Cypress CapSense AN64846, Начало работы

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CapSense Technology
AN64846 - Getting Started with CapSense
®
Doc. No. 001-64846 Rev. *X 12
2.1.2 Firmware Component
Firmware is a vital component of the CapSense system. It processes the raw count data and makes logical decisions.
The amount of firmware development required for your application depends on which CapSense controller family you
select.
Devices from the CapSense Express family are fully configurable either through hardware or through I
2
C and do not
require any firmware development on the CapSense controller. The finger touch data is sent to a host for higher level
processing; see
Figure 2-5 . These devices are appropriate for systems where simplicity of design and short time-to-
market are the key requirements.
Figure 2-5. Example CapSense Express System Implementation
Capacitance
Measurement
(Hardware)
Decision
Logic
(Firmware)
CapSense Controller
Decision
Logic
(Firmware)
Non-CapSense
Actions
Application
Functions
Host
Sensors
The programmable devices allow complex system-level integration. These controllers can process the raw count data
as well as perform other system functions.
See CapSense Selector Guide
for additional details. Cypress’ PSoC Creator™ and PSoC Designer accommodate
firmware development in C and assembly languages. See Software Tools for more information on this and other tools.
2.2 Capacitive Sensing Methods
Capacitance can be measured between two points using either self-capacitance or mutual capacitance. The left side
of
Figure 2-6 shows the self-capacitance method and the right side shows the mutual-capacitance method.
Figure 2-6. Self-Capacitance and Mutual-Capacitance Methods
Z
V
x
+
_ C
x
Self Capacitance
Tx Rx
V
1
+
_
Mutual Capacitance
C
x
V
2
+
_
I
2.2.1 Self Capacitance
Self-capacitance uses a single pin and measures the capacitance between that pin and ground. A self-capacitance
sensing system operates by driving current on a pin connected to a sensor and measuring the voltage. When a finger
is placed on the sensor, it increases the measured capacitance. Self-capacitance sensing is best suited for
single-touch sensors, such as buttons and sliders.
Cypress’s CapSense solutions use self-capacitance sensing because it enables efficient use of pins for single-touch
sensors and sliders.
In a CapSense self-capacitance system, the sensor capacitance measured by the controller is called C
S
. When a finger
is not on the sensor, C
S
equals the parasitic capacitance (C
P
) of the system. This parasitic capacitance is a simplification
of the distributed capacitance that includes the effects of the sensor pad, the overlay, the trace between the CapSense