Cypress CapSense AN64846 Getting Started
AN64846
Getting Started with CapSense
®
Document No. 001-64846 Rev. *X
Cypress Semiconductor
198 Champion Court
San Jose, CA 95134-1709
http://www.cypress.com
Copyrights
AN64846 - Getting Started with CapSense® Doc. No. 001-64846 Rev. *X 2
Copyrights
© Cypress Semiconductor Corporation, 2010-2018. This document is the property of Cypress Semiconductor Corporation
and its subsidiaries, including Spansion LLC (“Cypress”). This document, including any software or firmware included or
referenced in this document (“Software”), is owned by Cypress under the intellectual property laws and treaties of the
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as specifically stated in this paragraph, grant any license under its patents, copyrights, trademarks, or other intellectual
property rights. If the Software is not accompanied by a license agreement and you do not otherwise have a written
agreement with Cypress governing the use of the Software, then Cypress hereby grants you a personal, non-exclusive,
nontransferable license (without the right to sublicense) (1) under its copyright rights in the Software (a) for Software
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TO THE EXTENT PERMITTED BY APPLICABLE LAW, CYPRESS MAKES NO WARRANTY OF ANY KIND, EXPRESS
OR IMPLIED, WITH REGARD TO THIS DOCUMENT OR ANY SOFTWARE OR ACCOMPANYING HARDWARE,
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
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liable, in whole or in part, and you shall and hereby do release Cypress from any claim, damage, or other liability arising
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Cypress, the Cypress logo, Spansion, the Spansion logo, and combinations thereof, WICED, PSoC, CapSense, EZ-USB,
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AN64846 - Getting Started with CapSense® Doc. No. 001-64846 Rev. *X 3
Contents
1. Introduction .......................................................................................................................................................... 7
1.1 How to Use this Guide ................................................................................................................................. 7
1.2 Code Examples ........................................................................................................................................... 7
1.3 Cypress CapSense Differentiation ............................................................................................................... 7
1.4 CapSense Design Flow ............................................................................................................................... 8
1.5 Document Conventions ............................................................................................................................... 9
2. CapSense Technology ...................................................................................................................................... 10
2.1 CapSense System Overview ..................................................................................................................... 10
2.1.1 Hardware Component ................................................................................................................... 10
2.1.2 Firmware Component ................................................................................................................... 12
2.2 Capacitive Sensing Methods ..................................................................................................................... 12
2.2.1 Self Capacitance........................................................................................................................... 12
2.2.2 Mutual Capacitance ...................................................................................................................... 14
2.3 CapSense Sensing Technology ................................................................................................................ 15
2.3.1 Capacitance Conversion ............................................................................................................... 15
2.3.2 CapSense with Sigma Delta Modulator (CSD) ............................................................................. 15
2.4 CapSense Tuning ................................................................ ...................................................................... 17
2.4.1 Definitions ..................................................................................................................................... 18
2.4.2 SmartSense Auto-Tuning ............................................................................................................. 19
2.5 Signal-to-Noise Ratio (SNR) ...................................................................................................................... 21
2.5.1 Measuring SNR ............................................................................................................................ 22
2.6 CapSense Widgets .................................................................................................................................... 22
2.6.1 Buttons (Zero-Dimensional Sensors) ............................................................................................ 23
2.6.2 Sliders (One-Dimensional Sensors).............................................................................................. 24
2.6.3 Touchscreens and Trackpads (Two-Dimensional Sensors) ......................................................... 26
2.6.4 Proximity (Three-Dimensional Sensors) ....................................................................................... 26
2.7 Sensor Construction .................................................................................................................................. 27
2.7.1 Field-Coupled Via Copper Trace (PCB) ........................................................................................ 27
2.7.2 Field Coupled Via Spring/Gasket/Foam ........................................................................................ 27
2.7.3 Field Coupled Via Printed Ink ....................................................................................................... 28
2.7.4 Field Coupled via ITO Film on Glass ............................................................................................ 28
2.8 Liquid Tolerance ........................................................................................................................................ 28
2.8.1 Effect of Liquid Droplets and Liquid Stream on CapSense ........................................................... 29
2.8.2 Driven-Shield Signal and Shield Electrode ................................................................................... 31
2.8.3 Guard Sensor ............................................................................................................................... 31
2.8.4 Effect of Liquid Properties on the Liquid-Tolerance Performance ................................................. 32
2.9 Proximity Sensing ................................ ................................ ...................................................................... 33
2.9.1 Proximity-Sensing Applications Based on CapSense ................................................................... 33
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2.9.2 Proximity Sensing with CapSense ................................................................................................ 35
2.10 User Interface Feedback ........................................................................................................................... 35
2.10.1 Visual Feedback ........................................................................................................................... 35
2.10.2 Haptic Feedback ........................................................................................................................... 36
2.10.3 Audible Feedback ......................................................................................................................... 37
3. Design Considerations ..................................................................................................................................... 39
3.1 Overlay Selection ...................................................................................................................................... 39
3.1.1 Overlay Material............................................................................................................................ 39
3.1.2 Overlay Thickness ........................................................................................................................ 40
3.1.3 Overlay Adhesives ........................................................................................................................ 40
3.2 ESD Protection .......................................................................................................................................... 40
3.2.1 Preventing ESD Discharge ........................................................................................................... 41
3.2.2 Redirect ........................................................................................................................................ 41
3.2.3 Clamp ................................................................................................ ................................ ........... 42
3.3 Electromagnetic Compatibility (EMC) Considerations ............................................................................... 42
3.3.1 Radiated Interference and Emissions ........................................................................................... 42
3.3.2 Conducted Immunity and Emissions ............................................................................................. 53
3.4 Software Filtering ....................................................................................................................................... 54
3.4.1 Average Filter ............................................................................................................................... 54
3.4.2 IIR Filter ........................................................................................................................................ 56
3.4.3 Median Filter ................................................................................................................................. 57
3.4.4 Jitter Filter ..................................................................................................................................... 58
3.4.5 Event-Based Filters ...................................................................................................................... 60
3.4.6 Rule-Based Filters ........................................................................................................................ 60
3.5 Power Consumption .................................................................................................................................. 60
3.5.1 Active and Sleep Current .............................................................................................................. 60
3.5.2 Average Current ........................................................................................................................... 60
3.5.3 Response Time Versus Power Consumption ............................................................................... 61
3.6 Proximity Sensing Design .......................................................................................................................... 62
3.6.1 Implementing Proximity Sensing with CapSense ......................................................................... 62
