Cypress CY8C20xx7/S CapSense Series Design Manual

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AN78329 - CapSense® Design Guide

CY8C20xx7/S

Doc. No. 001-78329 Rev. *E

Cypress Semiconductor

198 Champion Court

San Jose, CA 95134-1709

Phone (USA): 800.858.1810

Phone (Intnl): 408.943.2600

www.cypress.com

Copyrights

© Cypress Semiconductor Corporation, 2012-2015. The information contained herein is subject to change without notice. Cypress Semiconductor Corporation assumes no responsibility for the use of any circuitry other than circuitry embodied in a Cypress product. Nor does it convey or imply any license under patent or other rights. Cypress products are not warranted nor intended to be used for medical, life support, life saving, critical control or safety applications, unless pursuant to an express written agreement with Cypress. Furthermore, Cypress does not authorize its products for use as critical components in life-support systems where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress products in life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress against all charges.

Trademarks

PSoC Designer™, Programmable System-on-Chip™, and SmartSense™ are trademarks and PSoC® and CapSense® are registered trademarks of Cypress Semiconductor Corp. All other trademarks or registered trademarks referenced herein are property of the respective corporations.

Source Code

Any Source Code (software and/or firmware) is owned by Cypress Semiconductor Corporation (Cypress) and is protected by and subject to worldwide patent protection (United States and foreign), United States copyright laws and international treaty provisions. Cypress hereby grants to licensee a personal, non-exclusive, non-transferable license to copy, use, modify, create derivative works of, and compile the Cypress Source Code and derivative works for the sole purpose of creating custom software and or firmware in support of licensee product to be used only in conjunction with a Cypress integrated circuit as specified in the applicable agreement. Any reproduction, modification, translation, compilation, or representation of this Source Code except as specified above is prohibited without the express written permission of Cypress.

Disclaimer

CYPRESS MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARD TO THIS MATERIAL, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. Cypress reserves the right to make changes without further notice to the materials described herein. Cypress does not assume any liability arising out of the application or use of any product or circuit described herein. Cypress does not authorize its products for use as critical components in life-support systems where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress‟ product in a life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress against all charges.

Use may be limited by and subject to the applicable Cypress software license agreement.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 2

Contents

1. Introduction .......................................................................................................................................................... 6

1.1 Abstract ....................................................................................................................................................... 6

1.2 Cypress‟s CapSense Documentation Ecosystem ........................................................................................ 6

1.3 CY8C20xx7/S CapSense Family Features .................................................................................................. 8

1.4 Document Conventions ............................................................................................................................. 10

2. CapSense Technology ...................................................................................................................................... 11

2.1 CapSense Fundamentals .......................................................................................................................... 11

2.2 Capacitive Sensing Methods in CY8C20xx7/S .......................................................................................... 12

2.2.1 CapSense Sigma Delta (CSD) ..................................................................................................... 12

2.2.2 CapSense Sigma Delta (CSD) PLUS ........................................................................................... 15

2.2.3 SmartSense_EMCPLUS Auto-Tuning .......................................................................................... 18

2.2.4 Selecting the User Module ............................................................................................................ 19

3. CapSense Design Tools .................................................................................................................................... 20

3.1 Overview ................................................................................................................................................... 20

3.1.1 PSoC Designer and User Modules ............................................................................................... 20

3.1.2 CY8C20xx7/S QuietZone Starter Kit ............................................................................................. 21

3.1.3 CapSense Data Viewing Tools ..................................................................................................... 21

3.2 User Module Overview .............................................................................................................................. 22

3.3 CapSense User Module Global Arrays ...................................................................................................... 22

3.3.1 Raw Count .................................................................................................................................... 23

3.3.2 Baseline ........................................................................................................................................ 23

3.3.3 Difference Count (Signal) ............................................................................................................. 23

3.3.4 Sensor State ................................................................................................................................. 23

3.4 CSD/CSDPLUS User Module Parameters ................................................................................................ 23

3.4.1 User Module High-Level Parameters ............................................................................................ 24

3.4.2 CSD/CSDPLUS User Module Low-Level Parameters .................................................................. 27

3.5 SmartSense_EMCPLUS User Module Parameters ................................................................................... 31

4. CapSense Performance Tuning with User Modules ....................................................................................... 33

4.1 General Considerations ............................................................................................................................. 33

4.2 Tuning the CSD/CSDPLUS User Module .................................................................................................. 35

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 3

4.1.1 Signal, Noise, and SNR ................................................................................................................ 33

4.1.2 Charge/Discharge Rate ................................................................................................................ 34

4.1.3 Importance of Baseline Update Threshold Verification ................................................................. 35

4.2.1 Recommended C

Value for CSD/CSDPLUS........................................................................... 36

MOD

Contents

4.2.2 I

Range .................................................................................................................................... 37

DAC

4.2.3 Autocalibration .............................................................................................................................. 37

4.2.4 I

4.2.5 Compensation I

Value ..................................................................................................................................... 37

DAC

Value ............................................................................................................. 37

DAC

4.2.6 Precharge Source ......................................................................................................................... 37

4.2.7 Prescaler ...................................................................................................................................... 37

4.2.8 Resolution ..................................................................................................................................... 38

4.2.9 Scanning Speed ........................................................................................................................... 38

4.2.10 High-Level API Parameters .......................................................................................................... 39

4.2.11 Set High-Level Parameters ........................................................................................................... 40

4.3 Using the SmartSense_EMCPLUS User Module ...................................................................................... 40

4.3.1 Guidelines for SmartSense_EMC_PLUS ...................................................................................... 40

4.3.2 Understanding the Difference ....................................................................................................... 40

4.3.3 Recommended C

Value for SmartSense_EMC_PLUS ........................................................... 41

MOD

4.3.4 SmartSense_EMCPLUS User Module Parameters ...................................................................... 41

4.3.5 SmartSense_EMCPLUS User Module Specific Guidelines .......................................................... 41

4.3.6 Scan Time of a CapSense Sensor................................................................................................ 42

4.3.7 SmartSense_EMCPLUS Response Time ..................................................................................... 43

4.3.8 Method to Ensure Minimum SNR Using the SmartSense_EMCPLUS UM ................................... 44

4.3.9 Firmware Design Guidelines ......................................................................................................... 45

4.4 Design Migration from CY8C20xx6A/AS to CY8C20xx7/S ........................................................................ 47

4.4.1 Discontinued Support/User Modules ............................................................................................ 47

4.4.2 Improvement and New Features................................................................................................... 47

4.4.3 Pin Compatibility ........................................................................................................................... 47

5. Design Considerations ..................................................................................................................................... 48

5.1 Overlay Selection ...................................................................................................................................... 48

5.2 ESD Protection .......................................................................................................................................... 49

5.2.1 Prevent ......................................................................................................................................... 49

5.2.2 Redirect ................................................................ ........................................................................ 49

5.2.3 Clamp ........................................................................................................................................... 49

5.3 Electromagnetic Compatibility (EMC) Considerations ............................................................................... 49

5.3.1 Radiated Interference ................................................................................................................... 49

5.3.2 Radiated Emissions ...................................................................................................................... 50

5.3.3 Conducted Immunity and Emissions ............................................................................................. 50

5.4 Software Filtering ....................................................................................................................................... 50

5.5 Power Consumption .................................................................................................................................. 51

5.5.1 System Design Recommendations ............................................................................................... 51

5.5.2 Sleep-Scan Method ...................................................................................................................... 51

5.5.3 Response Time versus Power Consumption ................................................................................ 51

5.5.4 Measuring Average Power Consumption ..................................................................................... 52

5.6 Pin Assignments ........................................................................................................................................ 52

5.7 GPIO Load Transient ................................................................................................................................. 53

5.7.1 Hardware Guidelines to Reduce GPIO Load Transient Noise ...................................................... 54

5.7.2 Firmware Guidelines to Compensate GPIO Load Transient Noise ............................................... 55

5.8 PCB Layout Guidelines ............................................................................................................................. 57

6. Liquid-Tolerant Design Considerations .......................................................................................................... 58

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 4

Contents

6.1 Shield Electrode and Guard Sensor .......................................................................................................... 58

6.1.1 Shield ............................................................................................................................................ 58

6.1.2 Guard Sensor ............................................................................................................................... 61

6.2 Design Recommendations ........................................................................................................................ 63

7. Proximity Sensing Design Considerations ..................................................................................................... 64

7.1 Types of Proximity Sensors ....................................................................................................................... 64

7.1.1 Button ........................................................................................................................................... 64

7.1.2 Wire .............................................................................................................................................. 64

7.1.3 PCB Trace .................................................................................................................................... 64

7.1.4 Sensor Ganging ............................................................................................................................ 64

7.2 Design Recommendations ........................................................................................................................ 65

8. Low-Power Design Considerations ................................................................................................................. 66

8.1 Additional Power Saving Techniques ........................................................................................................ 66

8.1.1 Set Drive Modes to Analog HI-Z ................................................................................................... 66

8.1.2 Putting it All Together ................................................................................................................... 67

8.1.3 Recommended I2C Slave Implementation in Sleep Mode ............................................................ 67

8.1.4 Sleep Mode Complications ........................................................................................................... 67

8.1.5 Pending Interrupts ........................................................................................................................ 67

8.1.6 Global Interrupt Enable ................................................................................................................. 68

8.2 Post Wakeup Execution Sequence ........................................................................................................... 68

8.2.1 PLL Mode Enabled ....................................................................................................................... 68

8.2.2 Execution of Global Interrupt Enable ............................................................................................ 68

8.2.3 Recommended I2C Slave Implementation in Sleep Mode ............................................................ 68

8.2.4 Sleep Timer .................................................................................................................................. 69

9. Resources .......................................................................................................................................................... 70

9.1 Website ..................................................................................................................................................... 70

9.2 Datasheet ................................................................................................ .................................................. 70

9.3 Technical Reference Manual ..................................................................................................................... 70

9.4 Development Kits ...................................................................................................................................... 71

9.4.1 CY8C20xx7/S QuietZone Starter Kit ............................................................................................. 71

9.4.2 Universal CapSense Controller Kit ............................................................................................... 71

9.4.3 Universal CapSense Module Boards ............................................................................................ 71

9.5 Sample Board Files ................................................................................................................................... 72

9.6 PSoC Programmer .................................................................................................................................... 74

9.7 CapSense Data Viewing Tools .................................................................................................................. 74

9.8 PSoC Designer .......................................................................................................................................... 74

9.9 Code Examples ......................................................................................................................................... 74

9.10 Design Support .......................................................................................................................................... 74

Revision History ......................................................................................................................................................... 75

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 5

1. Introduction

1.1 Abstract

This document provides design guidance for implementing capacitive sensing (CapSense) functionality with the CY8C20xx7/S family of CapSense controllers. The following topics are covered in this guide:

 Features of the CY8C20xx7/S family of CapSense controllers  CapSense principles of operation  Introduction to CapSense design tools  Guide to tune the CapSense system for optimal performance  System electrical and mechanical design considerations for CapSense  Low-power design considerations for CapSense  Additional resources and support for designing CapSense into your system

1.2 Cypress’s CapSense Documentation Ecosystem

Figure 1-1 and Table 1-1 summarize the Cypress CapSense documentation ecosystem. These resources allow

implementers to quickly access the information needed to successfully complete a CapSense product design.

Figure 1-1 shows the typical flow of a product design cycle with capacitive sensing; the information in this guide is

most pertinent to the topics highlighted in green. Table 1-1 provides links to the supporting documents for each of the numbered tasks in Figure 1-1.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 6

Introduction

3. CapSense device selection based on needed functionality

2. Specify system requirements and

characteristics

11. Preproduction build (prototype)

12. Test and evaluate system functionality and CapSense performance

Performance

satisfactory

13. Production

Yes

= Topics covered in this document

1. Understanding CapSense technology

4. Mechanical design

5. Schematic capture and

PCB layout

Design for CapSense

9. Programming PSoC

†

10. CapSense Configuration*

6. PSoC Designer project

creation†

7. Firmware

development†

8. CapSense tuning†

†

= Applicable to MBR family of devices only

= Applicable to programmable devices only

Figure 1-1. Typical CapSense Product Design Flow

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 7

Introduction

Numbered Design Task

in Figure 1-1

Supporting Cypress CapSense Documentation

● Getting Started with CapSense

● CY8C20xx7/S CapSense Device Datasheet

● Getting Started with CapSense

● CY8C20xx7/S CapSense Design Guide (this document)

● Getting Started with CapSense

● PSoC® Designer™ User Guides

● Assembly Language User Guide

● C Language Compiler User Guide

● CapSense Code Examples

● PSoC CY8C20xx7/S Technical Reference Manual

● PSoC Family-Specific CapSense Design Guide (this document)

● PSoC Family-Specific CapSense User Module Datasheets (CSD, CSDPLUS, and

SmartSense_EMC_PLUS)

● PSoC CY8C20xx7/S Technical Reference Manual

● AN2397 -CapSense Data Viewing Tools

● Programmer User Guide

● MiniProg3 User Guide

● ISSP Programming Specifications - CY8C20045, CY8C20055, CY8C20065,

CY8C20xx6A,CY8C20xx7

● AN59389 - Host Sourced Serial Programming for CY8C20xx6A, CY8C20xx6AS, CY8C20xx6L, and

CY8C20xx7/S

● CY8C20xx7/S CapSense Design Guide (this document)

● CapSense Code Examples

Table 1-1. Cypress Documents Supporting Numbered Design Tasks of Figure 1-1

1.3 CY8C20xx7/S CapSense Family Features

Cypress‟s CY8C20xx7/S is a low-power, high-performance, programmable CapSense controller family that features:

Advanced Touch Sensing Features

 Programmable capacitive sensing elements

 Supports a combination of CapSense buttons, sliders, and proximity sensors  Integrated API to implement buttons and sliders  Supports up to 31a capacitive sensors or six slidersb  Supports parasitic sensor capacitance range of 5 pF to 45 pF

 SmartSense™ Auto-tuning enables fast time to market

 Sets and monitors tuning parameters automatically at power on and at runtime  Design portability – self-tunes for changes in user interface design  Environmental compensation during runtime  Detects touches as low as 0.1 pF

 Enhanced noise immunity and robustness

It is assumed that two pins are used for I

See the CY8C20xx7/S datasheet for more information.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 8

C communication and one pin is used for C

connection.

MOD

Introduction

 SmartSense_EMCPLUS compensates for environment and noise variations automatically  SmartSense_EMCPLUS offers superior noise immunity for applications with challenging conducted and

radiated noise conditions

 Internal regulator provides stability against power supply noise and ripple up to 500 mV of supply VDD ripple

acceptable

 Integrated API of software filters for SNR improvement

 Ultra low power consumption

 Three different power modes for optimized power consumption  Active, sleep, and deep-sleep modes (deep-sleep current: 100 nA)  28 μA per sensor at 125 ms wake from sleep rate

 Driven shield available on five GPIO pins

 Delivers best-in-class water tolerant designs  Robust proximity sensing in the presence of metal objects  Supports longer trace lengths  Maximum load of 100 pF (3 MHz)

Device Features

 High-performance, low-power M8C Harvard-architecture processor

 Up to 4 MIPS with 24-MHz internal clock

 Flexible on-chip memory

 Up to 32 KB of flash and 2 KB of SRAM  Emulated EEPROM supported

 Precision, programmable clocking

 Internal main oscillator (IMO): 6/12/24 MHz ± 5%  Option for precision 32-kHz external crystal oscillator

 Enhanced general-purpose input/output (GPIO) features

 34 GPIOs with programmable pin configuration.  25 mA sink current/GPIO and 120 mA total sink current/device  Internal restive pull-up, HI-Z, open-drain, and strong drive modes on all GPIOs

 Peripheral features

 Three 16-bit timers  I2C - master (100 kHz) and slave (400 kHz)  SPI - master and slave - configurable range of 46.9 kHz to 12 MHz  10-bit incremental ADC - 0 to 1.2-V input range

 Operating conditions

 Wide operating voltage: 1.71 V to 5.5 V  Temperature range: –40 °C to +85 °C

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 9

Introduction

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.

