Cypress CapSense AN64846, Getting Started

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CapSense Technology
AN64846 - Getting Started with CapSense
®
Doc. No. 001-64846 Rev. *X 24
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. See
Mutual Capacitance to learn about Cypress products that support mutual-capacitance technology.
2.6.2 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. A centroid calculation example is given in Section
2.6.2.1 . 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.
2.6.2.1 Linear Sliders
In a linear slider, each CapSense controller I/O pin is connected to one slider segment. A zigzag pattern (double
chevron) as shown in
Figure 2-23 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.
Figure 2-23. Linear Slider
The centroid equation for a linear slider follows. The CapSense algorithm considers only three slider segments, the
segment with the maximum signal and its two adjacent segments, for the centroid calculation.
𝐶𝑒𝑛𝑡𝑟𝑜𝑖𝑑 𝑃𝑜𝑠𝑖𝑡𝑖𝑜𝑛 = (
𝑆
𝑋+1
− 𝑆
𝑋−1
𝑆
𝑋−1
+ 𝑆
𝑋
+ 𝑆
𝑋+1
+ 𝑋) ×
𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
𝑛−1
Equation 13
Where:
X
– Index of the segment that has the maximum signal
S
i
– Signal of the segment at index i
Resolution
– Total number of finger positions to be resolved on the slider
n
– Number of segments in the slider
Assuming a linear slider with 5 segments and a resolution of 100, let us now see an example of centroid calculation for
the signal pattern shown in
Figure 2-24 . This signal pattern indicates that the finger is touching segment 2.