3.6.2 Proximity Sensor Design .............................................................................................................. 64
3.6.3 Factors Affecting Proximity Distance ............................................................................................ 65
3.7 Pin Assignments ................................................................................................................................ ........ 69
3.8 PCB Layout Guidelines ............................................................................................................................. 71
3.8.1 Parasitic Capacitance, CP ................................................................ ............................................. 71
3.8.2 Board Layers ................................................................................................................................ 71
3.8.3 Board Thickness ........................................................................................................................... 72
3.8.4 Button Design ............................................................................................................................... 72
3.8.5 Slider Design ................................................................................................................................ 73
3.8.6 Sensor and Device Placement ..................................................................................................... 79
3.8.7 Trace Length and Width ............................................................................................................... 80
3.8.8 Trace Routing ............................................................................................................................... 80
3.8.9 Crosstalk Solutions ....................................................................................................................... 81
3.8.10 LEDs Close to CapSense Sensors ............................................................................................... 82
3.8.11 Vias ............................................................................................................................................... 82
3.8.12 Ground Plane ............................................................................................................................... 83
3.8.13 Power Supply Layout Recommendations ..................................................................................... 83
3.8.14 Shield Electrode and Guard Sensor ............................................................................................. 85
Contents
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3.8.15 CapSense System Design with Single Layer PCB ....................................................................... 88
3.8.16 CapSense System Design with ITO.............................................................................................. 88
3.9 PCB Assembly and Soldering.................................................................................................................... 88
4. CapSense Selector Guide ................................................................................................................................. 89
4.1 Defining CapSense Requirements ............................................................................................................ 89
4.2 CapSense Portfolio .................................................................................................................................... 91
4.2.1 Configurable CapSense Controllers (CapSense Express Family) ................................ ................ 91
4.2.2 Programmable CapSense Controllers .......................................................................................... 93
5. CapSense Resources ...................................................................................................................................... 100
5.1 CapSense Design Guides and Application Notes.................................................................................... 103
5.2 Additional CapSense Resources ............................................................................................................. 103
5.2.1 Cypress Document Manager ...................................................................................................... 103
5.2.2 Website ....................................................................................................................................... 103
5.3 Software Tools ......................................................................................................................................... 105
5.3.1 Integrated Development Environments ....................................................................................... 105
5.3.2 Data Monitoring Tools ................................................................................................................. 106
5.3.3 CapSense Tuner......................................................................................................................... 106
5.3.4 EZ-Click™ .................................................................................................................................. 107
5.3.5 Bridge Control Panel ................................................................................................................... 107
5.4 Development Kits .................................................................................................................................... 107
5.4.1 PSoC 4 Development Kits .......................................................................................................... 107
5.4.2 PSoC 3 and PSoC 5LP Development Kits .................................................................................. 107
5.4.3 CapSense Express Development Kits ........................................................................................ 108
5.4.4 PSoC 1 Development Kits .......................................................................................................... 108
5.4.5 Kits for Programming and Debugging ......................................................................................... 108
5.5 Design Support ........................................................................................................................................ 108
A. Springs ............................................................................................................................................................. 109
A.1 Finger-Introduced Capacitance ............................................................................................................... 109
A.1.1 Mounting Springs to the PCB ..................................................................................................... 111
A.2 CapSense and Mechanical Button Combination ..................................................................................... 112
A.3 Design Examples ..................................................................................................................................... 112
B. Schematic and Layout Checklist .................................................................................................................... 114
B.1 Schematic Checklist ................................................................................................................................ 114
B.1.1 Decoupling Capacitor ................................................................................................................. 114
B.1.2 Bulk Capacitor ............................................................................................................................ 114
B.1.3 Pin Assignment ........................................................................................................................... 114
B.1.4 C
MOD
........................................................................................................................................... 114
B.1.5 RB ............................................................................................................................................... 115
B.1.6 Series Resistor on CapSense Lines ........................................................................................... 115
B.1.7 Series Resistor on Communication Lines ................................................................................... 115
B.2 Layout Checklist ...................................................................................................................................... 116
B.2.1 Buttons ....................................................................................................................................... 117
B.2.2 Slider .......................................................................................................................................... 117
B.2.3 Overlay ................................ ....................................................................................................... 118
B.2.4 Sensor Traces ............................................................................................................................ 118
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B.2.5 Vias on Sensors.......................................................................................................................... 118
B.2.6 Ground Plane/Mesh .................................................................................................................... 118
B.2.7 Series Resistor ........................................................................................................................... 118
B.2.8 Shield Electrode.......................................................................................................................... 119
B.2.9 Guard Sensor ............................................................................................................................. 119
C. Clearance Between Sensor and Ground ....................................................................................................... 120
D. PSoC 1 In-Circuit Emulation (ICE) Pods ........................................................................................................ 123
D.1 Evaluation Pods ....................................................................................................................................... 123
D.2 In-Circuit Emulation (ICE) Pod Kits .......................................................................................................... 123
Glossary .................................................................................................................................................................... 124
Revision History ....................................................................................................................................................... 129
AN64846 - Getting Started with CapSense® Doc. No. 001-64846 Rev. *X 7
1. Introduction
<
1.1 How to Use this Guide
This guide is an ideal starting point for those who are new to capacitive touch sensing (CapSense®). You can use this
guide to:
Become familiar with the technology underlying CapSense solutions
Understand important design considerations, such as schematic, layout, and EMI (Electro Magnetic Interference)
Select the right device for your application
Find a CapSense resource to help with your design
When you are ready to design your application, consult the Design Guide specific to the CapSense device family you
have selected. Refer to the Glossary for the definitions of CapSense terms.
1.2 Code Examples
To access an ever-growing list of hundreds of PSoC code examples, visit our code examples webpage. You can also
explore the PSoC 4 video library here.