[Bracketed, Bold]

Displays keyboard commands in procedures: [Enter] or [Ctrl] [C]

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.

1.4 Document Conventions

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 10

2. CapSense Technology

2.1 CapSense Fundamentals

CY8C20xx7/S

MOD

Sensor 1

X,1

Sensor 2

X,2

Sensor 3

X,3

Sensor 4

X,4

Sensor n

X,n

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 CX,1 through CX,n for a design with n sensors. The circuitry internal to the CY8C20xx7/S device converts the magnitude of each CX into a digital code that is stored for post processing. The other component, C will be discussed in more detail in Capacitive Sensing Methods in CY8C20xx7/S.

Figure 2-1. CapSense Implementation in a CY8C20xx7/S PSoC Device

, is used by the CapSense controller‟s internal circuitry and

MOD

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:

0





Where: CF = The capacitance added by a finger in contact with the overlay over a sensor

= Free space permittivity

= Dielectric constant (relative permittivity) of overlay

A = Area of finger and sensor pad overlap D = Overlay thickness

Equation 2-1



CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 11

CapSense Technology

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 (CP). CP 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 (CX) connected to a sensor pin.

When a finger is not touching a sensor:

=  Equation 2-2

With a finger on the sensor pad, CX equals the sum of CP and CF:

= +  Equation 2-3

In general, CP is an order of magnitude greater than CF. CP usually ranges from 10 pF to 20 pF, but in extreme cases can be as high as 50 pF. CF usually ranges from 0.1 pF to 0.4 pF. The magnitude of CP 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 (CX) 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 (CX) 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.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 12

CapSense Technology

Sigma Delta

Modulator

High Z

input

BUS

Modulation

DAC

Precharge

clock

CY8C20xx7/S

REF

Switched-capacitor input

circuit converts

capacitance to current

External Connection

MOD

2.2nF/X7R/5V

Sigma delta converter converts current to digital counts

Analog Mux

Bus

Sw2

Sw1

Analog Mux Bus (AMUXBUS)

Discharge Clock

Break-

Before-Make

Circuitry

Sw2

Sw1

Figure 2-3. CSD Block Diagram

2.2.1.1 Switched-Capacitor Input

The CSD method in the CY8C20xx7/S device incorporates CX into a switched capacitor circuit, as Figure 2-3 shows.

Figure 2-4. Pin Configured as Switched-Capacitor Input

Two non-overlapping, out-of-phase clocks of frequency FSW (see Figure 2-6) control the switches Sw1 and Sw2. The continuous switching of Sw1 and Sw2 forms an equivalent resistance RS, as Figure 2-5 shows. The value of the equivalent resistance RS is:

CX F

Equation 2-4

RS=

Where: CX = Sensor capacitance FSW = Frequency of the switching clock

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 13

CapSense Technology

AMUXBUS

SENSOR

REF

(1V)

TSW = 1/F

Sw2 CLOSED Sw1 OPEN

Sw2 OPEN Sw1 CLOSED

Figure 2-5. Switched-Capacitor Input Sinking Current from AMUXBUS

The sigma delta converter maintains the voltage of AMUXBUS at a constant V

(this process is explained in Sigma

REF

Delta Converter). Figure 2-6 shows the voltage waveform across the sensor capacitance. Driving switches Sw1 and

Sw2 through non-overlapping precharge clocks thus results in an average current sink (I as Equation 2-5 shows. The magnitude of I

SENSOR

= V



REFRS

= CX FSWV

Equation 2-5

REF

is directly proportional to the magnitude of CX.

SENSOR

) from the AMUXBUS

SENSOR

Figure 2-6. Voltage across Sensor Capacitance (CX)

2.2.1.2 Sigma Delta Converter

The sigma delta converter converts the input current to a corresponding digital count. It consists of a sigma delta modulator and one current-sourcing digital-to-analog converter (I

The sigma delta modulator controls the 8-bit I I

and is referred as “I

DAC

external integrating capacitor C

” or “modulation I

DAC

, as Figure 2-3 on page 13 shows. The recommended value of C

MOD

The sigma delta modulator switches the modulation I across C

, to maintain the C

MOD

voltage at V

MOD

current in an on/off manner. This I

DAC

in this document. The sigma delta converter also requires an

DAC”

ON or OFF corresponding to the small voltage variations

DAC

REF

In maintaining the average AMUX voltage at a steady state value (V average charge current (I

DAC

) to I

by controlling the modulation bit stream duty cycle. The sigma delta converter

SENSOR

stores the bit stream over the duration of a sensor scan and the accumulated result is a digital output value, known as raw count, which is proportional to CX.

The sigma delta converter can operate from 9-bit to 16-bit resolutions. If „N‟ is the resolution of the sigma delta converter and I

raw count = (2

The raw count is interpreted by high-level algorithms to resolve the sensor state and detect touches. Figure 2-7 plots the CSD raw counts from a number of consecutive scans during which the sensor is touched and then released by a finger. As explained in CapSense Fundamentals, the finger touch causes CX to increase by CF, which in turn causes

is the value of the modulation I

DAC

 1)

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 14

REF FSW CX

DAC

current, the approximate equation for the raw count is

DAC

Equation 2-6

), as Figure 2-3 on page 13 shows.

DAC

is known as the modulation

DAC

), the sigma delta converter matches the

REF

is 2.2 nF.

MOD

CapSense Technology

raw counts to increase proportionally. By comparing the shift in steady state raw count level to a predetermined threshold, the high-level algorithms can determine whether the sensor is in the ON (touch) or OFF (no touch) state.

Figure 2-7. CSD Raw Counts during a Finger Touch

The hardware parameters or the CSD/CSDPLUS User Module Low-Level Parameters such as I

and FSW, and the

DAC

firmware parameters or the User Module High-Level Parameters should be tuned to optimum values for reliable touch detection. For a detailed discussion on tuning, see CapSense Performance Tuning with User Modules.

2.2.2 CapSense Sigma Delta (CSD) PLUS

Figure 2-8 shows a block diagram of the CSDPLUS method for converting sensor capacitance (CX) into digital

counts. The main difference between CSDPLUS and CSD method is the number of I I

and the CSD uses a single I

DACs

DAC

used; CSDPLUS uses two

DACs

The CSDPLUS 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. Each block is explained in the following sections.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 15

CapSense Technology

Sigma Delta

Modulator

High Z

input

BUS

Modulation

DAC

Compensation

DAC

Precharge

clock

CY8C20xx7/S

REF

Switched-capacitor input

circuit converts

capacitance to current

External Connection

MOD

2.2 nF/X7R/5 V

Sigma delta converter converts current to digital counts

Analog Mux

Bus

Sw2

Sw1

Analog Mux Bus (AMUXBUS)

Discharge Clock

Break-

Before-Make

Circuitry

Sw2

Sw1

Figure 2-8. CSDPLUS Block Diagram

2.2.2.1 Switched-Capacitor Input

The CSDPLUS method in CY8C20xx7/S devices incorporates CX into a switched-capacitor circuit, as Figure 2-9 shows.

Figure 2-9. Pin Configured as Switched-Capacitor Input

Two non-overlapping, out-of-phase clocks of frequency FSW (see Figure 2-11) control switches Sw1 and Sw2. The continuous switching of Sw1 and Sw2 forms an equivalent resistance RS, as Figure 2-10 shows. The value of the equivalent resistance RS is:

RS=

Equation 2-7

CX F

Where: CX = Sensor capacitance FSW = Frequency of the switching clock

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 16

CapSense Technology

AMUXBUS

SENSOR

REF

(1V)

TSW = 1/F

Sw2 CLOSED Sw1 OPEN

Sw2 OPEN Sw1 CLOSED

raw count = (2

 1)

REF FSW

DAC

 (2N 1)

COMP

DAC

Equation 2-9

Figure 2-10. Switched Capacitor Input Sinking Current from AMUXBUS

The sigma delta converter maintains the voltage of AMUXBUS at a constant V

(this process is explained in Sigma

REF

Delta Converter). Figure 2-11 shows the voltage waveform across the sensor capacitance. Driving switches Sw1 and

Sw2 through non-overlapping precharge clocks thus results in an average current sink (I as Equation 2-8 shows. The magnitude of I

SENSOR

= V



REFRS

= CX FSWV

Equation 2-8

REF

is directly proportional to the magnitude of CX.

SENSOR

) from the AMUXBUS

SENSOR

Figure 2-11. Voltage across Sensor Capacitance (CX)

2.2.2.2 Sigma Delta Converter

The sigma delta converter converts the input current to a corresponding digital count. It consists of a sigma delta modulator and two current-sourcing digital-to-analog converters (I

The sigma delta modulator controls the one 7-bit I modulation I compensation I

and is referred as “I

DAC

, is either always ON or always OFF. This I

DAC

” or “modulation I

DAC

current in an on/off manner. This I

DAC

DAC”

document. The sigma delta converter also requires an external integrating capacitor C

recommended value of C corresponding to the small voltage variations across C

is 2.2 nF. The sigma delta modulator switches the modulation I

MOD

, to maintain the C

MOD

In maintaining the average AMUX voltage at a steady state value (V average charge current (I

DAC

) to I

by controlling the modulation bit stream duty cycle. The sigma delta converter

SENSOR

stores the bit stream over the duration of a sensor scan and the accumulated result is a digital output value, known as raw count, which is proportional to CX.

The sigma delta converter can operate from 9-bit to 16-bit resolutions. If „N‟ is the resolution of the sigma delta converter, I

is the value of the modulation I

DAC

current and I

DAC

equation for the raw count is

Note that raw count values are always positive. Thus, I

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 17

should always be less than 2N V

COMP

), as Figure 2-8 on page 16 shows.

DACs

in this document. The other 7-bit I

is referred as “Compensation I

DAC

, as Figure 2-8 on page 16 shows. The

MOD

voltage at V

MOD

), the sigma delta converter matches the

REF

is the compensation I

COMP

REF

current, the approximate

DAC

REF FSW

is known as the

DAC

, known as the

DAC

” or “I

DAC

COMP

ON or OFF

DAC

” in this

CapSense Technology

The raw count is interpreted by high-level algorithms to resolve the sensor state and detect touches. Figure 2-12 plots the CSD raw counts from a number of consecutive scans during which the sensor is touched and then released by a finger. As explained in CapSense Fundamentals, the finger touch causes CX to increase by CF, which in turn causes raw counts to increase proportionally. By comparing the shift in steady state raw count level to a predetermined threshold, the high-level algorithms can determine whether the sensor is in the ON (touch) or OFF (no touch) state.

Figure 2-12. CSD Raw Counts during a Finger Touch

The hardware parameters or the CSD/CSDPLUS User Module Low-Level Parameters such as I and the firmware parameters or the User Module High-Level Parameters should be tuned to optimum values for reliable touch detection. For a detailed discussion on tuning, see CapSense Performance Tuning with User Modules.

DAC, ICOMP

, and FSW,

2.2.3 SmartSense_EMCPLUS Auto-Tuning

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. Tuning is an iterative process and can be time consuming. SmartSense_EMCPLUS Auto-Tuning was developed to simplify the user interface development cycle. The process is easy to use and significantly reduces the design cycle time by eliminating the tuning process throughout the entire product development cycle, from prototype to mass production. SmartSense_EMCPLUS tunes each CapSense sensor automatically at power-up and then monitors and maintains optimum sensor performance during runtime. This technology adapts for manufacturing variation in PCBs, overlays, and noise generators such as LCD inverters, AC line noise, and switch-mode power supplies, and automatically tunes them out.

2.2.3.1 Process Variation

The SmartSense_EMCPLUS User Module (UM) for the CY8C20xx7/S is designed to work with sensor parasitic capacitance in the range 5 pF to 45 pF, (typical sensor CP values are in the range 10 pF to 20 pF). The sensitivity parameter for each sensor is set automatically, based on the characteristics of that particular sensor. This improves the yield in mass production, because consistent response is maintained from every sensor regardless of CP variation between sensors within the specified range (5 pF to 45 pF). Parasitic capacitance of the individual sensors can vary due to PCB layout, PCB manufacturing process variation, or with vendor-to-vendor PCB variation within a multisourced supply chain. The sensitivity of a sensor depends on its parasitic capacitance; higher CP values decrease the sensor sensitivity and result in decreased finger touch signal amplitude. In some cases, the change in CP value detunes the system, resulting in less than optimum sensor performance (either too sensitive or not sensitive enough) or worst case, a nonoperational sensor. In either situation, you must retune the system, and in some cases requalify the UI subsystem. SmartSense_EMCPLUS Auto-Tuning solves these issues.

SmartSense_EMCPLUS Auto-Tuning makes platform designs possible. Imagine the capacitive touch-sensing multimedia keys in a laptop computer; the spacing between the buttons depends on the size of the laptop and keyboard layout. In this example, the wide-screen machine has larger spaces between the buttons than a standardscreen model. More space between buttons means increased trace length between the sensor and the CapSense controller, which leads to higher parasitic capacitance of the sensor. This means that the parasitic capacitance of the CapSense buttons can be different in different models (Figure 2-13 and Figure 2-14) of the same platform design. Though the functionality of these buttons is the same for all laptop models, the sensors must be tuned for each model. SmartSense_EMCPLUS enables you to do platform designs using the recommended best practices shown in the PCB Layout in Getting Started with CapSense, knowing the tuning will be done efficiently and automatically.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 18

CapSense Technology

Feasibility

Study

Schematics

Design

PCB Layout

Design

Mechanical Design

Review

System

Integration

Re-tuning for any

changes

Tuning process

Production Fine

Tuning

Design

Validation

Production

Typical Capacitive User Interface Design Cycle

Firmware

Development

Feasibility

Study

Schematics

Design

PCB Layout

Design

Mechanical Design

Review

System

Integration

Design

Validation

Production

SmartSense-based Capacitive User Interface Design Cycle

Firmware

Development

Figure 2-13. Design of Laptop Multimedia Keys for a 21-Inch Model

Figure 2-14. Design of Laptop Multimedia Keys for a 15-Inch Model with Identical Functionality and Button Size

2.2.3.2 Reduced Design Cycle Time

Usually, the most time-consuming task for a capacitive sensor interface design is firmware development and sensor tuning. With a typical touch-sensing controller, the sensor must be retuned when the same design is ported to different models or when there are changes in the mechanical dimensions of the PCB or the sensor PCB layout. A design with SmartSense_EMCPLUS solves these challenges because it needs less firmware development effort, no tuning, and no retuning. This makes a typical design cycle much faster. Figure 2-15 compares the design cycles of a typical touch-sensing controller and a SmartSense_EMC_PLUS-based design.

