Cypress CY8C20xx7/S CapSense Series, Руководство по конструкции

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CapSense Technology
CY8C20xx7/S CapSense
®
Design Guide Doc. No. 001-78329 Rev. *E 12
Figure 2-2. Section of Typical CapSense PCB with the Sensor Being Activated by a Finger
In addition to the parallel plate capacitance, a finger in contact with the overlay causes electric field fringing between
itself and other conductors in the immediate vicinity. The effect of these fringing fields is typically minor compared to
that of the parallel plate capacitor and can usually be ignored.
Even without a finger touching the overlay, the sensor I/O pin has some parasitic capacitance (C
P
). C
P
results from
the combination of the CapSense controller internal parasitics and electric field coupling between the sensor pad,
traces, and vias, and other conductors in the system such as ground plane, other traces, any metal in the product‟s
chassis or enclosure, and so on. The CapSense controller measures the total capacitance (C
X
) connected to a
sensor pin.
When a finger is not touching a sensor:
𝐶
𝑋
= 𝐶
𝑃
Equation 2-2
With a finger on the sensor pad, C
X
equals the sum of C
P
and C
F
:
𝐶
𝑋
= 𝐶
𝑃
+ 𝐶
𝐹
Equation 2-3
In general, C
P
is an order of magnitude greater than C
F
. C
P
usually ranges from 10 pF to 20 pF, but in extreme cases
can be as high as 50 pF. C
F
usually ranges from 0.1 pF to 0.4 pF. The magnitude of C
P
is of critical importance when
tuning a CapSense system and is discussed in CapSense Performance Tuning with User Modules .
2.2 Capacitive Sensing Methods in CY8C20xx7/S
CY8C20xx7/S devices support CSD, CSDPLUS, and SmartSense_EMCPLUS CapSense methods for converting
sensor capacitance (C
X
) into digital counts. The CSDPLUS method is the superset of the CSD method and has
several improvements when compared to CSD. These two methods are implemented in the hardware. The
SmartSense_EMCPLUS uses the autotuning algorithm implemented in firmware to automatically tune all the
CSDPLUS parameters. The CSD, CSDPLUS, and SmartSense_EMCPLUS methods are implemented in the form of
a PSoC Designer User Module and are described in the following sections.
2.2.1 CapSense Sigma Delta (CSD)
Figure 2-3 shows a block diagram of the CSD method for converting sensor capacitance (C
X
) into digital counts. This
method can be conceptually broken into two blocks
– switched-capacitor input, which converts capacitance to current
and sigma delta converter, which converts current to digital counts. These blocks are explained in the following
sections.