1.3 Cypress CapSense Differentiation
Capacitive touch sensing has changed the face of industrial design in products such as cellphones, PCs, consumer
electronics, automotive, and white goods. Cypress CapSense solutions bring elegant, reliable, and easy-to-use
capacitive touch sensing functionality to your design. Our capacitive touch sensing solutions have replaced more than
four billion mechanical buttons. A CapSense-based user interface design has the following advantages over a
mechanical-buttons based design:
Mechanical buttons are less reliable and wear out over time due to the physical movement. CapSense designs do
not involve moving parts.
Mechanical buttons pose problems when moisture seeps through the gaps in the assembly. CapSense-based front
panels can be completely sealed under the overlay.
Mechanical buttons require a small force to operate compared to the touch buttons and this force can increase
over time due to the accumulation of dirt in the gaps.
Mechanical buttons require multiple parts and increase the BOM cost whereas many CapSense designs consist
of only a PCB and an overlay with adhesive.
Mechanical buttons include the cost of tools required to make cutouts in the front panel. CapSense designs do not
require such cutouts.
Mechanical buttons yield poor aesthetics compared to the sleek and elegant touch buttons. CapSense designs
also offer more flexibility in designing the user interface in terms of button shape and graphical representation.
Cypress’ robust CapSense solutions leverage our flexible Programmable System-on-Chip (PSoC) architecture, which
accelerates time-to-market, integrates critical system functions, and reduces BOM cost. Cypress offers a wide range
of configurable and programmable CapSense controllers. Configurable CapSense controllers are hardware or I2C
configurable. Programmable devices provide complete flexibility to meet your exact design requirements, including
reducing BOM cost by integrating further system functionality. Following are some of the unique features offered by
CapSense products.
Introduction
AN64846 - Getting Started with CapSense® Doc. No. 001-64846 Rev. *X 8
Robust sensing technology
High noise immunity
High-performance sensing across a variety of overlay materials and thicknesses
SmartSense™ Auto-Tuning technology
Proximity sensing
Liquid-tolerant operation
Complete user interface solution including audio, visual, and haptic feedback
Low power consumption
Wide operating voltage range (1.71 V - 5.5 V)
Small form-factor packaging
Reduced BOM cost with integrated features like ADC, DAC, timer, counter, and PWM
1.4 CapSense Design Flow
Figure 1-1 depicts the typical flow of a CapSense product design. This flow is similar to any other electronic system
design flow except that CapSense designs involve an additional step called Tuning. This is the process of finding the
optimum values for various hardware and software parameters required for CapSense operation. These parameters
vary depending on the board layout, sensor dimensions, overlay properties, and application requirements such as
power consumption and response time. Therefore, this step is usually performed when the pre-production builds are
available. Many of the CapSense devices support Cypress’s Auto-tuning algorithm called SmartSense that
automatically sets parameters for optimal performance after the design phase and continuously compensates for
system, manufacturing, and environmental changes.
The enclosure or casing design is an integral part of a CapSense product design as the aesthetic feel and the
performance of the end product depend on the casing material and its design. Since the casing acts as an overlay for
the sensors, the touch-sensing performance depends on the overlay properties such as thickness and material type.
Therefore, it is important to test and evaluate the performance along with the overlay material, which is similar to the
one used in the end-product right from the prototype stage.
Introduction
AN64846 - Getting Started with CapSense® Doc. No. 001-64846 Rev. *X 9
Figure 1-1. Typical CapSense Product Design Flow
1.5 Document Conventions
Convention
Usage
Courier New
Displays file locations, user entered text, and source code:
C:\ ...cd\icc\
Italics
Displays file names and reference documentation:
Read about the sourcefile.hex file in the PSoC Designer User Guide.
File > Open
Represents menu paths:
File > Open > New Project
Bold
Displays commands, menu paths, and icon names in procedures:
Click the File icon and then click Open.
Times New Roman
Displays an equation:
2 + 2 = 4
Text in gray boxes
Describes Cautions or unique functionality of the product.
AN64846 - Getting Started with CapSense® Doc. No. 001-64846 Rev. *X 10
2. CapSense Technology
<
Cypress’ CapSense controllers use changes in capacitance to detect the presence of a finger on or near a touch
surface, as shown in Figure 2-1. This touch button example illustrates a capacitive sensor replacing a mechanical
button. The sensing function is achieved using a combination of hardware and firmware. See the Glossary for the
definitions of CapSense terms.
Figure 2-1. Illustration of a Capacitive Sensor Application
2.1 CapSense System Overview
CapSense touch sensing solutions include the entire system environment in which they operate. This includes:
Hardware components such as PCB and guard sensor
Firmware component to process the sensor data
2.1.1 Hardware Component
The CapSense controller resides within a larger system composed of a printed circuit board (PCB), and a touch-surface
called the overlay that protects the PCB.
Figure 2-2. Exploded View of the CapSense Hardware
CapSense Technology
AN64846 - Getting Started with CapSense® Doc. No. 001-64846 Rev. *X 11
The capacitive sensor pads of a sensor board are formed by the PCB traces. The most common PCB format is a twolayer board with sensor pads and a hatched ground plane (see Ground Plane) on the top, and the electrical components
on the bottom. The ground plane is also provided in the bottom layer surrounding the electrical components. The
electrical components include the CapSense controller and associated parts that convert the sensor capacitance into
digital raw counts. Figure 2-3 shows a cross-sectional view of a two-layer board stack-up. The four-layer design is an
option when the board area must be minimized. PCB layout plays a very important role in CapSense system
performance. Best practices are discussed in the device-specific Design Guides.
Figure 2-3. Two-Layer Stack-Up of a CapSense Board
2.1.1.1 Ground Plane
In general, a proper ground plane on the PCB reduces both RF emissions and interference. However, solid grounds
near CapSense sensors, or traces connecting these sensors to the PSoC pins, increase the parasitic capacitance of
the sensors. The increase in parasitic capacitance is unwanted as it reduces the sensitivity. It is thus recommended
that you use hatched ground planes surrounding the sensor and on the bottom layer of the PCB, below the sensors,
as Figure 2-4 shows. Typical hatching for the ground fill is 7-mil line, 45 mil spacing on the top layer, and 7-mil line, and
70-mil spacing on the bottom layer. The same hatch-fill on the top layer is driven with shield signal when liquid tolerance
is required. Refer to Liquid Tolerance to learn more.