Figure 2-15. Typical Capacitive Interface Design Cycle Comparison

2.2.4 Selecting the User Module

The SmartSense_EMCPLUS eliminates the tuning process that is required by CSD and CSDPLUS UM. It is recommended to use SmartSense_EMCPLUS to simplify the CapSense design process. However, there may be instances where the sensor parameters such as resolution and prescaler need to be controlled to optimize device power consumption or detect touch for a sensor with high CP. In such cases, use the CSD/CSDPLUS UM.

The following are the advantages of the CSDPLUS UM when compared to CSD:

 CSDPLUS UM provides higher SNR for a given sensor resolution (or scan time).  CSDPLUS UM requires less time to scan a sensor to achieve the same SNR. Because of the short scan time,

the average power consumptiona of the CapSense device with CSDPLUS UM will be lower than the CSD UM when they are tuned to achieve the same SNR.

However, if the sensor CP is below 10 pF, the CSDPLUS UM provides lower SNR when compared to the CSD UM and it is recommended to use the CSD UM instead of the CSDPLUS UM.

Note: When I

Here the device is assumed to enter sleep mode after scanning the sensor and wakeup periodically to reduce the device power

consumption.

is set to „0‟, the CSDPLUS UM behavior will be the same as the CSD UM.

COMP

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 19

3. CapSense Design Tools

3.1 Overview

Cypress offers a full line of hardware and software tools for developing your CapSense capacitive touch-sensing application. See Resources for ordering information.

3.1.1 PSoC Designer and User Modules

Cypress‟s exclusive integrated design environment, PSoC Designer, allows you to configure analog and digital

blocks, develop firmware, and tune and debug your design. Applications are developed in a drag-and-drop design environment using a library of user modules. User modules are configured either through the Device Editor GUI or by writing into specific registers with firmware. PSoC Designer comes with a built-in C compiler and an embedded programmer. A pro compiler is available for complex designs.

The CSD and CSDPLUS User Module implements capacitive touch sensors using switched-capacitor circuitry, an analog multiplexer, a comparator, digital counting functions, and high-level software routines (APIs). User modules for other analog and digital peripherals are available to implement additional functionality such as I2C, SPI, TX8, and timers.

Figure 3-1. PSoC Designer Device Editor

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CapSense Design Tools

3.1.1.1 Getting Started with CapSense User Modules

To create a new CY8C20xx7/S project in PSoC Designer:

1. Create a new PSoC Designer project with CY8C20xx7/S as the target device.

2. Select and place the CSDPLUS/SmartSense_EMCPLUS User Module.

3. Right-click the user module to access the User Module wizard.

4. Set button sensor count, slider configuration, pin assignments, and associations.

5. Set pins and global user module parameters.

6. Generate the application and switch to the Application Editor.

7. Adapt sample code from the user module datasheet to implement buttons or sliders. For a detailed procedure for creating a PSoC Designer project and configuring the User Module wizard, refer to the

datasheet of the specific user module. For code examples on CapSense user modules, see Code Examples.

3.1.2 CY8C20xx7/S QuietZone Starter Kit

The CY8C20xx7/S QuietZone Starter Kit features simple plug-in hardware to make prototyping easy. The kit is available from our module partner ArtaFlex at the following link: www.artaflexmodules.com/quietzone

Figure 3-2. CY8C20xx7/S QuietZone Starter Kit

3.1.3 CapSense Data Viewing Tools

Many times during CapSense design, you will want to monitor CapSense data (raw counts, baseline, difference counts, and so on) for tuning and debugging purposes. There are two CapSense data viewing tools, MultiChart and Bridge Control Panel. These tools are explained in the application note AN2397 – CapSense Data Viewing Tools.

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CapSense Design Tools

Raw Count

Difference Count

3.2 User Module Overview

Figure 3-3. User Module Block Diagram

User modules contain an entire CapSense system from physical sensing to data processing. The behavior of the user module is defined using a variety of parameters. These parameters affect different parts of the sensing system and can be separated into low-level and high-level parameters that communicate with one another using global arrays.

Low-level parameters define the behavior of the sensing method at the physical layer and relate to the conversion from capacitance to raw count such as the speed and resolutions for scanning sensors. Low-level parameters are unique to each type of sensing method and are described in CSD/CSDPLUS User Module Low-Level Parameters and SmartSense_EMCPLUS User Module Parameters.

High-level parameters, such as debounce counts and noise thresholds, define how the raw counts are processed to produce information such as the sensor ON/OFF state and the estimated finger position on a slider. These parameters are the same for all sensing methods and are described in User Module High-Level Parameters.

3.3 CapSense User Module Global Arrays

Before learning CapSense User Module parameters, you must be familiar with certain global arrays used by the CapSense system. These arrays should not be altered manually, but may be inspected for debugging purposes.

Figure 3-4. Raw Count, Baseline, Difference Count, and Sensor State

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CapSense Design Tools

3.3.1 Raw Count

The hardware circuit in the CapSense controller measures the sensor capacitance CX. The circuit stores the result in a digital form called raw count upon calling the user module API UMname_ScanSensor(), where UMname can be CSD, CSDPLUS, or SmartSenseEMC_PLUS.

The raw count of a sensor is proportional to its sensor capacitance. Raw count increases as the sensor capacitance value increases.

The raw count values of sensors are stored in the UMname_waSnsResult[] integer array. This array is defined in the header file UMname.h.

3.3.2 Baseline

Baseline can be considered as the raw count value corresponding to the parasitic capacitance of sensor, CP. Gradual environmental changes such as temperature and humidity affect the sensor CP and hence CX, which results in variations in the raw counts.

The user module uses a complex baselining algorithm to compensate for these variations. The algorithm uses baseline variables to accomplish this. The baseline variables keep track of any gradual variation in raw count values. Essentially, the baseline variables hold the output of a digital low-pass filter to which raw count values are input.

The baselining algorithm is executed by the user module API UMname_UpdateSensorBaseline, where UMname can be CSD, CSDPLUS, or SmartSense_EMC_PLUS.

The baseline values of sensors are stored in UMname_waSnsBaseline[] integer array. This array is defined in the header file UMname.h.

3.3.3 Difference Count (Signal)

The difference count, which is also known as the signal of a sensor, is defined as the difference in counts between a sensor‟s raw count and baseline values. When the sensor is inactive, the difference count is zero. Activating sensors (by touching) results in a positive difference count value.

The difference count values of sensors are stored in the UMname_waSnsDiff[] integer array, where UMname can be CSD, CSDPLUS, or SmartSense_EMC_PLUS. This array is defined in the header file UMname.h.

Difference count variables are updated by the user module API UMname_UpdateSensorBaseline().

3.3.4 Sensor State

Sensor state represents the active/inactive status of the physical sensors. The state of the sensor changes from 0 to 1 upon finger touch and returns to 0 upon finger release.

Sensor states are stored in a byte array named UMname_baSnsOnMask[] array, where UMname can be CSD, CSDPLUS, or SmartSense_EMCPLUS. This array is defined in the header file UMname.h. Each array element stores the sensor state of eight consecutive sensors.

Sensor states are updated by the user module API UMname_bIsAnySensorActive().

3.4 CSD/CSDPLUS User Module Parameters

The CSD/CSDPLUS User Module parameters are classified into high-level and low-level parameters. See Figure 3-5 for a list of CSDPLUS user module parameters and how they are classified. The only difference between the CSD and CSDPLUS UM parameter window is that the CSD UM does not have Compensation iDAC Value.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 23

CapSense Design Tools

Figure 3-5. PSoC Designer – CSDPLUS Parameter Window

3.4.1 User Module High-Level Parameters

3.4.1.1 Finger Threshold

The Finger Threshold parameter is used by the user module to judge the active/inactive state of a sensor. If the difference count value of a sensor is greater than the finger threshold value, the sensor is judged as active. This definition assumes that the hysteresis level is set to „0‟ and debounce is set to „1‟.

Possible values are 3 to 255. For the recommended value, see Set High-Level Parameters.

3.4.1.2 Hysteresis

The Hysteresis setting prevents the sensor ON state from chattering because of system noise. The function of hysteresis is given in Equation 3-1. This equation assumes that debounce is set to „1‟.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 24

CapSense Design Tools

Figure 3-6. Sensor State versus Difference Count with Hysteresis Set to Zero

Figure 3-7. Sensor State versus Difference Count with Hysteresis

   + ,  =      ,  =  Equation 3-1

Possible values are 0 to 255. For the recommended value, see Set High-Level Parameters.

3.4.1.3 Debounce

The Debounce parameter prevents spikes in raw counts from changing the sensor state from OFF to ON. For the sensor state to transition from OFF to ON, the difference count value must remain greater than the finger threshold value plus the hysteresis level for the number of samples specified.

Possible values are 1 to 255. A setting of „1‟ provides no debouncing. For the recommended value, see Set High-Level Parameters.

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3.4.1.4 Baseline Update Threshold

As previously explained, the baseline variables keep track of any gradual variations in raw count values. In other words, baseline variables hold the output of a digital low-pass filter to which the raw count values are input. The Baseline Update Threshold parameter is used to adjust the time constant of this low-pass filter.

Baseline update threshold is directly proportional to the time constant of this filter. The higher the baseline update threshold value, the higher the time constant.

Possible values are 0 to 255. For the recommended value, see Set High-Level Parameters.

3.4.1.5 Noise Threshold

The user module uses the Noise Threshold value to interpret the upper limit of noise counts in the raw count. For individual sensors, the baselining update algorithm is paused when the raw count is greater than the baseline and the difference between them is greater than this threshold.

For slider sensors, the centroid calculation is paused when the difference count is greater than the noise threshold value.

Possible values are 3 to 255. For proper user module operation, the noise threshold value should never be set higher than finger threshold minus hysteresis.

For the recommended value, see Set High-Level Parameters.

3.4.1.6 Negative Noise Threshold

The Negative Noise Threshold helps the user module to understand the lower limit of noise counts in the raw count. The baselining update algorithm is paused when the raw count is below the baseline and the difference between them is greater than this threshold.

Possible values are 0 to 255. For the recommended value, see Set High-Level Parameters.

3.4.1.7 Low Baseline Reset

The Low Baseline Reset parameter works in conjunction with the negative noise threshold parameter. If the sample count values are less than the baseline minus the negative noise threshold for the specified number of samples, the baseline is set to the new raw count value. It essentially counts the number of abnormally low samples required to reset the baseline. The sample value is used to correct the finger-on-at-startup condition.

Possible values are 0 to 255. For the recommended value, see Set High-Level Parameters.

3.4.1.8 Sensors Autoreset

The Sensors Autoreset parameter determines whether the baseline is updated at all times, or only when the difference counts are less than the noise threshold value.

When Sensors Autoreset is enabled, the baseline is updated constantly. This limits the maximum time duration of the sensor but prevents the sensors from permanently turning on when the raw count accidentally rises without anything touching the sensor. This sudden rise can be caused by a large voltage fluctuation in the power supply, a high-energy RF noise source, or a quick temperature change.

When Sensors Autoreset is disabled, the baseline is updated only when the difference counts are less than the noise threshold parameter.

Possible values are Enabled and Disabled. For the recommended value, see Set High-Level Parameters.

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CapSense Design Tools

raw count = GC C

 (2N 1)

COMP

DAC

Equation 3-2

raw count

CPCP+C

maximum raw count = 2

CapSense Signal

Slope of the line = G

3.4.2 CSD/CSDPLUS User Module Low-Level Parameters

The CSD/CSDPLUS User Module has several low-level parameters in addition to the high-level parameters. These parameters are specific to the CSD/CSDPLUS sensing method and determine how raw count data is acquired from the sensor.

3.4.2.1 I

The I

DAC

change in CX). A higher value of I

According to Equation 2-9, the raw count can be related to CX and I

Value

DAC

parameter sets the range and sensitivity of the capacitance measurement (the change in raw count per unit

corresponds to a higher range, but a lower sensitivity as explained here.

DAC

as follows.

DAC

Where GC is the capacitance to digital conversion gain, and is equal to (2N 1)

Consider a case where compensation I

value, that is, I

DAC

is zero. In this case, the raw count relation to CX

COMP

(Equation 3-2) can be represented by Figure 3-8.

Figure 3-8. Raw Count versus Sensor Capacitance

REF FSW

DAC

The change in raw count when a finger is placed on the sensor is called a CapSense signal. Figure 3-9 shows how the value of the signal changes with respect to the conversion gain.

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CapSense Design Tools

raw count

CPCP+C

maximum raw count = 2

Signal 2

Signal 1

Baseline 1

Baseline 2

GC3 > GC2 > G

raw count

CPCP+C

maximum raw count = 2

85 % of maximum

raw count

Signal

Figure 3-9. Signal Values for Different Conversion Gains

Figure 3-9 shows three plots corresponding to three conversion gain values GC3, GC2, and GC1. An increase in the

conversion gain results in a higher signal value. However, this increase in the conversion gain also moves the raw count corresponding to CP towards the maximum value of raw count (2N). For very high gain values, the raw count saturates as the plot of GC3 shows. Therefore, you should tune the conversion gain to get a good signal value while avoiding a saturation of raw count. It is recommended to tune the gain such that the raw count corresponding to CP is 85 percent of the maximum raw count, as Figure 3-10 shows.

This parameter can be changed at runtime using the corresponding API function. Possible values are 1 to 127.

Figure 3-10. Recommended Tuning if I

COMP

is Zero

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CapSense Design Tools

raw count

CPCP+C

maximum raw count = 2

85 % of maximum

raw count

Signal when

COMP

≠ 0

COMP

/ V

REF FSW

COMP

= 0

COMP

≠ 0

Signal when

COMP

= 0

Resolution

Scan Speed for Individual Buttons (µs)

Ultra Fast

Fast

Normal

Slow 9 57

125

205

125

205

380

125

205

380

720

205

380

720

1400

380

720

1400

2800

720

1400

2800

5600

1400

2800

5600

11000

2800

5600

11000

22000

3.4.2.2 Compensation I

The Compensation I

parameter sets the offset of the raw count versus CX graph. Compensation I

DAC

Value

may be used

DAC

to achieve a high sensitivity for high-CP sensors. Figure 3-11 shows a plot of raw count versus sensor capacitance when I

is non-zero.

COMP

Figure 3-11. Raw Count versus CX When I

is Non-zero

COMP

Increasing the value of the compensation I

increases the CX axis intercept to: 󰇡

DAC

COMP

󰇢

REF

This also increases the slope of the line when the raw count is again tuned to 85 percent of the maximum value. Therefore, the signal increases when the compensation I

value increases.

DAC

Note that this increase in signal may not proportionally increase the SNR because of the variations in noise counts when the compensation I

The dual I

mode complicates the tuning process because of the additional variable (I

DAC

is used.

DAC

) involved. However, you

COMP

can use this mode to increase the signal. The details are explained in Tuning the CSD/CSDPLUS User Module. This parameter can also be changed at runtime using the corresponding API function. Possible values are 1 to 127.

3.4.2.3 Resolution

This parameter determines the scanning resolution in bits. The maximum raw count for scanning resolution of N bits is 2N–1. Increasing the resolution improves sensitivity, but increases scan time and hence reduces response rate.

Possible values are 9 to 16 bits.

