Making capacitive touch sensors water tolerant

When a capacitive touch screen goes into sleep or standby mode to save energy, how can you design the system to wake up quickly without degrading its performance or burning a lot of power. Here are two options: a traditional method and a new MCU-based method.
Touch-sense keys are gaining popularity with both end users and ***a***developers of everyday human-interface applications because the technology is aesthetically appealing, easy to use, and avoids any mechanical design. In particular for developers, capacitive touch sensors are popular because they can be implemented using a copper pad as part of the standard printed circuit board’s design.

Employed as a user interface to electronic equipment, capacitive touch sensors may be arranged in a keypad matrix and are activated (controls a signal indicating activation) by a change in capacitance of the capacitive touch sensor when an object, usually a user’s fingertip, causes the capacitance of the capacitive touch sensor to change (see sidebar on for details).

The basics of capacitive touch-sensing technology

The recent success of the capacitive-sensor scroll wheel, now being used in quite a few devices, has given this technology an edge over other touch-sense technologies.

When any object with capacitive characteristics-such as a finger-comes close to a capacitive touch sensor, it acts as another capacitor due to its dielectric nature. This varies the effective capacitance of the system, which is used to detect the touch.

The finger acts as one of the parallel plates, while another parallel plate is connected to the sensor input ***a*** of the chip, as shown in the illustration below. The iron content in human blood creates strings of capacitors that are aligned to the surface of the body. When such strings of capacitors come in proximity with a conductor, a capacitance that is essentially coupled to ground is created, which causes a change in the measured voltage, deter ***a***mining the touch.

Making capacitive touch sensors water tolerant
Click on image to enlarge.

A typical capacitive touch-sense system is composed of three main functional blocks:

•    An analog block for capacitive sensing.

•    A controller for processing the data.

•    An interface block for communicating with a host processor.

Generally, the keypad matrix of capacitive touch sensors is fabricated on a substrate with a protective covering, such as glass or a clear plastic resin cover, over the capacitive touch sensors. The protective covering may also have alpha-numeric characters thereon, to identify the purpose of each of the associated capacitive touch sensors.

When an object having capacitance, such as a user’s fingertip, comes in close proximity to the sensor element, the capacitance value of the sensor element changes. This capacitance change is electronically detected so as to generate a signal indicating activation of that capacitive touch sensor by the object in close proximity thereto. This electronic detection must be performed by an electronic device, which requires power to operate.

But there is a problem. Current technology requires that the electronic device, when in a sleep mode, be awakened before it scans the keypad matrix of capacitive touch sensors. When the electronic device is in a low-power standby or sleep mode, one of two things occur: either response time to the detection of the capacitance degra ***a***des or power consumption of the electronic device suffers.

Clearly, what is needed is a way to provide automated scan and control of capacitive keypads-irrespective of whether the major power-consuming logic circuits of an electronic device are in a run, idle, or sleep mode. Being able to scan and control the capacitive touch sensors of the keypad matrix will allow the power-consuming logic circuits of an electronic device to remain in the low-power sleep mode until awakened for processing data or controlling a function. Not having to program for the various power modes of the electronic device simplifies the code in the user software (firmware) application.

This article describes two approaches, both based on voltage variation, that can be used for implementing capacitive touch sensing.

The first approach is implemented with a charge time measurement unit (CTMU) peripheral that can be implemented on-chip with the microcontroller. Cap-touch solutions using the CTMU peripheral will have a faster response, as it has different ranges of current source that help to charge the analog channels at a faster rate-thereby improving the response time of the cap-touch system.

The second approach is the more traditional one which uses the capacitance voltage division (CVD) technique, which uses a microcontroller’s on-chip analog-to-digital (A/D) converter and doesn’t need a dedicated capacitive-sensing peripheral. The CVD technique is widely used, since it can be implemented with any microcontroller that has an A/D converter. However, the response time is slower when compared with implementations using a CTMU.