Figure 2-4. Ground Fill on a PCB
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 I2C 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-tomarket 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 CS. When a finger
is not on the sensor, CS equals the parasitic capacitance (CP) 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
CapSense Technology
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controller pin and the sensor pad, the vias through the circuit board, and the pin capacitance of the CapSense controller.
CP is related to the electric field around the sensor pad. Although the following diagram shows field lines only around
the sensor pad, the actual electric field is more complicated.
Figure 2-7. CP and Electric Field
When a finger touches the sensor surface, it forms a simple parallel plate capacitor with the sensor pad through the
overlay. The result is called finger capacitance, CF, and is defined by Equation 1. CF is a simplification of a distributed
capacitance that includes the effects of the human body and the return path to the circuit board ground.
Equation 1
Where:
ε
0
= Free space permittivity
ε
r
= Dielectric constant of overlay
A = Area of finger and sensor pad overlap
D = Overlay thickness
Figure 2-8. CapSense System Equivalent Model
With a finger on the sensor surface, CS equals the sum of CP and CF.
Equation 2
CapSense Technology
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2.2.2 Mutual Capacitance
Figure 2-9 shows the button sensor layout for mutual-capacitance sensing. Mutual-capacitance sensing measures the
capacitance between two electrodes. One of the electrodes is called the transmit (TX) electrode and the other electrode
is called the receive (RX) electrode.
In a mutual-capacitance measurement system, a digital voltage (signal switching between V
DDD
and GND) is applied
to the TX pin and the amount of charge received on the RX pin is measured. The amount of charge received on the
RX electrode is directly proportional to the mutual capacitance (CM) between the two electrodes.
When a finger is placed between the TX and RX electrodes, the mutual-capacitance decreases to C
1
M
as shown in
Figure 2-10. Because of the reduction in mutual-capacitance, the charge received on the RX electrode also decreases.
The CapSense system measures the amount of charge received on the RX electrode to detect the touch/no touch
condition.
Figure 2-9. Mutual Capacitance Sensing Working
Overlay
RXTX TX
RX Electrode
PCB
RX
Electrode
TX Electrode
C
M
C
M
Figure 2-10. Mutual Capacitance with Finger Touch
Overlay
RXTX TX
RX Electrode
PCB
C
1
M
C
1
M
The mutual-capacitance effect is best suited to multi-touch systems such as touchscreens and trackpads. Cypress
offers mutual-capacitance-based trackpad solutions for consumer applications and TrueTouch® touchscreen solutions
for automotive and home appliance applications. Contact your local Cypress sales office directly for more information.
To find your local sales office, click here.
CapSense Technology
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2.3 CapSense Sensing Technology
This section explains the self-capacitance measurement method implemented in CapSense devices.
2.3.1 Capacitance Conversion
The CapSense hardware converts the sensor capacitance into a digital count, called raw count. The raw count is
interpreted as either a TOUCH or NO TOUCH state for the sensor, as Figure 2-11 shows. The numerical value of the
raw count is the digital representation of the sensor capacitance, and increases as the capacitance increases as
explained in Section 2.3.2 . The raw count is directly proportional to the average current drawn out by the sensor
capacitance, which increases when the sensor capacitance increases. Sensitivity is a measure of how much the output
will change for a given change on the input. The sensitivity of the CapSense sensor has units of counts-per-pF.
Figure 2-11. Sensing Algorithm Output
2.3.2 CapSense with Sigma Delta Modulator (CSD)
Cypress’s CSD method shown in Figure 2-12 uses a switched capacitor circuit on the front end of the system to convert
the sensor capacitance (CP) to an equivalent resistor (see Figure 2-13). A Sigma-Delta modulator then converts the
current measured through the equivalent resistor into a digital count. When a finger (CF) is on the sensor, the
capacitance increases and the equivalent resistance decreases. This causes an increase in current through the resistor,
which the Sigma-Delta modulator balances by controlling the Modulation IDAC, resulting in an increase in the digital
count. The Sigma Delta converter maintains the voltage of Analog Mux Bus (AMUXBUS) at a constant V
REF
(typically
1.2 V) with the help of a CMOD capacitor. An external resistor (RS) in-series with the sensor along with the AMUXBUS
resistance (R
bus
) helps to improve noise immunity. The recommended value for RS is 560 Ω.
Figure 2-12. CSD Block Diagram
C
P
isensor
Sigma-Delta
Converter
Sense Clock
CMOD
HI-Z
Input
SW1
SW2
CSD
GND
= External Connection
AMUX
Bus
Rbus
Vref
2.2nF/X7R/5V
Modulation
IDAC
R
S
C
F
Compensation
IDAC
Modulator Clock
GND
Raw Count Output
CapSense Technology
AN64846 - Getting Started with CapSense® Doc. No. 001-64846 Rev. *X 16
Figure 2-13. Equivalent Resistor Model
C
S
R
eq
AMUXBUS
SW
1
SW
2
AMUXBUS
I
CS
I
CS
I
CS
Two non-overlapping, out-of-phase clocks of frequency FSW (see Figure 2-14) control the switches SW1 and SW2. FSW
should be selected such that the sensor completely charges and discharges.
Figure 2-14. Voltage Across Sensor Capacitance
V
t
V
REF
(1.2V)
0
T
SW
= 1/F
SW
SW
1
OPEN
SW
2
CLOSED
SW
1
CLOSED
SW
2
OPEN
The continuous switching of SW1 and SW2 forms an equivalent resistance Req, as Figure 2-13 shows. The value of the
equivalent resistance RS is:
Equation 3
Where:
CS = Total sensor capacitance = Parasitic capacitance (CP) + Finger capacitance (CF)
FSW = Frequency of the switching clock or Sense Clock Frequency
Equation 4 gives the average current taken by the sensor capacitor from the AMUXBUS.