Table 3-1. Resolution and Scan Speed

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3.4.2.4 Scanning Speed

This parameter sets the sensor scanning speed by setting the clock input to the Sigma Delta Converter. Although a faster scanning speed provides a good response time, slower scanning speeds give the following advantages:

 Improved SNR  Better immunity to power supply and temperature changes

 Less demand for system interrupt latency; you can handle longer interrupts

Possible values are Ultra Fast, Fast, Normal, and Slow.

3.4.2.5 Shield Electrode Out

A shield electrode is used to reduce parasitic capacitance and achieve liquid tolerance. Refer to Liquid-Tolerant

Design Considerations in this document and the “Shield Electrode and Guard Sensor” section in Getting Started with

CapSense for more details on shield electrode. This parameter selects where to route the output of the shield

electrode. Possible values are P0[0], P1[2], P0[2], P2[2], and P2[4].

3.4.2.6 Precharge Source

This parameter selects the clock source for precharge switches, referred to as precharge clock in the Switched-

Capacitor Input section.

Possible values are PRS and Prescaler. The selection of Prescaler sets the switching clock frequency as IMO/prescaler. The selection of PRS passes the divided IMO clock through a pseudo-random generator, providing a spread-spectrum clock. Use the PRS source in most cases to get better EMI immunity and lower emissions as PRS averages the switching frequency over a wide range.

3.4.2.7 Prescaler

This parameter sets the prescaler ratio and determines the precharge switch output frequency. This parameter also affects the PRS output frequency.

Possible values are 1, 2, 4, 8, 16, 32, 64, 128, and 256.

3.4.2.8 PRS Resolution

This parameter changes the PRS sequence length. When very short scan times are needed, an 8-bit PRS must be used to avoid excessive noise. The scan time is

determined by the Resolution (not to be confused with PRS resolution) parameter. For scan times less than or equal to 380 μs, PRS resolution should be set to 8 bits; for scan times of greater than 380 μs, PRS resolution should be set to 12 bits. The default setting is 8 bits.

3.4.2.9 Autocalibration

When Autocalibration is enabled, the modulation and compensation I raw count baseline at approximately 85 percent of 2N, where “N” is the Resolution setting of the sensor. Enabling autocalibration overrides the device editor settings of I

DAC

and I

COMP

When autocalibration is disabled, the raw count value depends on I capacitance, IMO frequency, prescaler, precharge source, and V

REF

Autocalibration consumes ROM and RAM resources and increases start time. Autocalibration does not automatically select the I increase the I

range value. If the raw count value after calibration is less than half of the resolution range, you should

DAC

range or reduce the precharge frequency. Autocalibration works to improve marginally functional

DAC

configurations.

3.4.2.10 I

The I

Range parameter scales the I

DAC

Range

current output. For example, selecting 8x will scale the I

DAC

the 4x range. Possible values are 4x and 8x.

are automatically calibrated to establish a

DACs

range, I

DAC

value, resolution, sensor

DAC

parameters set in the device editor.

output to twice

DAC

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High Level

Low Level

3.5 SmartSense_EMCPLUS User Module Parameters

Figure 3-12. PSoC Designer SmartSense_EMCPLUS Parameters

3.5.1.1 Immunity Level

This parameter defines the immunity level of the user module against the external noise. Selecting a High immunity level provides maximum immunity against the external noise. A Medium immunity level provides moderate immunity. Setting the immunity level to Medium consumes twice the scan time and RAM memory, and setting the immunity level to High consumes three times the scan time and RAM memory for sensor implementation compared to the Low immunity mode.

Possible values are Low, Medium, and High.

Figure 3-13. PSoC Designer SmartSense_EMCPLUS Global Setting

3.5.1.2 Threshold Setting Mode

Selecting the Manual threshold mode provides flexibility in setting the finger threshold for each sensor. Selecting Automatic threshold mode causes the SmartSense_EMCPLUS user module to automatically set the thresholds for each sensor. Automatic threshold mode consumes more RAM than Manual threshold mode.

Possible values are Manual and Automatic.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 31

CapSense Design Tools

Low Level

High Level

Figure 3-14. PSoC Designer SmartSense_EMC_PLUS Sensor Setting

3.5.1.3 Sensor Sensitivity

This parameter is used to increase and decrease the sensitivity of a sensor. Possible values are 0.1 pF, 0.2 pF, 0.3 pF, and 0.4 pF

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 32

4. CapSense Performance Tuning with User Modules

Optimal user module parameter settings depend on board layout, button dimensions, overlay material, and application requirements. These factors are discussed in Design Considerations. Tuning is the process of identifying the optimal parameter settings for robust and reliable sensor operation.

4.1 General Considerations

4.1.1 Signal, Noise, and SNR

A well-tuned CapSense system reliably discriminates between ON and OFF sensor states. To achieve this level of performance, the CapSense signal must be significantly larger than the CapSense noise. The CapSense signal is compared to the CapSense noise by using a quantity called signal-to-noise ratio (SNR). Before discussing the meaning of SNR for CapSense, it is first necessary to define signal and noise in the context of touch sensing.

4.1.1.1 CapSense Signal

The CapSense signal is the change in the sensor raw count when a finger is placed on the sensor, as demonstrated in Figure 4-1. The output of the sensor is a digital counter with a value that tracks the sensor capacitance. In this example, the average level without a finger on the sensor is 5925 counts. When a finger is placed on the sensor, the average output increases to 6060 counts. Because the CapSense signal tracks the change in counts due to the finger, Signal = 6060 – 5925 = 135 counts.

Figure 4-1. CapSense Signal and Noise

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CapSense Performance Tuning with User Modules

Vref

5*Rx*C

min

>= 10*Rx*C

Vref

4.1.1.2 CapSense Noise

CapSense noise is the peak-to-peak variation in sensor response when a finger is not present, as demonstrated in

Figure 4-1. In this example, the output waveform without a finger is bounded by a minimum of 5912 counts and a

maximum of 5938 counts. Because the noise is the difference between the minimum and the maximum values of this waveform, Noise = 5938 – 5912 = 26 counts.

4.1.1.3 CapSense SNR

CapSense SNR is the simple ratio of signal and noise. Continuing with the example, if the signal is 135 counts and noise is 26 counts, the SNR is 135:26, which reduces to an SNR of 5.2:1. The minimum recommended SNR for CapSense is 5:1, which means the signal is five times larger than the noise. Filters are commonly implemented in firmware to reduce noise. See Software Filtering for more information.

4.1.2 Charge/Discharge Rate

To achieve maximum sensitivity in the tuning process, the sensor capacitor must be fully charged and discharged during each cycle. The charge/discharge path switches between two states at a rate set by a user module parameter called Precharge Clock in the CSDPLUS User Module.

The charge/discharge path includes series resistance that slows down the transfer of charge. The rate of change for this charge transfer is characterized by an RC time constant involving the sensor capacitor (C) and series resistance (R), as shown in Figure 4-2.

Figure 4-2. Charge/Discharge Waveforms

Set the charge/discharge rate to a level that is compatible with this RC time constant. In general, you should allow a period of 5 RC for each transition, with two transitions per period (one charge, one discharge). The equations for minimum time and maximum frequency are:



= 10 ×  Equation 4-1



10×





For example, assume the series resistor includes a 560-Ω external resistor and up to 800 Ω of internal resistance,

Equation 4-2



and the sensor capacitance is typical: RX = 1.4 kΩ CX= 24 pF The value of the time constant and maximum front-end switching frequency in this example is: Ts

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 34

= 0.34 µs

min

CapSense Performance Tuning with User Modules

= 3 MHz

max

4.1.3 Importance of Baseline Update Threshold Verification

Temperature and humidity both cause the average number of counts to drift over time. The baseline is a reference count level for CapSense measurements that is an important part of compensating for environmental effects. Highlevel decisions, such as Finger Present and Finger Absent states, are based on the reference level established by the baseline. Because each sensor has unique parasitic capacitance associated with it, each capacitive sensor has its own baseline.

Baseline tracks the change in counts at a rate set by the Baseline Update Threshold parameter. Make sure to match the update rate to the intended application. If the update rate is too fast, the baseline will compensate for any changes introduced by a finger, and the moving finger will not be detected. If the update rate is too slow, relatively slow environmental changes may be mistaken for fingers. During development, you should verify the Baseline Update Threshold settings.

4.2 Tuning the CSD/CSDPLUS User Module

Figure 4-3 is a flow chart showing the tuning process for CSD/CSDPLUS UM parameters. CSD/CSDPLUS UM

parameters can be separated into two broad categories: low-level (hardware) parameters and high-level API parameters. The parameters in these categories affect the behavior of the capacitive sensing system in different ways. However, there is a complementary relationship between the sensitivity of each sensor as determined by the hardware parameter settings and many of the high-level parameter settings. You must consider this fact when you change any hardware parameter to make sure that the corresponding high-level parameters are adjusted accordingly. Tuning CSD/CSDPLUS User Module parameters should always begin with the hardware parameters.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 35

CapSense Performance Tuning with User Modules

Measure Cp of sensors

using EP64134

Start

If max Cp < 45 pF set

Idac Range to 4X.

Otherwise set to 8X.

Enable Autocalibration

Set Precharge Source to

PRS

Set Prescaler according

to Table 4-1

Set Resolution according to Figure 29 and Table 4-2

Set Scanning Speed to

Fast

Set PRS Resolution to

12 bits if scan time >

380 µs according to

Table 4-3. Otherwise set

to 8 bits.

Set Shield

Electrode Out

according to needs

of design

Establish digital

communication interface and

obtain raw counts from each

sensor during finger

activation cycle, for example.

Figure 30

Is SNR from all sensors

> 5:1?

Increase Resolution and/or

experiment with Scanning

Speed until optimal SNR is

obtained

Is SNR from all sensors

> 5:1?

Revise PCB design

according to AN2292 and

AN2318

Does scan time meet

design requirements?

Yes

Reduce Resolution or

use faster Scanning

Speed

Yes

Set Finger Threshold

to 80% of signal

Set Noise Threshold

to 40% of signal

Set BaselineUpdate Threshold to double

the Noise Threshold

Set Sensors

Autoreset according to

needs of design

Set Hysteresis to 10%

of signal

Set Debounce

according to needs of

design

Set Negative Noise Threshold to same

value as Noise

Threshold

Set Low Baseline

Reset to 10

End

Hardware

Parameters

High-Level API

Parameters

Figure 4-3. Tuning the CSD/CSDPLUS User Module

Hardware parameters configure the hardware that the CSD/CSDPLUS method uses to convert the physical capacitance of each sensor into a digital code. This section describes these parameters and provides guidance about how each should be tuned based on system characteristics and other parameters.

By default, hardware parameters are global settings that apply to all CapSense sensors in a design. In designs where total parasitic capacitance of each sensor (CP), sensor sensitivity, or both, vary over a wide range, there may not be global hardware parameter settings that are suitable for all sensors. In such cases, the SetIdacValue(i), SetPrescaler(i), and SetScanMode(i) API functions can be used to configure the respective hardware parameters for each sensor, where (i) is the sensor index before calling the ScanSensor(i) API function.

Table 4-1 and Table 4-3 provide tuning recommendations for several key hardware parameters based on sensor CP.

CP values depend on the characteristics of the PSoC, PCB layout, and proximity of other components in the assembled product. Because of this, CP must be measured in its original position with the system in its final assembled state; that is, in the same enclosure and with the same overlay as the system will have in service. The best way to measure CP is to use the code example "Measuring Absolute Sensor Capacitance with a CY8C20xx6A CapSense Controller" available in the CapSense Controller Code Examples Design Guide. This project measures the absolute capacitance of each sensor in a system using the PSoC itself, thus taking into account all factors affecting CP. See the documentation associated with the code example for instructions on its setup and use.

4.2.1 Recommended C

The recommended C C

stability over temperature and the capacitor should have a voltage rating not less than 5 V.

MOD

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 36

Value for CSD/CSDPLUS

MOD

value for a CSD-based design is 2.2 nF. X7R or NPO type capacitors are recommended for

MOD

CapSense Performance Tuning with User Modules

CP (pF)

Precharge Source = PRS

Precharge Source = Prescaler

Prescaler

IMO = 24 MHz

Prescaler

IMO = 12 MHz

Prescaler

IMO = 6 MHz

Prescaler

IMO = 24 MHz

Prescaler

IMO = 12 MHz

Prescaler

IMO = 6 MHz

<6 1 Note 1

Note 1

2 1 1

7–11

2 1 Note 1

4 2 1

12–15

2 1 Note 1

4 2 1

16–19

4 2 1 8 4 2 20–22

4 2 1 8 4 2 23–26

4 2 1 8 4 2 27–30

4 2 1 8 4 2 31–34

4 2 1 8 4 2 35–37

8 4 2

16 8 4

4.2.2 I

Range

DAC

For projects where the maximum sensor CP is less than 45 pF, use 4X; otherwise, use 8X.

4.2.3 Autocalibration

Autocalibration should always be set to „Enabled‟ in CY8C20xx7/S CSD/CSDPLUS designs. The ability of the autocalibration algorithm to successfully set the I recommended size.

4.2.4 I

DAC

Value

This parameter determines the current output of I enabled, as recommended, this parameter is overridden and has no effect. When autocalibration is disabled, raising this parameter lowers the raw count baseline and vice versa. Also, as recommended in IDAC Value in

CSD/CSDPLUS User Module Low-Level Parameters, I

of 2N.

4.2.5 Compensation I

DAC

Value

This parameter determines the current output of the Compensation I parameter is applicable only for the CSDPLUS UM. When autocalibration is enabled, as recommended, this parameter is overridden and has no effect. When autocalibration is disabled, raising this parameter lowers the raw count baseline and vice versa. If auto-calibration is disabled, use the following method to tune compensation I value.

Initially, tune the raw counts to 85 percent of the maximum raw counts by only using the modulation I compensation I tune the modulation I with a higher value of compensation I

at zero). Then, increase the compensation I

DAC

to get 85 percent of the maximum raw counts. Note down the SNR. Repeat this procedure

DAC

, until maximum SNR is achieved. Note, that a higher compensation I

DAC

value results in higher signal (see Compensation IDAC Value in CSD/CSDPLUS User Module Low-Level Parameters for more details). Also, note that the best SNR is generally achieved with I autocalibration also sets the I

DAC

and I

such that I

COMP

relies on the prescaler being set properly and C

DAC

when autocalibration is disabled. When autocalibration is

DAC

value should be chosen such that raw count is 85 percent

DAC

when autocalibration is disabled. This

DAC

to 10 percent of the modulation I

DAC

value = I

DAC

= I

and raw counts are near 85 percent of 2N.

COMP

MOD

value, and re-

DAC

value. Enabling

COMP.

being the

(keep the

DAC

4.2.6 Precharge Source

This parameter selects the sensor switching clock source. The available options are Prescaler, which uses the IMO through a divider, or PRS, which passes the divided IMO clock through a pseudo random generator, providing a spread-spectrum clock. PRS provides superior noise immunity and lower noise emissions and is therefore the recommended default setting for Precharge Source. In some instances, the prescaler precharge source can provide higher SNR. However, when using copper circuitry, this SNR improvement is usually marginal and rarely justifies foregoing the benefits of PRS.

