end of the spectrum are the fixed-function devices that do nothing but scan sensors and output data. At the highly integrated and flexible end of the spectrum are programmable sensing devices that allow a wide range of capacitive-sensing functions to be implemented, including buttons, sliders, touch pads and proximity sensors.

Additionally, this flexibility both simplifies last-minute design changes as well as supports noncapacitive sensing functions that are typically done by one or more devices. For example, a handset may require functions that include capacitive sensing by the key mat, ambient light sensing through a photodiode, tilt sensing via an accelerometer and motor driving in a small motor that runs when the handset is set in vibrate mode. All of those functions can be integrated into a single chip with flexible software development done in C.

As another example of the value added by a programmable approach, let's consider the following scenario. Since all sensing and control functions are under software control, it is possible to configure the capacitive sensors for proximity detection during low-power standby mode, and to reconfigure the same sensors as touch sensors in normal operation mode. In standby mode, the proximity sensors are scanning for the presence of a finger in a zone of 1cm or 2cm above any of the capacitive sensors.

When an approaching finger is sensed, the sensors can be reconfigured in software so that the proximity-sensing function is replaced by the touch-sensing function. The handset will continue in this mode of operation until the user stops interacting with the capacitive sensor. When that happens, proximity sensors set the handset back to standby mode.

Transparent capacitance
The latest trend in touch sensing in mobile handsets is the use of indium tin oxide (ITO) on glass or plastic film. ITO is a conductive material that is transparent when applied as a thin film. It has been used for years in resistive touchscreens. Recent advances in MCUs have made capacitive versions of the same touchscreens practical. Since resistive touchscreens rely upon mechanical deflection of the touch surface, they eventually wear out and need replacement. Capacitive versions of the ITO touchscreen do not require mechanical deflection. Elimination of this mechanical failure mode is one of the advantages of capacitive-based ITO touchscreens over the standard resistive ones. n

About the author
Mark Lee
is a senior applications engineer at Cypress Semiconductor Corp. in Lynnwood, Washington. He earned a PhD in electrical engineering from the University of Washington.

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