Equation 4
Equation 5 shows the raw count equation when only Modulation IDAC (single IDAC mode) is used and Equation 6
shows the raw count when both Modulation IDAC and Compensation IDAC (Dual IDAC mode) are used. The dual IDAC
mode gives more sensitivity and produces more count for a specific finger capacitance compared to the single IDAC
mode.
R
Equation 5
Equation 6
Where:
N = Scan resolution of the Sigma-Delta modulator in bits. CSD supports up to 16-bit resolution.
The CapSense architecture and its features vary slightly depending on the device. For example, some devices do not
support dual IDAC mode. The architecture presented here is available in many of the CapSense controllers. Refer to
the device-specific Design Guide to learn about the sensing technology implemented for that device.
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The fourth-generation CapSense functionality in PSoC 4 S-Series, PSoC 4100S Plus, and PSoC 4100PS devices is
an improved version of the previous generation. The main differences between the two generations of CapSense
architecture are listed in Table 2-1.
Table 2-1. Comparison of CapSense Architecture
Feature
Third-Generation
CapSense
Fourth-Generation
CapSense
Advantages of Fourth-Generation
over Third-Generation CapSense
Sensing Modes
Self-Cap and Mutual Cap
Self-Cap, Mutual Cap and
ADC mode
Multiple functionality
IDAC Modes
Sourcing or Sinking Mode
Sourcing Mode
-
Self-Cap Range
5 pF – 60 pF
5 pF – 200 pF
Supports high-CP-design applications
VREF
1.2 V
0.6 V to VDDA-0.6 V
Improved SNR
IDAC LSB Size
1.2 µA, 2.4 µA
37.5 nA, 300 nA, 2.4 µA
Improved Sensitivity
Split IDAC Capability
Requires two IDACs
Requires one IDAC1
Improved Sensitivity
EMI Reduction - Digital
-
Spread Spectrum - CSD
Controlled
Spread Spectrum clock is generated by
hardware and CPU is completely free.
10-bit ADC
No
Yes
ADC using CSD hardware
HW State Machine
No
Yes
CPU no longer required for Initialization
or for Spread Spectrum SenseClk
generation
The CapSense hardware in PSoC 4 S-Series, PSoC 4100S Plus, and PSoC 4100PS support both self-capacitanceand mutual capacitance-based capacitive sensing. The hardware also supports input voltage measurement when
CapSense scanning is not in progress. Refer to the CapSense chapter in the respective device datasheet for a detailed
explanation of the CapSense hardware in PSoC 4 S-Series, PSoC 4100S Plus, and PSoC 4100PS devices. Refer to
PSoC 4100S and PSoC 4100S Plus: PSoC 4 Architecture Technical Reference Manual (TRM), PSoC 4000S Family:
PSoC® 4 Registers Technical Reference Manual (TRM), and PSoC 4100S Plus Registers Technical Reference Manual
(TRM) for the basic knowledge of fourth-generation CapSense architecture.
2.4 CapSense Tuning
Optimal CapSense system performance depends on the board layout, button dimensions, overlay material, and
application requirements. In addition to these factors, switching frequency and threshold levels must be carefully
selected for robust and reliable performance. Tuning is the process of determining the optimum values for these
parameters. Tuning is required to maintain high sensitivity to touch and to compensate for process variations in the
sensor board, overlay material, and environmental conditions.
Many of the CapSense devices support SmartSense, Cypress’ Auto-tuning algorithm, which automatically sets
parameters for optimal performance and continuously compensates for system, manufacturing and environmental
changes. See SmartSense Auto-Tuning for more information.
This section gives an introduction about the tuning process. For details on all the parameters involved in CapSense
operation and the step-by-step tuning procedure, refer to the device-specific Design Guide. Cypress provides many
tools to make tuning and data monitoring easy. Refer to Data Monitoring Tools to learn more about these tools.
The following section defines some terms that will help you understand the tuning process.
1
Requires one IDAC if compensation and modulation IDAC split is 50-50; if it is not 50-50, it requires two IDACs.
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2.4.1 Definitions
Raw Count: As seen in Figure 2-19, sensor capacitance is converted into a count value by the CapSense
algorithm. The unprocessed count value is referred to as raw count. Processing of the raw count results in ON/OFF
states for the sensor.
Baseline: The raw count value of a sensor may vary gradually due to changes in the environment such as
temperature and humidity. Therefore, the raw count is low-pass filtered to create a new count value known as
baseline that keeps track of and compensates for the gradual changes in raw count. The baseline is less sensitive
to sudden changes in the raw count caused by a touch. Therefore, the baseline value provides the reference level
for computing the signals (explained below). Figure 2-15 shows the concept of raw count, baseline and signal.
Figure 2-15. Raw Count and Baseline
baseline
raw count
signal = raw count - baseline
sensor OFF sensor OFFsensor ON
Difference Count or Signal: Subtracting the baseline level from the raw count produces the difference count that
is used in the ON/OFF decision process.
The thresholds are offset by a constant amount from the baseline level. The thresholds have the following
functions:
Noise Threshold: A parameter used to differentiate signal from noise. If the raw count is above noise threshold,
then the baseline is not updated and the difference count indicates the difference between raw count and baseline.
If the raw count is below the noise threshold, then the baseline is updated and the difference count is zero. See
Figure 2-16 for details.
Finger Threshold: A parameter used with Hysteresis to determine the state of the sensor, as Equation 6 and
Figure 2-16 show.
Equation 6
Hysteresis: A parameter used with Finger Threshold to determine the state of the sensor, as Equation 6 and
Figure 2-16 shows. Hysteresis provides immunity against noisy transitions of sensor state.
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Figure 2-16. Hysteresis
Sensor ONSensor OFF Sensor OFF
Finger Threshold + Hysteresis
Finger Threshold - Hysteresis
Finger Threshold
2.4.2 SmartSense Auto-Tuning
2.4.2.1 What Is SmartSense?
Tuning the touch sensing user interface is a critical step in ensuring proper system operation and a pleasant user
experience. The typical design flow involves tuning the sensor interface in the initial design phase, during system
integration, and finally production fine-tuning before the production ramp. Because tuning is an iterative process, it can
be time-consuming. SmartSense Auto-Tuning helps to simplify the user interface development cycle. In addition, the
method is easy to use and reduces the design cycle time by eliminating the tuning process throughout the product
development cycle, from prototype to mass production.