4.2.7 Prescaler

Prescaler is the divider applied to the IMO to develop the precharge clock. This is the most critical hardware UM parameter for properly tuning a CSD design. Prescaler depends on the selected precharge source, IMO, and the CP of the sensors being scanned. Table 4-1 gives recommended prescaler settings based on these parameters.

Table 4-1. Prescaler Setting Based on Precharge Source, IMO, and CP

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 37

CapSense Performance Tuning with User Modules

CP (pF)

Precharge Source = PRS

Precharge Source = Prescaler

Prescaler

IMO = 24 MHz

Prescaler

IMO = 12 MHz

Prescaler

IMO = 6 MHz

Prescaler

IMO = 24 MHz

Prescaler

IMO = 12 MHz

Prescaler

IMO = 6 MHz

38–41

8 4 2

16 8 4

42–45

8 4 2

16 8 4

46–49

8 4 2

16 8 4

50–52

8 4 2

16 8 4

53–56

8 4 2

16 8 4

57–60

8 4 2

16 8 4

CP (pF)

CF = 0.1 pF

CF = 0.2 pF

CF = 0.4 pF

CF = 0.8 pF

10 9 8

7–12

10 9 13–24

25–48

>49

Note 1 This combination of Precharge Source, Prescaler, and CP is not recommended.

4.2.8 Resolution

Available choices are 9 to 16 bits. Raising the resolution raises sensitivity, SNR, and noise immunity at the expense of scan time. The maximum raw count (full scale range) for scanning resolution N is 2N-1. Table 4-2 gives recommended resolution settings based on CP and the finger capacitance CF. CF is the change in capacitance of a sensor when a finger is placed on the sensor. CF depends on overlay thickness, sensor size, and proximity of the sensor to other large conductors. Figure 4-4 gives CF values as a function of overlay thickness and circular sensor diameter.

Figure 4-4. Finger Capacitance (CF) Based on Overlay Thickness and Circular Sensor Diameter

Table 4-2. Resolution Setting Based on Finger Capacitance and CP

4.2.9 Scanning Speed

This parameter controls the integration time for each LSB of the scan result. The choices are Ultra Fast, Fast, Normal, and Slow. Fast is generally a good starting point. In some, but not all cases, slower scanning speed can yield

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 38

CapSense Performance Tuning with User Modules

Resolution

(bits)

Scanning Speed

Ultra Fast

Fast

Normal

Slow

125

205

125

205

380

125

205

380

720

205

380

720

1400

380

720

1400

2800

720

1400

2800

5600

1400

2800

5600

11000

2800

5600

11000

22000

Noise

Signal

higher SNR at the expense of longer scan time and more power consumption. Table 4-3 shows the actual scan time in microseconds for a single sensor based on resolution and scanning speed.

Table 4-3. Scan Time for a Single Sensor in µs Based on Resolution and Scanning Speed

4.2.10 High-Level API Parameters

High-level API parameters determine the behavior of high-level firmware algorithms that discriminate between sensor activations and noise, and compensate for signal drift caused by environmental conditions. To determine proper values for these parameters, you must establish a digital communication interface with the system to monitor raw counts, baseline, and difference counts during a finger activation event for each sensor. This data is stored in arrays named CSDPLUS_waSnsBaseline[], CSDPLUS_waSnsResult[], and CSDPLUS_waSnsDiff[], respectively. The highlevel API parameter settings are based primarily on ambient noise and finger signal strength, as indicated by this data. Noise and signal strength depend on EMI environment, PCB layout, overlay thickness, and other physical characteristics of the system. Therefore, the data used as the basis for setting these parameters must be taken in its original position with the system in its final assembled state and in the same EMI environment as will exist in use.

Figure 4-5 shows the typical raw counts obtained from a sensor during a finger activation cycle; that is, the sensor is

activated and then deactivated. Labels are superimposed over the data that indicate how noise and signal are to be calculated based on the raw data. Where appropriate, the high-level parameter descriptions that follow include information about how to set each parameter based on these noise and signal values. According to CapSense design best practice, the SNR must be at least 5:1 for robust CapSense system operation. If SNR is less than 5:1, the hardware parameters must be adjusted, the PCB layout changed according to the guidelines of Getting Started with

CapSense to raise SNR to at least 5:1, or both.

Figure 4-5. Typical Raw Counts from a Sensor During Finger Activation Cycle

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CapSense Performance Tuning with User Modules

4.2.11 Set High-Level Parameters

The following recommendations are a starting place for selecting the optimal parameter settings:

 Finger Threshold: Set to 80 percent of raw counts with sensor ON  Noise Threshold: Set to 40 percent of raw counts with sensor OFF  Negative Noise Threshold: Set equal to noise threshold  Baseline Update Threshold: Set to two times noise threshold  Hysteresis: Set to 10 percent of raw counts with sensor ON  Low Baseline Reset: Set to 50  Sensors Autoreset: Based on design requirements  Debounce: Based on design requirements

4.3 Using the SmartSense_EMCPLUS User Module

SmartSense_EMCPLUS allows you to create a CapSense design that requires no tuning, as long as the sensor parasitic capacitance is in the range from 5 pF to 45 pF with a minimum 0.1-pF finger touch. You can create a SmartSense_EMCPLUS design by using the SmartSense_EMCPLUS User Module in PSoC Designer 5.1 or higher versions. This section also shows you how to migrate an existing CSD CapSense design to SmartSense_EMC_PLUS.

4.3.1 Guidelines for SmartSense_EMC_PLUS

Follow these guidelines when using the SmartSense_EMCPLUS User Module in an application:

 SmartSense_EMCPLUS requires that the capacitive user interface design follows the layout and system design

best practices documented in the Design Considerations section.

 All of the CSD/CSDPLUS UM parameters (such as I

resolution) are determined at runtime by the SmartSense_EMCPLUS User Module. You should not use APIs that modify these CSD parameters in firmware, unless you know exactly what effect it has in your design.

 To migrate an existing design from CSD/CSDPLUS to SmartSense_EMC_PLUS:

 Ensure that all APIs that set or modify the CSD/CSDPLUS parameters are first removed from the program.  Ensure that the parasitic capacitance of all CapSense sensors in the design is between 5 pF and 45 pF over

environmental and PCB production process variations.

 Make sure recommended C

the C

port pin selected in the user module wizard.

MOD

capacitor (X7R, 2.2-nF, and voltage rating more than 5 V) is connected to

MOD

value, prescaler period, clock divider, scan speed, and

DAC

4.3.2 Understanding the Difference

The differences between the SmartSense_EMCPLUS User Module and the standard CSD/CSDPLUS User Module are:

 The SmartSense_EMCPLUS User Module supports the same APIs that a standard CSD/CSDPLUS User

Module supports. Thus, no change is required in placing, configuring, starting, or calling other APIs except the User Module instance name.

 There is no need to set any User Module parameters for tuning, as all the parameters related to tuning are

automatically set at runtime by the SmartSense_EMCPLUS User Module.

 The C

than 5 V is recommended in all CapSense applications.

 The SmartSense_EMCPLUS algorithm maintains the signal SNR of each sensor between 5:1 and 11:1 to ensure

robust CapSense operation while maximizing performance.

 The scanning time of the SmartSense_EMCPLUS User Module is restricted by the algorithm to be between

410 µs and 2.8 ms per sensor in 24-MHz operating mode, based on the parasitic capacitance of the sensor.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 40

capacitor value is restricted to 2.2 nF. Use of an X7R or NPO capacitor with a voltage rating higher

MOD

CapSense Performance Tuning with User Modules

4.3.3 Recommended C

The recommended C are recommended for C

value for a SmartSense_EMC_PLUS-based design is 2.2 nF. X7R or NPO type capacitors

MOD

stability over temperature. The capacitor should have voltage rating not less than 5 V.

MOD

Value for SmartSense_EMC_PLUS

MOD

4.3.4 SmartSense_EMCPLUS User Module Parameters

Only four parameters must be set for this user module. These are:

 Sensors Autoreset  Debounce  Modulator Capacitor Pin  Sensitivity Level

4.3.4.1 Sensors Autoreset

This parameter determines whether the baseline is updated at all times or only when the signal difference is below the noise threshold. When set to Enabled, the baseline is updated constantly. This setting limits the maximum time that a sensor may remain ON (typically it is 5 to 10 seconds), but it prevents the sensors from permanently turning ON when the raw count suddenly rises without anything touching the sensor because of any failure condition of the system.

4.3.4.2 Debounce

The Debounce parameter adds a debounce counter to the sensor‟s active transition. For the sensor to be declared as active from inactive state, a finger touch signal should be present on the sensor for debounce number of consecutive scans. This parameter affects all of the sensors similarly.

4.3.4.3 Modulator Capacitor Pin

This parameter selects the pin to which the 2.2 nF/X7R/voltage rating more than 5 V C The available pins are P0[1] and P0[3].

Note An external 2.2-nF capacitor is mandatory for SmartSense_EMCPLUS to work correctly.

capacitor is connected.

MOD

4.3.4.4 Sensitivity Level

Sensitivity is used to increase or decrease strength of signal from a sensor. A lower value for sensitivity (0.1 pF) leads to a stronger signal from the sensor. Designs with thicker overlays require stronger signals from sensors for proper implementation. The available options for sensitivity selection are High (0.1 pF), Medium High (0.2 pF), Medium Low (0.3 pF), and Low (0.4 pF).

To produce a stronger signal from a sensor (high sensitivity), the SmartSense_EMCPLUS UM must use more time for sensor scanning. This means that setting 0.1-pF (High) sensitivity for a sensor will consume more scan time compared to the sensor that has the sensitivity level set to 0.2 pF (Medium High).

Tuning best practice is to find the highest sensitivity value for the sensor to produce the required 5:1 SNR. You may start the tuning with the highest sensitivity value (0.4 pF) and reduce the value as required to meet the 5:1 SNR

4.3.5 SmartSense_EMCPLUS User Module Specific Guidelines

All guidelines applicable to the SmartSense_EMC UM apply to the SmartSense_EMCPLUS UM. For general guidelines about CapSense design and SmartSense_EMC_PLUS-based design, see the Getting Started with CapSense design guide. This section documents a few important aspects of the SmartSense_EMCPLUS UM.

4.3.5.1 Sensor Scan Time, Response Time, and Memory Utilization

When a sensor is implemented using the SmartSense_EMCPLUS UM, the scan time of a sensor, response time of the sensor, and RAM memory usage depends on the immunity mode selected in the user module.

 With immunity mode Medium, sensor scan time is two times higher than a sensor with immunity mode Low. With

immunity mode High, the scan time of a sensor is three times higher than the scan time of a sensor with immunity mode Low.

 Increase in scan time proportionally increases the response time of a sensor. With immunity mode Medium,

response time is two times higher than that of a sensor with immunity mode Low. Similarly, response time of a sensor with immunity mode High is three times higher than that of a sensor with immunity mode Low.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 41

CapSense Performance Tuning with User Modules

Sensitivity = 0.2 pF

Sensitivity = 0.3 pF

Sensitivity = 0.4 pF

CP (pF)

ST (µs)

CP (pF)

ST (µs)

CP (pF)

ST (µs)

8 to 10

340

8 to 17

340

8 to 10

170

10 to 23

680

17 to 35

680

10 to 23

340

23 to 41

1360

35 to 41

1360

23 to 41

680

41 to 45

2730

41 to 45

2730

41 to 45

1360

Sensitivity = 0.2 pF

Sensitivity = 0.3 pF

Sensitivity = 0.4 pF

CP (pF)

ST (µs)

CP (pF)

ST (µs)

CP (pF)

ST (µs)

8 to 10

680

8 to 17

680

8 to 10

340

 To implement a robust electromagnetic compliant algorithm, the SmartSense_EMCPLUS UM uses RAM

memory. As a result, the highest immunity mode (High) needs approximately three times the RAM memory used in immunity mode Low. Immunity mode Medium uses only about two times more RAM memory than that of immunity mode Low.

4.3.5.2 IMO Tolerance and Time Critical Task

IMO tolerance for SmartSense_EMC-enabled parts is +5 percent and –20 percent.  When implementing time-critical algorithms and logic, you must consider IMO tolerance to make sure that the

firmware logic or algorithm does not break.

 If a project uses interrupts, you should consider IMO tolerance while analyzing interrupt latency, ISR execution

time, and so on.

 Every timing analysis that depends on the IMO (for example, a timer clocked by IMO, delay created using loop in

firmware, and API execution time) must consider the IMO tolerance to ensure robust application firmware.

4.3.5.3 I2C Operating Speed

I2C interface operation frequency is limited to a maximum of 80 percent of the actual operating frequency of the user module in the SmartSense_EMC-enabled parts. This limitation is caused by the 20-percent IMO tolerance.

 This means, when clock speed of 400 kHz is selected in the I2C user module, the I2C interface can be operated

to maximum of 320 kHz. Similarly, operating frequency is limited to a maximum of 80 kHz and 40 kHz when 100-kHz and 50-kHz clock modes, respectively, are selected in the I2C user module.

 While using the I2C slave interface, the master clock should operate within the reduced specification mentioned

earlier. Not doing this will lead to data corruption, I2C bus conjunction, or inconsistent behavior from the I2C user

module.  Using the I2C master module impacts only the throughput of the interface. When using I2CSBUF user module, it is strongly recommended use auto-nack mode while CPU accessing the 32-

byte dedicated buffer and when device put into sleep or deep sleep modes. See the I2CSBUF user module datasheet details information.

4.3.6 Scan Time of a CapSense Sensor

To maintain the consistent finger response sensitivity over a wide range of parasitic capacitance, the SmartSense_EMCPLUS User Module automatically determines the hardware parameters of the user module. As a result, sensor scan time does not remain constant. For a design in mass production, it could vary based on the parasitic capacitance variation of the PCB.

The total scan time of a sensor is decided by four factors. They are parasitic capacitance of sensor, IMO frequency, CPU operating frequency, and sensitivity level of the SmartSense_EMCPLUS User Module.

Scan time of a sensor can be found using Equation 4-3 and the following tables.   =   󰇛󰇜+   () Equation 4-3 The following tables show the sampling time value with various IMO and sensitivity levels.

Table 4-4. Sampling Time for a Sensor with IMO = 24 MHz

Table 4-5. Sampling Time for a Sensor with IMO = 12 MHz

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 42

CapSense Performance Tuning with User Modules

10 to 23

1360

17 to 35

1360

10 to 23

680

23 to 41

2730

35 to 41

2730

23 to 41

1360

41 to 45

5460

41 to 45

5460

41 to 45

2730

Sensitivity = 0.2 pF

Sensitivity = 0.3 pF

Sensitivity = 0.4 pF

CP (pF)

ST (µs)

CP (pF)

ST (µs)

CP (pF)

ST (µs)

8 to 11

680

8 to 10

680

8 to 11

680

11 to 23

1360

10 to 17

1360

11 to 23

1360

23 to 42

2730

17 to 35

2730

23 to 41

2730

42 to 45

5460

35 to 41

5460

41 to 45

5460

41 to 45

10920

CPU CLK

Processing Time (PT) in µs

142 6 284

568

Table 4-6. Sampling Time for a Sensor with IMO = 6 MHz

Table 4-7 shows the value for processing time with various CPU frequencies.

Table 4-7. Processing Time for a Sensor

For example, if a CapSense system is designed with a 24-MHz IMO frequency, a 6-MHz CPU clock (IMO/4), and a SmartSense_EMCPLUS sensitivity level of 0.3 pF, the scan time of the sensor that has parasitic capacitance around 15 pF can be calculated from the previous tables using Equation 4-1.

