Cypress CY8C20xx7/S CapSense Series, Руководство по конструкции

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CY8C20xx7/S CapSense
®
Design Guide Doc. No. 001-78329 Rev. *E 11
2. CapSense Technology
2.1 CapSense Fundamentals
CapSense is a touch-sensing technology, which works by measuring the capacitance of each I/O pin on the
CapSense controller that has been designated as a sensor. As shown in
Figure 2-1 , the total capacitance on each
sensor pin can be modeled as equivalent lumped capacitors with values of C
X
,1 through C
X
,n for a design with n
sensors. The circuitry internal to the CY8C20xx7/S device converts the magnitude of each C
X
into a digital code that
is stored for post processing. The other component, C
MOD
, is used by the CapSense controller‟s internal circuitry and
will be discussed in more detail in Capacitive Sensing Methods in CY8C20xx7/S .
Figure 2-1. CapSense Implementation in a CY8C20xx7/S PSoC Device
CY8C20xx7/S
C
MOD
Sensor 1
C
X,1
Sensor 2
C
X,2
Sensor 3
C
X,3
Sensor 4
C
X,4
Sensor n
C
X,n
As shown in Figure 2-1 , each sensor I/O pin is connected to a sensor pad by traces, vias, or both, as necessary. The
overlay is a nonconductive cover over the sensor pad that constitutes the product‟s touch interface. When a finger
comes into contact with the overlay, the conductivity and mass of the body effectively introduces a grounded
conductive plane parallel to the sensor pad. This is represented in
Figure 2-2 . This arrangement constitutes a parallel
plate capacitor, whose capacitance is given by:
𝐶
𝐹
=
𝜀
0
𝜀
𝑟
𝐴
𝐷
Equation 2-1
Where:
C
F
= The capacitance added by a finger in contact with the overlay over a sensor
ε
0
= Free space permittivity
ε
r
= Dielectric constant (relative permittivity) of overlay
A = Area of finger and sensor pad overlap
D = Overlay thickness