2.4.2.2 What Does SmartSense Do?
SmartSense tunes each CapSense sensor automatically at power-up and then monitors and maintains optimum sensor
performance during runtime. The number of parameters to be tuned is reduced from 17 in CSD to 4 with SmartSense.
Power-up tuning: SmartSense tunes the parameters of each sensor based on the individual sensor parasitic
capacitance to get the desired sensitivity for the sensor.
Runtime tuning: Noise in the system is measured dynamically. The thresholds are adjusted accordingly for each
sensor to overcome false triggering due to dynamic variations in noise in the CapSense system.
2.4.2.3 How and Where is SmartSense Helpful?
SmartSense technology adapts for manufacturing variations in PCBs, overlays, and noise generators, such as LCD
inverters, AC line noise, and switch-mode power supplies and automatically tunes them out. SmartSense handles
changes in system environment, such as temperature, humidity, and noise sources such as RF, SMPS, LCD Inverter,
and AC line noise.
The following sections describe scenarios in which SmartSense is instrumental in adapting to the external noise. By
maintaining a robust signal-to-noise ratio, the false triggering of buttons is prevented.
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2.4.2.3.1 Different Noise Levels in Different Designs
SmartSense technology dynamically tunes itself (adjusts noise and finger thresholds) for different noise environments.
In Figure 2-17, Design A and Design B have different noise levels. To maintain a minimum SNR of 5:1, you must adjust
the dynamic threshold. SmartSense does this automatically, allowing seamless transition from one model to another
with minimal or no tuning required.
Figure 2-17. Different Noise Levels in Design A and B Being Compensated Automatically
2.4.2.3.2 Noise Spikes During Production
SmartSense technology also automatically tunes out the noise spikes (in production) that may not be seen during the
design stage, as indicated in Figure 2-18. This is a powerful SmartSense feature that prevents false button presses in
the end system, which prevents a failure analysis for a mass production design.
Figure 2-18. Finger Threshold Dynamically Adjusted to Prevent False Button Touches
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2.4.2.4 When Is Manual-Tuning Advantageous?
SmartSense allows a device to calibrate itself for optimal performance and complete the entire tuning process
automatically. This technology will meet the needs of most designs, but, in the case where SmartSense will not work
or there are specific SNR or power requirements, the CapSense CSD parameters can be manually adjusted to meet
system requirements. This is called manual tuning. Some advantages of manual tuning, as opposed to SmartSense
Auto-tuning are:
Strict control over parameter settings: SmartSense sets all of the parameters automatically. However, there may
be situations where you need strict control over the parameters. For example, use manual tuning if you need to
strictly control the time CSD takes to scan a group of sensors. This can be done to reduce EMI in systems.
Supports higher parasitic capacitances: SmartSense supports parasitic capacitances as high as 45 pF for a 0.2-
pF finger capacitance, and as high as 35 pF for a 0.1-pF finger capacitance. If the parasitic capacitance is higher
than the value supported by SmartSense, use manual tuning.
Refer to the device-specific Design Guide for the step-by-step procedure on manual tuning.
2.5 Signal-to-Noise Ratio (SNR)
Signal is a generic engineering term that can have many meanings. For CapSense applications, signal is defined as
the change in the raw count between the OFF and ON states. Signal is also called Difference Count.
Noise is another term that has many meanings. The following discussion presents a definition of CapSense noise that
uses a simple mathematical model of the sensor output over time.
When the sensor is in the OFF state, the counts, X(t), can be modeled by an average count and a noise component.
Equation 7
X0 is the average of X(t)
N0(t) is the noise component for t during the OFF state
The same model applies when the sensor is in the ON state.
Equation 8
X1 is the average of X(t)
N1(t) is the noise component for t during the ON state
X0 is called the baseline level of the raw counts. The difference between X1 and X0 is called the signal, S.
Equation 9
The noise components N0(t) and N1(t) are similar but not identical. For example, N1(t) usually contains a higher level
of AC line noise in finger sensing applications compared to N0(t). This occurs because the human body acts as an
antenna to 50-Hz and 60-Hz line noise, and the finger contact with the sensor overlay couples the noise into the
CapSense system.
We define the noise level N as the worst case measured peak noise in the OFF state.
Equation 10
Thus, CapSense SNR, is defined as the ratio of signal (S) to noise (N).
Equation 11
Based on the experiments and knowledge from many CapSense applications, Cypress recommends a minimum SNR
of 5:1 to ensure sufficient margin between noise and signal for robust ON/OFF operation.
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Figure 2-19. Signal and Noise
2.5.1 Measuring SNR
SNR should be measured in the noise environment where CapSense is intended to be used. In other words, measure
the system SNR under worst-case noise conditions.
The first step in measuring SNR is to monitor the raw count for each sensor. This can be done using data logging to a
text file and plotting in a spreadsheet, or using the Cypress Bridge Control Panel and Miniprog3 or by using the Tuner
tool, which directly displays the SNR, available with the CapSense component in PSoC Creator (see Data Monitoring
Tools for more details). Refer to AN2397 – CapSense Data Viewing Tools that teaches how to monitor raw count using
these tools. Whatever the method, the raw count should be observed for SNR measurement. The difference count
should not be used in the measurement of SNR since it is a function of the baseline update process, which involves
filtering (filling the "bucket") and nonlinear threshold events.
Another factor to consider is how the signal is produced. The worst-case ON and OFF scenario should be used when
measuring SNR. If the system is designed to sense the presence of a finger, then measure SNR with a light touch of
the sensor area, and position the contact point slightly off-center. For automated testing, a worst-case finger touch
(0.1 pF) can often be simulated by an equivalent metal disc that is the size and shape of a small coin.
As an example of measuring SNR, consider the raw count waveform in Figure 2-19.
X0 = 5925 counts
X1 = 6055 counts
S = 130 counts
N = 5940 - 5910 = 30 counts
SNR = 130:30 = 4.3:1
2.6 CapSense Widgets
CapSense widgets consist of one or more CapSense sensors, which as a unit represent a certain type of user interface.