Sampling time for the previously mentioned configuration (24 MHz of IMO and 0.3 pF of sensitivity) is chosen from

Table 4-4; it is 680 µs. Processing time for the previously mentioned configuration (CPU clock of 6 MHz) Is chosen

from Table 4-7; it is 284 µs. Thus, the total scan time in this configuration is 680 + 284 = 964 µs. Scan time for more than one sensor is the sum

of the scan time of each sensor.

4.3.7 SmartSense_EMCPLUS Response Time

Consider the following application with standard CSD along with typical CapSense scanning firmware.

 Three CapSense sensors with parasitic capacitance of sensor between 5 pF and 10 pF  IMO of 12 MHz and CPU clock of 12 MHz  Sensor sensitivity level of 0.4 pF  Debounce = 3

According to the previous tables, scanning of each sensor requires 482 µs and three sensors have a scan time of

1.45 ms. The following firmware example requires 1 ms for additional firmware execution; thus, the loop execution time will be 2.45 ms.

while (1) { SmartSense_EMC_PLUS_ScanAllSensors(); SmartSense_EMCPLUS _UpdateAllBaselines();

if(SmartSense_EMCPLUS _bIsAnySensorActive() ) { //1ms firmware routines } }

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CapSense Performance Tuning with User Modules

This means that, when a CapSense sensor is activated, firmware produces the sensor ON status within 7.35 ms (the sensor should be active for debounce number of consecutive scans). This is often referred to as the response time of a CapSense system.

If the scan time varies with respect to the parasitic capacitance to maintain consistent, what is the impact on response time if the parasitic capacitance of the sensor changes because of the process variation? Response time may be increased (slow response) in this case. This can have a negative impact on sensor performance. Guidelines to build a robust firmware design are provided in the next section.

4.3.8 Method to Ensure Minimum SNR Using the SmartSense_EMCPLUS UM

SmartSense_EMCPLUS is an advanced electromagnetic compliance design of a CSD-based SmartSense user module that does not require a tedious tuning process. However, there are two simple steps to ensure robustness of the design while using the SmartSense_EMCPLUS UM.

1. Set up a real-time monitoring tool to monitor the CapSense user module parameters to measure sensor signals.

The sensor raw count (SmartSense_EMCPLUS_waSnsResult), sensor normalized signal

(SmartSense_EMCPLUS_baSnsSignal), and sensor finger threshold

(SmartSense_EMCPLUS_baBtnFThreshold) must be observed during the tuning process. Do not use the LCD

or any other numerical displays to monitor data because they are slow and do not allow visualizing the data

dynamics. Recommended data monitoring tools are multi-chart or the I2C USB Bridge Control Panel.

2. Set the sensitivity level to 0.4 pF (Low), and calculate the SNR. Figure 4-6 shows a typical raw count graph with

a finger touch. According to CapSense best practices, SNR for a robust design should be greater than 5:1. If

measured SNR is more than 10:1, reduce sensitivity level value to the next possible step until SNR is more than

5:1 and less than 10:1.

Figure 4-6. Raw Count Graph for a Typical Sensor with a Finger Touch

3. If you are using the automatic finger threshold feature in the design, the process is complete after the previous

step. If you are using the flexible finger threshold feature, you should also set individual finger thresholds for

each sensor to complete the process. To set finger threshold, monitor sensor signal

(SmartSense_EMCPLUS_baSnsSignal) and set finger threshold value to roughly 80 percent of the sensor signal

value when the sensor is touched. This completes the process. Figure 4-7 shows a typical sensor signal and

finger threshold value.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 44

CapSense Performance Tuning with User Modules

Figure 4-7. Sensor Signal for a Typical Sensor with a Finger Touch

4.3.9 Firmware Design Guidelines

The response time of the CapSense sensors may change due to the increased parasitic capacitance of the sensor. It is also important to watch the loop execution time (see the following example code), which may also increase. When the parasitic capacitance of all sensors is less than 10 pF, the firmware routine is executed at a rate of 2.45 ms. This rate will change if the sensor scan time is increased because of the increase in the parasitic capacitance of the sensor based on the process variation.

The following is example code for toggling a port pin based on the main loop execution time.

while (1) { SmartSense_EMC_PLUS_ScanAllSensors(); SmartSense_EMC_PLUS_UpdateAllBaselines();

if(SmartSense_EMC_PLUS_bIsAnySensorActive() ) { //1ms firmware routines }

PRT0DR_Shadow ^= 0x01; PRT0DR = PRT0DR_Shadow; }

The period of the signal on the Port_0[1] pin is 4.9 ms (the period is twice the loop time as the port pin is toggled). If the parasitic capacitance of one sensor is increased to approximately 15 pF, the scan time will change to 1.78 ms; thus, the period of the signal on Port_0[1] will be 5.6 ms.

If the parasitic capacitance of the sensor is close to the boundary of the SmartSense_EMCPLUS capacitance banks (for example, 9 pF, which is very close to the 10-pF boundary), SmartSense_EMCPLUS may choose a neighboring scan time in an application because of process variation. Because of this, different production parts of the same design can have two different main loop execution times and response times.

Based on the above discussion, firmware should not rely on the scan time of the sensor for implementing other features (for example, software PWM, software delay, and so on). Programs implementing a watchdog timer (WDT) should consider this fact while setting the WDT expiration time

A simple firmware implementation example to get a consistent main loop execution time using the Timer16 User Module follows.

// Main program BYTE bTimerTicks = 0;

#pragma interrupt_handler myTimer_ISR_Handler; void myTimer_ISR_Handler( void );

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CapSense Performance Tuning with User Modules

void main() { M8C_EnableGInt;

SmartSense_EMC_PLUS_Start(); SmartSense_EMC_PLUS_ScanAllSensors(); SmartSense_EMC_PLUS_SetDefaultFingerThresholds() ;

Timer16_EnableInt(); Timer16_SetPeriod (TIMEOUT_10MS) ; Timer16_Start();

while( 1 ) { /* Scan all 3 sensors and update Baseline */ SmartSense_EMC_PLUS_ScanAllSensors(); SmartSense_EMC_PLUS_UpdateAllBaselines();

/* Wait till timer expires or sleep here */

while (bTimerTicks != 1) ; bTimerTicks = 0 ;

if(CSDAUTO_bIsAnySensorActive() ) { //1ms firmware routines }

// Toggle Port_0[1] PRT0DR_Shadow ^= 0x01 ; PRT0DR = PRT0DR_Shadow ; } }

// Timer16 ISR program void myTimer_ISR_Handler(void) { bTimerTicks++; }

In the previous example, the program waits for the timer to expire even if the sensor scanning is complete. The timer period should be chosen based on the worst-case main loop execution time. This is the sum of the worst-case scan times of the individual CapSense sensors. If the parasitic capacitance of the sensor is close to the boundary of the SmartSense_EMCPLUS capacitance bank, choose a higher scan time (using Table 4-5) for the calculation.

The SmartSense_EMCPLUS UM enables you to easily implement the capacitive touch-sensing user interface into a system. It removes the difficulties of the tuning process and helps to increase the yield in production against manufacturing process variations of the PCB and other variations. Therefore, the preferred option is to migrate the existing CSD-based CapSense designs to SmartSense_EMCPLUS and to use SmartSense_EMCPLUS for new designs.

The main loop execution time and scan time of SmartSense_EMCPLUS vary based on the process variations. Though it does not affect the performance of CapSense in any way, the firmware developer should consider this when implementing CapSense PLUS applications with SmartSense_EMCPLUS Auto-Tuning technology.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 46

CapSense Performance Tuning with User Modules

System Requirements

I2CSBUF

EzI2Cs

I2CHW

I2Cm

Wake on I2C slave address match

Yes

N/A

Master requires clock stretch

Yes

N/A

I2C data buffer size

1-32 bytes

1-255 bytes

N/A

I2C master

Yes

Package

Pin Compatibility to CY8C20xx6A/AS

16-SOIC

New package offering with CY8C20xx7/S

16-QFN

Pin to pin compatible

24-QFN

One pin - Pin 23 is Vss in CY8C20xx7/S while it is an I/O in CY8C20xx6A/AS

32-QFN

Two pins - Pin 28 is I/O and Pin 29 is Vss in CY8C20xx7/S; in CY8C20xx6A/AS, Pin 28 is Vss and Pin 29 is an I/O

30-WLCSP

Pin to pin compatible

48-QFN

Two pins - Pins 36 and 45 are NC while in CY8C20xx6A/AS both are I/O

4.4 Design Migration from CY8C20xx6A/AS to CY8C20xx7/S

The CY8C20xx7/S family of CapSense controllers provides superior noise immunity based on QuiteZoneTM technology and improved sensitivity of 0.1 pF with a 5:1 SNR. This section summarizes key points to migrate an existing CY8C20xx6A/AS design.

4.4.1 Discontinued Support/User Modules

 The USB interface has been discontinued in the CY8C20xx7/S family.  On-chip debug (OCD) support has been discontinued for the CY8C20xx7/S family.  CSA and CSA_EMC user modules are not supported in the CY8C20xx7/S family.

4.4.2 Improvement and New Features

 CSD-based improved CapSense sensing engine to provide an improved sensitivity of 0.1 pF.  New improved I2C slave interface (I2CSBUF User Module) with dedicated 32-byte buffer eliminating clock

stretching by the slave.  Improved I2C interface also supports wakeup interrupt from I2C slave address match event. Use Table 4-8 to select the correct I2C User Module for your design. Table 4-8. I2C Features

4.4.3 Pin Compatibility

Table 4-9. Pin Compatibility

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 47

5. Design Considerations

When designing capacitive touch-sense technology into your application, it is crucial to keep in mind that the

Material



Breakdown Voltage

(V/mm)

Minimum Overlay Thickness

at 12 kV (mm)

Air

1.0

1200–2800

Wood – dry

1.2–2.5

3900

Glass – common

7.6–8.0

7900

1.5

Glass – Borosilicate (Pyrex®))

6.0

13,000

0.9

PMMA Plastic (Plexiglas®)

2.8

13,000

0.9

ABS

2.4–4.1

16,000

0.8

Polycarbonate (Lexan®)

2.9–3.0

16,000

0.8

Formica

4.6–4.9

18,000

0.7

FR-4

4.8

28,000

0.4

PET Film – (Mylar®)

3.2

280,000

0.04

Polymide film – (Kapton®)

2.9-3.9

290,000

0.04

CapSense device exists within a larger framework. Careful attention to every level of detail from PCB layout to user interface to end-use operating environment will lead to robust and reliable system performance. For more in-depth information, see Getting Started with CapSense.

5.1 Overlay Selection

In CapSense Fundamentals, Equation 4-1 was presented for finger capacitance

0





Where:

= Free space permittivity

= Dielectric constant of overlay

A = Area of finger and sensor pad overlap (mm2) D = Overlay thickness (mm)

To increase CapSense signal strength, choose an overlay material with a higher dielectric constant, decrease the overlay thickness, and increase the button diameter.

Table 5-1. Overlay Material Dielectric Strength



Conductive material cannot be used as an overlay because it interferes with the electric field pattern. For this reason, do not use paints containing metal particles in the overlay.

An adhesive is typically used to bond the overlay to the CapSense PCB. A transparent acrylic adhesive film from 3M™ called 200MP is qualified for use in CapSense applications. This special adhesive is dispensed from paperbacked tape rolls (3M product numbers 467MP and 468MP).

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 48

Design Considerations

ESD Protection Device

Input

Capacitance

Leakage

Current

Contact

Discharge

Maximum Limit

Air Discharge

Maximum Limit

Manufacturer

Part Number

Littelfuse

SP723

5 pF

2 nA

8 kV

15 kV

Vishay

VBUS05L1-DD1

0.3 pF

0.1 µA <

±15 kV

±16 kV

NXP

NUP1301

0.75 pF

30 nA

8 kV

15 kV

5.2 ESD Protection

Robust ESD tolerance is a natural byproduct of thoughtful system design. By considering how contact discharge will occur in your product, particularly in your user interface, it is possible to withstand an 18-kV discharge event without incurring any damage to the CapSense controller.

CapSense controller pins can withstand a direct 2-kV event. In most cases, the overlay material provides sufficient ESD protection for the controller pins. Table 5-1 lists the thickness of various overlay materials required to protect the CapSense sensors from a 12-kV discharge, as specified in IEC 61000-4-2. If the overlay material does not provide sufficient protection, ESD countermeasures should be applied in the following order: Prevent, Redirect, Clamp.

5.2.1 Prevent

Make sure that all paths on the touch surface have a breakdown voltage greater than potential high-voltage contacts. Also, design your system to maintain an appropriate distance between the CapSense controller and possible sources of ESD. If it is not possible to maintain adequate distance, place a protective layer of a high breakdown voltage material between the ESD source and the CapSense controller. One layer of 5-mil-thick Kapton® tape will withstand 18 kV.

5.2.2 Redirect

If your product is densely packed, it may not be possible to prevent the discharge event. In this case, you can protect the CapSense controller by controlling where the discharge occurs. A standard practice is to place a guard ring on the perimeter of the circuit board that is connected to chassis ground. As recommended in PCB Layout Guidelines, providing a hatched ground plane around the button or slider sensor can redirect the ESD event away from the sensor and CapSense controller.

5.2.3 Clamp

Because CapSense sensors are purposely placed close to the touch surface, it may not be practical to redirect the discharge path. In this case, including series resistors or special-purpose ESD protection devices may be appropriate.

The recommended series resistance value is 560 Ω. A more effective method is to provide special-purpose ESD protection devices on the vulnerable traces. ESD

protection devices for CapSense need to be low capacitance. Table 5-2 lists devices recommended for use with CapSense controllers.

Table 5-2. Low-Capacitance ESD Protection Devices Recommended for CapSense

5.3 Electromagnetic Compatibility (EMC) Considerations

5.3.1 Radiated Interference

Radiated electrical energy can influence system measurements and potentially influence the operation of the processor core. The interference enters the PSoC chip at the PCB level, through CapSense sensor traces and any other digital or analog inputs. Layout guidelines for minimizing the effects of RF interference include:

 Ground Plane: Provide a ground plane on the PCB.  Series Resistor: Place series resistors within 10 mm of the CapSense controller pins.

 The recommended series resistance for CapSense input lines is 560 Ω.

 The recommended series resistance for communication lines such as I2C and SPI is 330 Ω.  Trace Length: Minimize trace length whenever possible.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 49

Design Considerations

Type

Description

Application

Average

Finite impulse response filter (no feedback) with equally weighted coefficients

Periodic noise from power supplies

IIR

Infinite impulse response filter (feedback) with a step response similar to an RC filter

High frequency white noise (1/f noise)

Median

Nonlinear filter that computes median input value from a buffer of size N

Noise spikes from motors and switching power supplies

Jitter

Nonlinear filter that limits current input based on previous input

Noise from thick overlay (SNR < 5:1), especially useful for slider centroid data

Event-Based

Nonlinear filter that causes a predefined response to a pattern observed in the sensor data

Commonly used during non-touch events to block CapSense data transmission.

