CapSense - Simple, Flexible, Integrated

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Elegance and Durability

Capacitive sensing is fast becoming the solution of choice for front panel display and media control applications. Popularized by leading edge music players, mobile phones and white goods, capacitive sensing touchscreens, touchpads, keypads, sliders and buttons offer a cleaner, more modern design as well as a more durable, hermetically sealed solution for almost any electronic product.

 

Flexibility and Integration

The Cypress CapSense capacitive sensing interface is the industry’s most flexible. Based on the popular PSoC® architecture, Cypress CapSense solutions allow users to modify hardware up to and even during production. And unlike “hard” touch-sensing alternatives, CapSense PLUS enables designers to integrate multiple system functions such as LED driver/backlight control, motor control, voltage monitoring, temperature sensing and control, power sequencing, I/O expansion, gyro and accelerometer sensing, and ambient light sensing. PSoC CapSense devices also provide popular communication interfaces such as I²C, SPI, UART and USB. By integrating these mixed analog and digital functions into a self-contained programmable system on-chip, you can minimize time to market, board space and system cost for your CapSense-enabled product.

 

 

Easy to Use Tools

Cypress offers two software tools for its PSoC products. The full featured PSoC Designer™ tool offers a complete integrated development environment while the revolutionary PSoC Express™ visual embedded design tool allows designers to implement CapSense without writing a single line of code. Both provide easy, real-time monitoring and calibration with pre-engineered user modules and drivers for two capacitive sensing methods— CapSense Sigma-Delta (CSD) and CapSense Successive Approximation (CSA).

 

 

Two Industry-Leading CapSense Methods

The CSD method enables buttons, sliders, touchpads and touch screens to operate in wet conditions. High-level decision logic provides compensation for environmental factors, such as temperature, humidity and power supply voltage change. A separate shield electrode can be used to reduce stray capacitance, providing more reliable operation in the presence of water film or droplets. The CY8C21x34 and CY8C24x94 device families and the CY3213A-CapSense development kit are optimized for the CSD method.

The CSA method delivers industry-leading interference immunity and power efficiency, making it ideal for portable consumer applications. The CY8C20x34 device family and CY3203A-CapSense development kit are optimized for the CSA method.

 

 

 

CapSense Successive Approximation (CSA) Training Kit
CY3203A-CAPSENSE

 

The CY3203A-CapSense Training Kit includes software, hardware and example projects to help designers learn how to implement PSoC CapSense in their own design using the CY8C20x34 family. A training board is included that is hardwired for buttons and sliders as well as LCD control and I²C communication. The training kit includes the following:

• Training board (CY8C20x34)
• PSoC Designer and Example Project CD
• CSA user module
• Mini programmer unit
• LCD module
• USB cable

 

CapSense Sigma-Delta (CSD) Training Kit
CY3213A-CAPSENSE

 

The CY3213A-CapSense CSD CapSense Training Kit includes software, hardware and example projects to help designers learn how to implement PSoC CapSense in their own design using the CY8C21x34 device family. A training board is included that is hardwired for buttons and sliders as well as LCD control and I²C communication. The training kit includes the following:

• Training board (CY8C21x34)
• PSoC Designer and Example Project CD
• CSD user module
• Mini programmer unit
• LCD module
• USB cable

 

 

Technology
Sensor technologies allow designers to add a simple and robust interface to otherwise mechanical applications.
Several sensor technologies exist today such as resistive film sensing and electric field sensing; each has benefits and drawbacks.

The most popular sensor technology is resistive film sensing, where two conductive, resistive plates are separated by a space (as shown in the diagram above). When one of these plates is contacted, the resistance of the system increases. The voltage across the system increases in turn. Measuring the voltage on the driven axis allows for positional measurement. This process is repeated on the other axis to give a two-dimensional measurement. One of the drawbacks of this sensor technology is that is relies upon the elastic properties of the film to return to a known state when contact force is removed. Eventually the elasticity of the sheet will degrade and the sensor will become unusable.

In electric field sensing, the system emits a weak electric field from an antenna, then looks for changes in currents at one or more receivers. Practical systems require multiple transmitter and receiver locations; this adds to the cost of systems based on electric field sensing.

A capacitive sensor detects the proximity of a conductive object. The capacitance measured by the sensor is a function of the distance from the sensor to the object. A capacitive sensor based on a relaxation oscillator requires a number of other support functions to be practical, such as a programmable current source, an analog multiplexer, and an auto-calibration system. This kind of sensor design is now greatly simplified by mixed-signal programmable system-on-chip devices.

 

Capacitive Sensing
Capacitive sensing is used in interface applications to build non-contact switches (or sensors). When protected by an insulating layer, capacitive sensors provide an elegant design overlay and provide robustness in severe environments.

Very simply, a capacitive sensor is a pair of adjacent plates (electrodes).

When a conductive object is placed in proximity to these electrodes, there is capacitance between the electrodes and the conductive object. The conductive object is a finger in the following case, though this technique could be applied to any conductive object. Examples include conductive door plates or position sensors.