CapSense widgets are broadly classified into four categories – Buttons (Zero-Dimensional), Sliders (One-Dimensional),
Touchpads/Trackpads (Two-Dimensional), and Proximity sensors (Three-Dimensional). Figure 2-20 shows button,
slider, and proximity sensor widgets. This section explains the basic concepts of different CapSense widgets. For a
detailed explanation of sensor construction, see Sensor Construction.
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Figure 2-20. Several types of Widgets
Button Sensor Slider Sensor Proximity Sensor
2.6.1 Buttons (Zero-Dimensional Sensors)
CapSense buttons replace mechanical buttons in a wide variety of applications such as home appliances, medical
devices, white goods, lighting controls and many other products. It is the simplest type of CapSense widget, consisting
of a single sensor. A CapSense button gives one of two possible output states: active (finger is present) or inactive
(finger is not present). These two states are also called ON and OFF states, respectively. To get higher reliability, lower
cost, and appealing industrial design, use CapSense buttons instead of mechanical buttons.
2.6.1.1 Simple Buttons
The simplest capacitive sensor consists of a copper pad connected to a CapSense controller pin with a trace. A button
is defined as the combination of the copper sensor pad and the nonconductive overlay material. The button is
surrounded by a grounded copper hatch separated by an annular gap. Each button requires one I/O pin of the
CapSense controller.
Figure 2-21. Typical Simple Buttons
2.6.1.2 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, as Figure 2-22 shows. This allows a design to have more buttons than there are
I/O pins on the CapSense controller when using a self-capacitance method.
Figure 2-22. 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 12
Using a matrix button design can significantly reduce the number of I/O pins required. For example, the matrix in Figure
2-22 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.
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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.
Equation 13
Where:
X – Index of the segment that has the maximum signal
Si – 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.
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Figure 2-24. Example Signal Pattern on a Linear Slider
0 0
150
100
110
0 1 2 3 4
Segment Index
Signal
Segment 2 has the maximum signal of 150 and its adjacent segments 1 and 3 have the signals of 100 and 110
respectively. Segments 0 and 4 do not have a signal since the finger is away from them. Now, the calculated centroid
position is 51 (rounded) as shown in the following equation. Note that the CapSense algorithm subtracts the Noise
Threshold (see Definitions to learn about Noise Threshold) from the signal before calculating the centroid, which is not
shown here for simplification. Refer to the CapSense component datasheet to learn more.
Equation 14
2.6.2.2 Diplexed Sliders
In a diplexed slider, each CapSense controller I/O pin is connected to two different slider segments. This allows a
design to have twice as many slider segments as there are I/O pins. For example, a diplexed 16-segment slider requires
only eight CapSense controller I/O pins.
Figure 2-25. 16-Segment Diplexed Slider
For a diplexed slider to work properly, the slider segments must be connected to the CapSense controller I/O pins in a
pre-determined order. The first half of the segments are connected to the CapSense controller I/O pins sequentially (0,
1, 2 …7) and operate similar to a linear slider. The second half of the segments are connected to the same CapSense
controller I/O pins in a non-sequential order. This order exploits the fact that activation of one segment results in partial
actuation of neighboring segments. While slider actuation of one half of the slider results in aliasing on to the other half,
the levels will be scattered in the untouched half. Sensing algorithms search for strong adjacent segment actuation and
ignore scattered actuation to determine finger position on the slider accurately.
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2.6.2.3 Radial Sliders
Radial sliders are similar to linear sliders in that finger position is estimated using data from adjacent sensors; however,
radial sliders are continuous (does not have a beginning or end), as Figure 2-26 shows. Refer to the CapSense
component datasheet to learn about the centroid equation for a radial slider.
Figure 2-26. Radial Slider
Area contacted by finger
2.6.3 Touchscreens and Trackpads (Two-Dimensional Sensors)
A trackpad (also known as touch pad) has two linear sliders arranged in an X and Y pattern, enabling it to locate a
finger’s position in both X and Y dimensions. Figure 2-27 shows a typical arrangement of a track pad sensor.
Figure 2-27. Trackpad Sensor Arrangement
2.6.4 Proximity (Three-Dimensional Sensors)
Proximity sensors detect the presence of a hand or other conductive object before it makes contact with the touch
surface. Imagine a hand stretched out to operate a car audio system in the dark. The proximity sensor causes the
buttons of the audio system to glow through backlight LEDs when the user's hand is near. One implementation of a
proximity sensor consists of a long trace on the perimeter of the user interface, as shown in Figure 2-28. Another way
to implement a proximity sensor is by ganging sensors together. See Proximity Sensing to learn more.
Figure 2-28. Proximity Sensor
Proximity Sensor
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2.7 Sensor Construction
A capacitive sensor can be constructed using different materials depending on the application requirement. In a typical
sensor construction, a conductive pad or surface that senses the user touches is connected to the pin of the capacitive
controller using a conductive trace or link. This whole arrangement is placed below a non-conductive overlay material
and the user interacts on top of the overlay. A very common method of sensor construction is to etch copper pads and
traces on a FR4 PCB. However, in touchscreen applications, Indium Tin Oxide (ITO) is used to construct transparent
sensors. This section presents various methods of constructing a sensor and the features of each method so that you
can choose one that fits your requirements.
2.7.1 Field-Coupled Via Copper Trace (PCB)
Figure 2-29. Field Coupled Using PCB
Features of a PCB-based design:
Most common implementation
Copper pads etched on the surface of the PCB act as sensor pads
Electric field emanates from the copper sensor pad to ground plane
No mechanical moving parts
A nonconductive overlay serves as the touch surface for the button
Ideal topology for simple flat panel designs
Low BOM cost
2.7.2 Field Coupled Via Spring/Gasket/Foam
Figure 2-30. Field Coupled Via Spring
Features of a design based on springs/gaskets/foam:
Electrical field coupled from PCB to overlay using a compressed spring, or conductive gasket or foam (Closed-cell
conductive foam should be used. Materials that absorb moisture should be avoided.)
