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20 Document No. 001-64846 Rev. *I Getting Started with CapSense

2.5.1.2 Matrix Buttons

In applications requiring a high number of buttons such as a calculator keypad or a QWERTY keyboard, capacitive

sensors can be arranged in a matrix. This allows a design to have more buttons than there are I/O pins on the

CapSense controller.

Figure 2-14. Typical Matrix Buttons

A matrix button design consists of two groups of capacitive sensors: Row sensors and Column sensors. When a

button is touched, it can be resolved by identifying the row and column sensors that are both in the TOUCH state.

The number of buttons supported by the matrix is equal to the product of the number of rows and the number of

columns.

     Equation 8

Using a matrix button design can significantly reduce the number of I/O pins required. For example, the matrix in

Figure 2-14 implements 12 buttons, but requires only seven I/O pins for sensors. Additional dedicated pins need to be

assigned to external components, depending on the sensing method selected.

Matrix buttons can only be sensed one at a time. When more than one row or column sensor is in the TOUCH state,

then the finger location cannot be resolved, and the situation is considered an invalid condition. Some applications

require multiple buttons to be sensed simultaneously, such as a keyboard with a Shift, Ctrl, and Alt key. In this case,

the Shift, Ctrl, and Alt keys should be designed as individual buttons, or should be changed to a mutual-capacitance

sensor design.

2.5.2 Sliders (One-Dimensional Sensors)

Sliders are used for controls requiring gradual adjustments. Examples include a lighting control (dimmer), volume

control, graphic equalizer, and speed control. A slider is built using an array of capacitive sensors called segments

that are placed adjacent to one another. Actuation of one segment results in partial actuation of physically adjacent

segments. By using an interpolation method called a centroid, you can achieve a higher resolution than the number of

slider segments. In a typical application, a slider with five segments can resolve at least 100 physical finger positions

on the slider. High resolution makes for smooth transitions in light or sound as a finger glides across a slider.

2.5.2.1 Linear Sliders

In a linear slider, each CapSense controller I/O pin is connected to one slider segment. A zigzag pattern (double

chevron) is recommended for slider segments. This layout ensures that when a segment is touched, the adjacent

segments are also partially touched. Sensor data from multiple sensors improves the estimation of finger position.

The maximum number of slider segments is a function of the number of available CapSense controller pins and the

required response time.

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