Rule-Based

Nonlinear filter that causes a predefined response to a pattern observed in the sensor data

Commonly used during normal operation of the touch surface to respond to special scenarios such as accidental multibutton selection

 Current Loop Area: Minimize the return path for current. Hatched ground instead of solid fill should be provided

within 1 cm of the sensors and traces to reduce the impact of parasitic capacitance.  RF Source Location: Partition systems with noise sources such as LCD inverters and switched-mode power

supplies (SMPS) to keep them separated from CapSense inputs. Shielding the power supply is another common

technique for preventing interference.

5.3.2 Radiated Emissions

Selecting a low frequency for the switched-capacitor clock helps to reduce radiated emissions from the CapSense sensor. This clock is controlled in firmware using the prescaler option. Increasing the prescaler value decreases the frequency of the switching clock.

5.3.3 Conducted Immunity and Emissions

Noise entering a system through interconnections with other systems is referred to as conducted noise. These interconnections include power and communication lines. Because CapSense controllers are low-power devices, conducted emissions must be avoided. The following guidelines will help reduce conducted emission and immunity:

 Use decoupling capacitors as recommended by the datasheet.  Add a bidirectional filter on the input to the system power supply. This is effective for both conducted emissions

and immunity. A pi-filter can prevent power supply noise from effecting sensitive parts, while also preventing the

switching noise of the part from coupling back onto the power planes.  If the CapSense controller PCB is connected to the power supply by a cable, minimize the cable length and

consider using a shielded cable.  Place a ferrite bead around power supply or communication lines to filter out high-frequency noise.

5.4 Software Filtering

Using software filters is one of the techniques for dealing with high levels of system noise. Table 5-3 lists the types of filters that are useful for CapSense.

Table 5-3. Table of CapSense Filters

Table 5-4 details the RAM and flash requirements for different software filters. The amount of flash required for each

filter type depends on the performance of the compiler. The requirements listed here are for both the ImageCraft compiler and the ImageCraft Pro compiler.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 50

Design Considerations

Filter Type

Filter

Order

RAM

(Bytes per sensor)

Flash (Bytes)

ImageCraft Compiler

Flash (Bytes)

ImageCraft Pro Compiler

Average

2–8 6 675

665

IIR

1 2 429

412

2 6 767

622

Median

3 6 516

450

516

450

Jitter filter on raw counts

N/A 2 277

250

Jitter filter on slider centroid

N/A 2 131

109

Table 5-4. RAM and Flash Requirements

5.5 Power Consumption

5.5.1 System Design Recommendations

For many designs, minimizing power consumption is an important goal. There are several ways to reduce the power consumption of your CapSense capacitive touch-sensing system.

 Set GPIO drive mode for low power  Turnoff the high-power blocks  Optimize CPU speed for low power  Operate at a lower VDD

In addition to these suggestions, applying the sleep-scan method can be very effective.

5.5.2 Sleep-Scan Method

In typical applications, the CapSense controller does not need to always be in the active state. The device can be put into the sleep state to stop the CPU and the major blocks of the device. Current consumed by the device in sleep state is much lower than the active current.

The average current consumed by the device over a long period can be calculated by using the following equation.





󰇛



×

󰇜

+( 

)

×

Equation 5-1

The average power consumed by the device can be calculated as follows:





= × 

Equation 5-2



5.5.3 Response Time versus Power Consumption

As illustrated in Equation 5-2, the average power consumption can be reduced by decreasing I decreased by increasing sleep time. Increasing sleep time to a very high value will lead to poor CapSense button response time. As a result, the sleep time must be based on system requirements.

In any application, if both power consumption and response time are important parameters to be considered, an optimized method can be used that incorporates both continuous-scan and sleep-scan modes. In this method, the device spends most of its time in sleep-scan mode where it scans the sensors and goes to sleep periodically, as explained in the previous section, thereby consuming less power. When a user touches a sensor to operate the system, the device jumps to continuous-scan mode where the sensors are scanned continuously without invoking sleep, thereby giving very good response time. The device remains in continuous-scan mode for a specified timeout period. If the user does not operate any sensor within this timeout period, the device jumps back to the sleep-scan mode.

or VDD. I

AVE

may be

AVE

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 51

Design Considerations

5.5.4 Measuring Average Power Consumption

The following instructions describe how to determine average power consumption when using the sleep-scan method:

1. Build a project that scans all of the sensors without going to sleep (continuous-scan mode). Include a pin-toggle

feature in the code before scanning the sensors. Toggling the state of the output pin serves as a time marker that

can be tracked with an oscilloscope.

2. Download the project to the CapSense device and measure the current consumption. Assign the measured

current to I

3. Get the sleep current information from the datasheet and assign it to I

4. Monitor the toggling output pin in the oscilloscope and measure the time period between two toggles. This gives

the active time. Assign this value to t

5. Apply sleep-scan to the project. The period of the sleep-scan cycle, T, is set by selecting the sleep timer

frequency in the global resources window as shown in Figure 5-1.

6. Subtract active time from the sleep-scan cycle period to get the sleep time. T

7. Calculate the average current using Equation 5-1.

8. Calculate average power consumption using Equation 5-2.

ACT

SLP

ACT

SLP

= T – t

ACT

Figure 5-1. Global Resources Window

5.6 Pin Assignments

An effective method to reduce interaction between CapSense sensor traces and communication and non-CapSense traces is to isolate each by port assignment. Figure 5-2 shows a basic version of this isolation for a QFN package. Because each function is isolated, the CapSense controller is oriented such that there is no crossing of communication, LED, and sensing traces.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 52

Design Considerations

Recommended for LED Drive

Recommended for CapSense Sensors and CMOD

P0.1, P2.5, P2.3, P2.1, P4.1, P3.7, P3.5, P3.3, P3.1, P1.7, P1.5, P1.3, P1.1

P1.0, P1.2, P1.4, P1.6, P3.0, P3.2, P3.4, P3.6, P4.0, P4.2, P2.0, P2.2, P2.4, P0.0, P0.2, P0.4, P0.6, P0.3

Figure 5-2. Recommended Port Isolation for Communication, CapSense, and LEDs

All GPIOs of the CapSense controller are capable of implementing the CapSense sensor and drive LEDs. However, if the GPIOs LED drive and CapSense sensors are implemented in the same project, it is recommended to use GPIO according to Table 5-5 for best performance. If a design is implementing only CapSense sensors or only LED drive, the restrictions mentioned in Table 5-5 are not applicable.

Table 5-5. Recommended GPIOs for LED Drive and CapSense Sensors

The architecture of the CapSense controller imposes a restriction on current budget for even and odd port pin numbers. An odd pin can be any port pin having an odd number as pin number. For a CapSense controller, if the current budget of odd port pin is 100 mA, the total current drawn though all odd port pins should not exceed 100 mA. In addition to the total current budget limitation, there is also a maximum current limitation for each port pin that is defined in the CapSense controller datasheet.

All CapSense controllers provide high-current sink and source capable port pins. When using high-current sink or source from port pins, it is recommended to use the ports that are closest to device ground pin to minimize the noise.

5.7 GPIO Load Transient

When GPIOs sink a large current (>10 mA) to the ground of the chip by driving port pins to strong-low, noise will be introduced into the CapSense system. The instantaneous change in the amount of the current flow to ground through the GPIOs is referred as GPIO load transient. The noise introduced into the CapSense system due to the GPIO load transient is called GPIO load transient noise, as shown in Figure 5-3. This section shows you how to reduce the GPIO load transient noise using hardware techniques, and compensate the noise using firmware techniques.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 53

Design Considerations

GPIO Load Transient

Noise

LED is ON

LED is OFF

GPIO PAD

CapSense GND

GPIO GND

VSS

GPIO PIN

CS Sensor

AMUX

Figure 5-3. GPIO Load Transient Noise in a CapSense System

When current is sunk through a GPIO pin, the voltage at CapSense ground (GPIO PAD) will not be zero because of the non-zero bond-wire resistance, R3. Because of the non-zero ground potential, the sensor will not be completely discharged when LED is sinking current; this will cause an increase in the sensor raw count.

Figure 5-4. Ground Structure in CY8C20xx7/S

Note: R1, R2, and R3 are bond wire resistances. For a robust CapSense design, the worst-case GPIO transient noise should be less than 30 percent of the finger

touch signal. The worst-case noise appears in the CapSense system when the GPIO state is changed from a nocurrent-flow state (for example, all LEDs OFF) to a maximum current flow state (for example, all LEDs ON).

The GPIO load transient noise increases with the sensor scan resolution. CapSense sensors with a high parasitic capacitance or proximity sensors require higher sensor-scan resolution to achieve an SNR greater than 5:1. In such systems, the effect of GPIO load transient is more pronounced. In some cases, the noise due to GPIO load transient might be higher than the signal due to finger touch and cause sensor false-triggers. The following section shows how to reduce GPIO load transient noise.

5.7.1 Hardware Guidelines to Reduce GPIO Load Transient Noise

 Reduce Sensor CP

The sensor CP determines the sensor scan resolution parameter. The larger the C

the higher will be the

resolution parameter required to achieve an SNR greater than 5:1. Setting a high-resolution parameter causes

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 54

Design Considerations

the amplitude of the GPIO load transient-noise to increase. Therefore, it is recommended to minimize the sensor

CP by following the layout guidelines mentioned in the Getting Started with CapSense design guide.  Reduce LED sink current

The GPIO load transient noise is directly proportional to the LED sink current. It is recommended to keep the

LED sink current within the limits as specified in the device datasheet. If the GPIO has to sink a current, which is

more than the maximum value specified in datasheet, use an external transistor or a driver IC.  Select appropriate pins for LED

All CapSense controllers provide high-current sink- and source-capable port pins. When using high-current sink

or source from port pins, you should use the ports recommended in Table 5-5.

5.7.2 Firmware Guidelines to Compensate GPIO Load Transient Noise

To prevent sensor false triggers due to GPIO load transients, the sensor baseline can be updated using rule-based algorithms. One of the methods to compensate the baseline is explained here.

Figure 5-5 shows a condition in which false triggers is seen due to GPIO load transient.

1. At instant 1, there is no finger on the sensor and the LED is in the OFF condition.

2. At instant 2, a finger is on the sensor and the shift in raw count is greater than the finger threshold.

3. Because the shift in raw count is greater than the finger threshold, the LED is turned ON at instant 3.

4. When the LED is turned ON, because of GPIO load-transient, the raw count further shifts.

5. At instant 4, even if the finger is removed, the raw count will not return to initial value because of the shift in the

raw count due to the GPIO load-transient. If this shift is greater than the finger threshold, the LED will remain ON

permanently indicating a sensor false-trigger. To prevent the sensor and the LED from remaining in ON condition permanently, the sensor baseline should be

compensated, which is explained in the below steps.

CY8C20xx7/S CapSense® Design Guide Doc. No. 001-78329 Rev. *E 55

Design Considerations

No Touch

Touch

No Touch

Finger Threshold

Shift Due to

Finger Touch

Shift Due to GPIO Load

Transient

Rawcount Stuck due to

GPIO Load Transient

LED OFF LED ON

Sensor Status Baseline Rawcount

Figure 5-5. CapSense Sensor Variables when Baseline is Not Compensated

Figure 5-6 shows a condition in which false triggers is eliminated by compensating the sensor baseline.

1. At instant 1, there is no finger on the sensor and the LED is in the OFF condition.

2. At instant 2, a finger is on the sensor and the shift in raw count (difference count) is greater than the finger

threshold.

3. Because the shift in difference count is greater than the finger threshold, the LED is turned ON at instant 3.

4. When the LED is turned ON, the noise due to GPIO load transient is calculated. That is, noise = raw count (when

LED is ON) – raw count (when LED is OFF)

This noise count due to GPIO load transient is added to the baseline, and as a result, when the finger is

removed, the difference count value will be zero and the LED will be turned OFF.

5. After the LED is turned off, the raw count will return to the initial value and the baseline is reset due to the low-

baseline reset algorithm.

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Design Considerations

No Touch

Touch

No Touch

Finger Threshold

Shift Due to

Finger Touch

Shift Due to GPIO Load

Transient

Difference Count is Zero Due to

Baseline Compensation

LED OFF

LED ON

Sensor Status Baseline Rawcount

LED OFF

Baseline

Compensated

Rawcount Returns to Initial Value

After LED is Turned OFF

Baseline is reset due to Low

Baseline Reset Algorithm

Figure 5-6. CapSense Sensor Variables when Baseline is Compensated

5.8 PCB Layout Guidelines

Detailed PCB layout guidelines are available in Getting Started with CapSense.

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6. Liquid-Tolerant Design Considerations

Some CapSense capacitive touch-sensing applications require reliable operation in the presence of water or other

PCB

Shield Electrode

Guard Sensor

Sensor Pads

liquids such as ketchup and blood. White goods, automotive applications, and industrial applications are examples of systems that must perform in environments that include water, ice, humidity changes, or any other liquids. For such applications, shield electrodes and guard sensors can provide robust touch sensing.

6.1 Shield Electrode and Guard Sensor

Figure 6-1. PCB Layout with Shield Electrode and Guard Sensor

6.1.1 Shield

Shield electrodes protect CapSense button sensors from detecting false touches caused by water drops or liquid spills. When water drops or any other liquid spills are present on the overlay surface, the coupling between the shield electrode and sensor pad is increased by CLD, as shown in Figure 6-2.

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Liquid-Tolerant Design Considerations

Figure 6-2. Capacitance Measurement with Liquid Drop

CLD = Capacitance between the liquid drop and shield electrode

The purpose of the shield electrode is to set up an electric field around the touch sensors that helps attenuate the effects of water. The shield electrode works by mirroring the voltage of the touch sensor on the shield.

Follow these guidelines to ensure proper shield operation:

 Schematic  Layout  Firmware development

6.1.1.1 Schematic

Select the proper pin to drive the shield electrode out signal. The following pins should be used to drive the shield electrode out signal.

 Port pins: P0[0], P1[2], P0[2], P2[2], or P2[4]

6.1.1.2 Layout

Follow the layout guidelines given in Getting Started with CapSense.

6.1.1.3 Firmware Development

There is a shield drive GUI, as shown in Figure 6-3, to minimize firmware development.

Figure 6-3. Driven Shield GUI (Shield Inactive)

Step 1: Enable the CapSense Shield Signal from the GUI. Click on “Ebuf0”, which is the EnableShieldDriver switch, as shown in Figure 6-4; the “Disable” option is selected by

default. Select CapsenseShieldSignal to enable the shield driver.

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Liquid-Tolerant Design Considerations

Shield Buffer

Output Port Pin

Buf 0

Port_2_4

Buf 1

Port_2_2

Buf 2

Port_0_2

Buf 3

Port_0_0

Buf 4

Port_1_2

Figure 6-4. Enabling CapsenseShieldSignal from GUI

Step 2: Route the shield electrode out to the shield pin. There are five possible shield drive buffers, labeled Buf 0 through Buf 4; their mapping is shown in the following table.

The shield drive buffers are OFF by default; select the desired shield drive buffer. Figure 6-5 shows the enabling of Buf 0 such that the shield signal is driven out on Port_2_4.

Figure 6-5. Schematic View of Output Select

Step 3: Choose polarity of the shield signal. There are two polarity choices, see Figure 6-8:

 Normal Mode (default): Shield driver signal in phase with sensor scan signal  Reverse Polarity: Shield driver signal 180° out of phase with sensor scan signal

Normal Mode shows the best performance in liquid-tolerant designs.