Conductive material itself acts as capacitive sensor pad
No mechanical moving parts. Springs and foam do not move
Any non-conductive overlay serves as the button touch surface
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Ideal topology for curved, sloping, or otherwise irregular front panels
Ideal for designs where touch sensor surface is physically separated from silicon or mother board
Ideal for designs where CapSense and mechanical button combination is desired
2.7.3 Field Coupled Via Printed Ink
Features of a design based on printed ink:
Electric field coupled with printed patterns on a flexible substrate using conductive ink
High series resistance due to higher sheet resistance (ohms-per-square) of printed ink compared to copper.
However, the series resistance of the materials such as silver loaded inks, ITO, and PEDOT depend on the
thickness variation as their sheet resistance is relatively low.
High parasitic capacitance due to thin PCB substrate
No mechanical moving parts, but substrate is flexible
Coupled to the touch sensor surface with a nonconductive overlay
Ideal topology for flexible front panels
Flexible PCB can be one-layer or two-layer film
2.7.4 Field Coupled via ITO Film on Glass
Features of a design based on ITO film:
Electric field coupled with printed or deposited patterns on glass
Higher series resistance of ITO films compared to copper
No mechanical moving parts
Ideal topology for graphical front panels
2.8 Liquid Tolerance
CapSense is used in a variety of applications such as home appliances, automotive, and industrial applications. These
applications require robust CapSense operation even in the presence of mist, moisture, water, ice, and humidity
changes. In a CapSense design, false sensing of touch may happen due to the presence of a film of liquid or liquid
droplets on the touch surface. Cypress’s CapSense sensing method can compensate for variation in raw count due to
mist, moisture, water, ice, and humidity changes and provide a robust, reliable, CapSense operation.
Figure 2-31. CapSense-based Touch User Interface in a Washing Machine with Liquid Tolerance
To compensate for changes in raw count due to mist, moisture, and humidity changes, the CapSense sensing method
continuously adjusts the baseline of the sensor to prevent sensor false triggers. To compensate for changes in raw
count due to a liquid droplet or liquid flow, you should implement a Shield Electrode and a Guard Sensor to provide
robust touch sensing, as Figure 2-32 shows. All sensor pins can optionally have a 560-Ω series resistor for improved
noise immunity.
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When liquid droplets are present on the touch surface and if the shield electrode is implemented, the CapSense system
can reliably work even in the presence of liquid droplets and report sensor ON/OFF status. When there is a liquid flow
or a liquid pool on the touch surface, the CapSense system detects the liquid by using a guard sensor and disables the
scanning for all other sensors in the system to prevent false triggers. Therefore, when there is a liquid flow or liquid
pool on the touch surface, the CapSense system will not detect a finger touch as long as the liquid is present on the
touch surface.
Figure 2-32. Shield Electrode (SH) and Guard Sensor (GUARD) Connected to CapSense Controller
CS1
CS3
CS2
SH
GUARD
Shield Electrode
Guard Sensor
Button Sensor
560 Ω
560 Ω
560 Ω
560 Ω
CapSense
Controller
2.8.1 Effect of Liquid Droplets and Liquid Stream on CapSense
To understand the effect of a liquid droplet or a liquid stream on a CapSense sensor, consider a CapSense system in
which the hatch fill around the sensor is connected to ground, as Figure 2-33 (a) shows. Surrounding the sensor with
a hatch fill connected to ground improves the noise immunity of the sensor. The parasitic capacitance of sensor is
denoted as CS in Figure 2-33 (b).
Figure 2-33. Typical CapSense System Layout
BTN1
Hatch Fill
Connected to
Ground
Button Sensor
560 Ω
CapSense
Controller
CapSense
Controller
560 Ω
C
S
(a)
(b)
As shown in Figure 2-34, when a liquid droplet falls on the touch surface, due to its conductive nature, it provides a
strong coupling path for the electric field lines to return to ground and therefore adds a capacitance CLD in parallel to
CP. When the sensor is charged and discharged, the capacitance CLD draws some amount of charge from the AMUX
bus because of the nonzero voltage difference across the capacitance CLD. This increases the overall capacitance seen
by the CapSense circuitry and results in an increase in the sensor raw count. In some cases, where the liquid is highly
conductive (salty water or water with high mineral content), the increase in raw count when a liquid droplet falls on the
touch surface might be equal to the increase in raw count due to a finger touch and thus causes false triggers, as Figure
2-35 shows.
Figure 2-34. Capacitance Added by Liquid Droplet when the Hatch Fill Is Connected to Ground
BTN1
Hatch Fill
Connected to
Ground
Button Sensor
560 Ω
CapSense
Controller
CapSense
Controller
560 Ω
C
S
C
LD
Liquid Droplet
CS – Sensor Parasitic Capacitance
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C
LD
– Capacitance added by Liquid Droplet
Figure 2-35. Effect of Liquid Droplet when the Hatch Fill Around the Sensor Is Connected to Ground
To compensate the capacitance added by the liquid droplet to the CapSense circuitry, you should drive the hatch fill
surrounding the sensor with the driven-shield signal.
As shown in Figure 2-36, when the hatch fill surrounding the sensor is connected to the driven-shield signal and when
a liquid droplet falls on the touch surface, because the voltage on both the sides of liquid droplet is kept at the same
potential, the capacitance C
LD
added by the liquid droplet is nullified. Because the voltage difference across the
capacitance C
LD
is zero, it will not draw any charge from the AMUX bus and, therefore, the increase in the raw count
when a liquid droplet falls on the sensor will be very small, as Figure 2-37 shows.
Figure 2-36. Capacitance Added by Liquid Droplet when the Hatch Fill around the Sensor Is Connected to Shield
BTN1
Hatch Fill
Connected to
Driven Shield
Button Sensor
560 Ω
CapSense
Controller
CapSense
Controller
560 Ω
C
S
C
SH
C
HG
C
LD
Liquid Droplet
Shield
Sensor Waveform
Driven Shield
Waveform
Shield
CS – Sensor Parasitic Capacitance
CSH – Capacitance between Sensor and Hatch Fill
C
HG
– Capacitance between Hatch Fill and Ground
C
LD
– Capacitance Added by Liquid Droplet
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