The most common form of capacitance sensor array is a set of capacitors where one side of each is grounded. The presence of a conductive object increases the capacitance of the switch to ground, and determining sensor activation is only a matter of measuring change in capacitance.

 

CapSense
PSoC® CapSense takes advantage the unique features of the PSoC to enable efficient design of capacitive sensor scan applications. These features include:

• Wide analog MUX allowing all channels to be serviced by a common comparator and current source
• Internal programmable current source
• Automatic connection of comparator to sensor discharge switch

The PSoC architecture allows designers to incorporate multiple capacitive sensing design elements into an application. Buttons, sliders, touchpads and proximity detectors are supported simultaneously with the same device in the same circuit.

Internal hardware eliminates the need for external components to set charging current or to calibrate sensitivity.

The flexibility of PSoC and CapSense allows designers to move across platforms and adapt to design changes quickly. All calibration is completed in software through an easy-to-use, graphical development environment. Application changes do not require migration to other devices because PSoC is highly configurable.

Non-CapSense applications are easily accomplished using PSoC's digital and analog resources. Use PSoC to scan switches and use the activation status to drive LEDs, control a motor, drive a speaker, etc.

 

Relaxation Oscillator
The PSoC® architecture supports a relaxation oscillator technique for capacitive sensing. The relaxation oscillator is built from a current source (A), a sensor capacitor (B), a comparator (C), and a discharge switch (D).

The sensor capacitor is charged by the current source. When the voltage across the capacitor reaches the threshold, the comparator is tripped, the discharge switch is closed, and the sensor capacitor is discharged. This creates a saw tooth waveform across the capacitor similar to that shown below:

This saw tooth waveform is used to clock a pulse-width modulator (E) which, in turn, enables a 16-bit timer (F).

Together, these circuits create the relaxation oscillator capacitive sensing circuit.

As a finger approaches the sensor, the capacitance of the sensor increases. A larger capacitor takes more time to charge given a constant current. Since the pulse-width modulator is clocked by this signal and enables the timer, a longer charge time on the sensor capacitor yields a longer high-time on the pulse-width modulator and larger value from the 16-bit timer. If the difference in counts from one scan to the next exceeds a predetermined threshold, the presence of a conductive object is detected.

 

Buttons
CapSense buttons represent the most basic function of a capacitive sensing application. Detecting the presence or absence of a conductive object (such as a finger) can be easily accomplished through a variety of materials and thicknesses.

Use CapSense buttons for media, volume, brightness, power status, and other control functions. CapSense buttons can replace discrete, mechanical buttons in virtually any application.

CapSense buttons are capable of sensing though up to 5mm of plastic or glass.

 

Sliders
Sliders offer a higher level of functionality to your interface design. Use a slider to discern position to a much greater resolution than is capable from the individual sensor elements alone. Sliders are capable of providing a resolution 100 times greater.

This increased resolution is made possible using a mathematical operation called interpolation. Capacitance change is measured on all slider elements and the capacitance values on adjacent elements are used to determine position.

It is also possible to achieve even greater I/O efficiency or resolution by diplexing pins. Diplexing is a method where each PSoC® CapSense pin is attached to two sensor elements. These sensor elements are ordered in a scattered fashion to eliminate confusion as to which side of the slider is active.

 

Proximity Detection
Capacitive sensing is by definition proximity detection. For standard buttons, the thickness of the overlay is the proximity setting. The sensor response is highest when a finger is pressing on the overlay.

In true proximity sensing, no contact is required between the sensor overlay and the user's finger or hand. In this application, it is necessary to increase the sensitivity of the sensor over that required for buttons. Increased sensitivity is realized by acquiring data from the sensor for a greater time. Longer acquisition times allow very small changes in capacitance that arise from more distant conductive objects to be magnified.

Obviously, when the acquisition time is increased for such applications, the update rate is slower. However, proximity detection applications require that sensors only detect presence, not fine, rapid movements. Therefore, it is possible to detect conductive objects over greater distances while achieving the kind of update rate and response time that proximity sensing requires.

There are also some changes that can occur in hardware to increase the sensitivity of a proximity sensing apparatus. Larger sensors have greater sensitivity to larger conductive objects, such as a hand. Removing the ground plane from the underside of the sensor PCB increases the sensitivity, but allows the field lines to direct themselves toward the user, rather than toward a ground plane. Increasing the space between the sensor and the surrounding ground plane also directs field lines toward the user.

Cypress currently offers two PSoC® device families providing CapSense technology. The CY8C21x34 family has up to 28 CapSense inputs and supports communication via I²C, SPI, PS/2, or UART. The CY8C24x94 family has up to 48 CapSense inputs and adds Full-Speed USB to the list of communication interfaces supported.

 

 

 Featured Products

Part Number		Description	Data Sheet	App. Notes
CY3203A-CAPSENSE		CapSense Successive Approximation (CSA) Training Kit
CY3213A-CAPSENSE		CapSense Sigma-Delta (CSD) Training Kit

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