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Liquid-Tolerant Design Considerations

PCB

Shield Electrode

Guard Sensor

Sensor Pads

6.1.2 Guard Sensor

Figure 6-6. Schematic View of Polarity Select

Figure 6-7. PCB with Shield and Guard Sensor

A guard sensor is a copper trace, as shown in Figure 6-9, that surrounds all the sensors on the PCB, which is used to detect the presence of a continuous water stream or a large liquid spill. When a water stream or a large liquid spill is present on the sensing surface, a large-capacitance CST is added to the system, as shown in Figure 6-8. This capacitance may be several times larger than CLD. Because of this, the effect of the shield electrode is completely masked and the raw counts measured by the sensor will be the same as or even higher than a finger touch. In this situation, a guard sensor will help; when it detects a water stream, it will block the other sensors from triggering.

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Liquid-Tolerant Design Considerations

Figure 6-8. Capacitance Measurement with Large Liquid Spills or Water Stream

 C  C  C

- Capacitance between the water stream/liquid spill and shield electrode

- Capacitance between the water stream/liquid spill and the system ground

- Capacitance between the water stream/liquid spill and the guard sensor

The guard sensor should be implemented in firmware. One CapSense pin and a counter (hardware/software) are required to implement the guard sensor.

When a water stream or a large liquid spill is present on the board, the guard sensor detects this event and disables the touch sensor processing logic. Additional guard sensor "dead" time prevents unlocking the sensor prematurely. When the liquid is gone, the guard counter suppresses touch processing for a short time (guard sensor counter). This eliminates false touch detection from any water/liquid that remains on the board.

Figure 6-9 is a flow chart that shows how the guard sensor should be implemented in firmware.

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Liquid-Tolerant Design Considerations

Start of main function

End of main function

Initialize CapSense block and other blocks

Scan all sensors including Guard sensor

Update baseline of all sensors

Is Guard Sensor ON? Turn off all the sensors

Guard sensor

counter elapsed?

Report sensor status normally

Yes

Figure 6-9. Flow Chart to Implement the Guard Sensor

6.2 Design Recommendations

The following system-level and PCB layout recommendations apply to CapSense systems that will be exposed to liquids or humid environments:

 Shield-electrode copper-hatch recommendations:

 Top layer – 7-mil trace and 45-mil grid (15-percent fill)

 Bottom layer – 7-mil trace and 70-mil grid (10-percent fill)

 The shield electrode between buttons should be at least 10 mm wide  Place sensor surface vertically or at an angle to the horizontal so that water drops or liquid spills naturally move

off the sensors and large liquid drops do not accumulate.  Use liquid-repellent and nonabsorbent overlay material. This minimizes liquid streaks and films on the device

panel. This is especially important if the liquid is highly conductive, such as seawater.  You must use a guard sensor in situations where the application may be subjected to continuous water streams

or large liquid spills. A guard sensor is not required if the device will be subjected only to rain, mist, or humid

conditions.

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7. Proximity Sensing Design Considerations

Proximity sensors detect the presence of a hand or other conductive object before it makes contact with the

Proximity Sensor

capacitive touch surface. Imagine a hand stretched out to operate a car audio system in the dark. Proximity detection enables the system to light up with the approach of a finger. For example, the buttons of an audio system will glow with its backlight LEDs when the user's hand is near.

7.1 Types of Proximity Sensors

7.1.1 Button

A button with large CP and small difference counts can work as a proximity sensor. The sensitivity of a proximity sensor implemented as a button is much higher than a regular capacitive touch-sensing button.

7.1.2 Wire

A single length of wire works well as a proximity sensor. Because detecting a hand relies on the capacitance change from electric field changes, any stray capacitance or objects affecting the electrical field around the wire will affect the range of the proximity sensor. Using a wire sensor is not an optimal solution for mass production because of manufacturing cost and complexity.

7.1.3 PCB Trace

A long PCB trace can form a proximity sensor. The trace can be a straight line, or it can surround the perimeter of a system‟s user interface, as shown in Figure 7-1. This method is appropriate for mass production, but it is not as sensitive as a wire sensor.

Figure 7-1. Proximity Sensor Using PCB Trace

7.1.4 Sensor Ganging

Another way to implement a proximity sensor is to gang sensors together. This is accomplished by combining multiple sensors into one large sensor, using firmware to connect the sensors from the internal analog multiplexer bus in PSoC Designer. Be careful not to exceed the CP limit of your design when using this method.

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Proximity Sensing Design Considerations

7.2 Design Recommendations

 Using a shield electrode effectively extends the detection distance of the proximity sensor. This is particularly

helpful when the proximity sensor must operate in the presence of metal.  A wire sensor increases the beneficial effect of the shield electrode because it can be located farther from the

shield electrode.

 Avoid large, solid ground-fill areas inside the proximity sensor, because this decreases the sensitivity.  Slower scan speeds provide increased sensitivity at all distances. For a proximity sensor, the scanning speed

should be very slow.

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8. Low-Power Design Considerations

Power consumption is an important aspect of microcontroller designs. Among the several techniques to reduce the average current used by the CapSense controller, sleep mode is the most popular. The CapSense controller uses sleep mode when it is not required to perform any function, similar to a cell phone backlight dimming after an idle period. This is done to reduce the average current consumed by the device, a necessity of all battery applications.

The CapSense controller enters sleep mode by writing a „1‟ to the SLEEP bit within the CPU_SCR0 register (Bit 3). This is accomplished by calling the M8C_Sleep macro.

While in sleep mode:

 the central CPU is stopped  the IMO is disabled  the bandgap voltage reference is powered down  the flash memory model is disabled

The only circuits left in operation are the supply voltage monitor and the 32-kHz internal oscillator. Power-saving techniques other than the standard sleep mode are:

 I2C sleep mode (see section Entry into I2C Sleep Mode)  Disable CapSense (PSoC) analog block references  Disable CT and SC blocks  Disable CapSense (PSoC) analog output buffers  Set drive modes to analog HI-Z

Sleep mode has negative effects for a design. If not used carefully, it can cause an unpredictable operation. The PSoC must be correctly awakened from sleep when necessary and the user must be aware that the device is sleeping to allow extra processing.

8.1 Additional Power Saving Techniques

All of the power-saving techniques, with the exception of sleep mode, is application-based. Some of them produce undesirable results. Each technique is discussed in detail in the following sections.

8.1.1 Set Drive Modes to Analog HI-Z

The state of the CapSense controller drive modes can affect power consumption. You can change the drive modes only on pins that do not cause adverse effects to the system. The change must occur in a sequence that does not produce line glitches. This sequence depends on the current drive mode of the pin and the state of the port data register. With the CapSense controller drive mode structure, the pin must temporarily be in either Resistive Pull-up or Resistive Pull-down drive mode when switching between HI-Z or Strong drive modes. The temporary drive mode is the opposite of the previous value on the pin. Therefore, if the pin was driven high, then the temporary drive mode must be Resistive Pull-down. This ensures that the drive mode of the pin is not resistive, which eliminates any possible glitch.

The drive modes are set manually in software, before going to sleep. Three registers, PRTxDM0, PRTxDM1, and PRTxDM2, control the drive modes. One bit per register is assigned to a pin. Therefore, to change the drive mode of a single pin, three register writes are needed. However, this is convenient because an entire port is changed by the same three register writes. The correct pit pattern for Analog HI-Z is 110b. Use the following code to set port zero to Analog HI-Z from Strong, by first going to Resistive Pull-down.

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Low-Power Design Considerations

PRT0DM0 = 0x00; // low bits PRT0DM1 = 0xff; // med bits PRT0DM2 = 0xff; //high bits

8.1.2 Putting it All Together

The following code is a sample of a typical sleep preparation sequence for a 28-pin part. In this sequence, interrupts are disabled, the analog circuitry is turned off, all drive modes are set to Analog HI-Z, and interrupts are re-enabled.

void PSoC_Sleep(void) { M8C_DisableGInt; PRT0DM0 = 0x00; // port 0 drives PRT0DM1 = 0xff; PRT0DM2 = 0xff; PRT1DM0 = 0x00; // port 1 drives PRT1DM1 = 0xff; PRT1DM2 = 0xff; PRT2DM0 = 0x00; // port 2 drives PRT2DM1 = 0xff; PRT2DM2 = 0xff; M8C_EnableGInt; M8C_Sleep; }

8.1.3 Recommended I2C Slave Implementation in Sleep Mode

When I2C is used in sleep mode, certain implementation guidelines must be followed to keep the I2C bus from locking or corrupted transactions from occurring.

8.1.3.1 Entry into I2C Sleep Mode

In general, the I2C slave must be put into a FORCE_NACK mode before entering the sleep mode. These steps need to be followed to enter the sleep mode correctly:

 Select the mode of operation (clock stretch or NACK during sleep to wakeup) through CLK_STRETCH_EN bit of

the I2C_BP_EZ_CFG register

 Set the FORCE_NACK bit of the I2C_XCFG register  Poll status bit I2C_XSTAT.READY_TO_NACK for logic „1‟  Set the I2C_ON bit in the SLP_CFG2 register  Call the M8C_Sleep function (this sets the SLEEP bit (Bit 3) within the CPU_SCR0 register)

Note: Data retention during sleep and deep sleep modes in the 32-byte I2C buffer is guaranteed only if power to the I2C block is enabled by asserting I2C_ON bit in the SLP_CFG2 register.

8.1.4 Sleep Mode Complications

The CapSense controller can exit sleep either from a reset or through an interrupt. The CapSense controller has three types of resets: External Reset, Watchdog Reset, and Power-On Reset. Any of these resets takes the CapSense controller out of sleep mode. After the reset deasserts, the CapSense controller begins executing code starting at Boot.asm. Available interrupts to wake the CapSense controller are: Sleep Timer, Low-Voltage Monitor, GPIO, Analog Column, and Asynchronous. Sleep mode complications arise when using interrupts to wake the CapSense controller or attempting digital communication while asleep. These considerations are discussed in detail in the following sections.

8.1.5 Pending Interrupts

If an interrupt is pending, enabled, and scheduled to occur after a write to the SLEEP bit in the CPU_SCR0 register, the system will not go to sleep. The instruction still executes, but the CapSense controller does not set the SLEEP bit. Instead, the interrupt is serviced, which effectively causes the CapSense controller to ignore the sleep instruction. To avoid this, interrupts should be globally disabled while sleep preparation occurs and then re-enabled just before writing the SLEEP bit.

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Low-Power Design Considerations

8.1.6 Global Interrupt Enable

The Global Interrupt Enable register (CPU_F) need not be enabled to wake the CapSense controller from interrupts. The only requirement to wake up from sleep by an interrupt is to use the correct interrupt mask within the INT_MSKx registers, as in the following example. If global interrupts are disabled, the ISR that wakes the CapSense controller is not executed but the CapSense controller still exits sleep mode.

In this case, you must manually clear the pending interrupt or enable global interrupts to allow the ISR to be serviced. Interrupts are cleared within the INT_CLRx registers.

//Set Mask for GPIO Interrupts M8C_EnableIntMask(INT_MSK0, INT_MSK0_GPIO)

// Clear Pending GPIO Interrupt INT_CLR0 &= 0x20;

8.2 Post Wakeup Execution Sequence

If the CapSense controller is awakened through a reset, execution starts at the beginning of the boot code. If the CapSense controller is awakened by an interrupt service routine, the first instruction to execute is the one immediately following the sleep instruction. This is because the instruction immediately following the sleep instruction is pre-fetched before the CapSense controller is fully asleep. Therefore, if global interrupts are disabled, the instruction execution will continue where it left off before sleep was initiated.

8.2.1 PLL Mode Enabled

If PLL mode is enabled, the CPU frequency must be reduced to the minimum of 3 MHz before going to sleep. This is because the PLL always overshoots as it attempts to relock after the CapSense controller wakes up and is reenabled. Additionally, you should wait 10 ms after wakeup before normal CPU operation begins to ensure proper execution. This implies that, to use sleep mode and the PLL, the software must be able to execute at 3 MHz. A simple write to the OSC_CR0 register can reduce CPU speed. However, this register just sets a divider of SYSCLK, which means that the CPU speed will vary between part families with different SYSCLKs. Typically, SYSCLK is 24 MHz

OSC_CR0 &= 0xf8; // CPU = 3 Mhz IMO = 24 Mhz

8.2.2 Execution of Global Interrupt Enable

Avoid interrupts on the instruction boundary of writing the SLEEP bit. This can cause all firmware preparations for going to sleep to be bypassed, if a sleep command is executed on a return from interrupt (reti) instruction. To prevent this, interrupts are temporarily disabled before sleep preparations and then re-enabled before going to sleep. Because of the timing of the Global Interrupt instruction, an interrupt cannot occur during the next instruction, which in this case is setting the SLEEP bit.

8.2.3 Recommended I2C Slave Implementation in Sleep Mode

When I2C is used in sleep mode, certain implementation guidelines must be followed to keep the I2C bus from locking or corrupted transactions from occurring.

8.2.3.1 Wakeup from I2C Sleep Mode using HW Address Match

To enable the wakeup through I2C, set the HW Addr EN bit so that the I2C slave block wakes the system if and only if the address matches. The I2C block responds to the transactions on the I2C bus when the system is in sleep mode but when the system is in sleep mode, the system clock shuts down.

Therefore, if the I2C block must respond to the transactions on the I2C bus, then the block does not have system clock to work on. As a result, the incoming SCL clock is used as the clock for the block to respond to the transactions on the bus.

When the address matches, the behavior depends on the CLK_STRETCH_EN setting. If this bit is set high, SCL is pulled low until the IMO is operational. If this bit is set low, the slave NACKS all I2C transactions addressed to it until the CPU wakes up and sets the ACK bit. When clock stretch mode is disabled, after a wakeup IRQ, any other start condition will not be recognized by the device until the IMO is operational. All transactions in this duration will receive a NACK. Figure 8-1 depicts the wakeup sequence through I2C.

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Low-Power Design Considerations

Figure 8-1. Wakeup Sequence

8.2.4 Sleep Timer

The CapSense controller offers a sleep timer and a Sleep Timer User Module. These are used while the CapSense controller is asleep and both perform similar functions. The actual sleep timer runs off the internal low-speed oscillator, which is never turned off. At selectable intervals of 1 Hz, 8 Hz, 64 Hz, and 512 Hz, the timer generates an interrupt. It is often useful to periodically wake the CapSense controller up to do some processing or check for activity. An example of this is to periodically wake up to scan a sensor. The Sleep Timer User Module uses the sleep timer to generate some additional functionality. This functionality includes a background tick counter to generate periodic interrupts, a delay function for program loops, a settable down counter, and a loop governor to control loop time. A simple block diagram for this functionality is shown in Figure 8-2. .

Figure 8-2. Sleep Timer User Module Block Diagram

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9. Resources

9.1 Website

Visit Cypress‟s CapSense Controllers website to access all of the reference material discussed in this section. Find a variety of technical resources for the CapSense CY8C20xx7/S family of devices on the CY8C20xx7/S web

page.

9.2 Datasheet

The datasheet for the CapSense CY8C20xx7/S family of devices are available at www.cypress.com.  CY8C20xx7/S Datasheet

9.3 Technical Reference Manual

Cypress created the following technical reference manual to give quick and easy access to information on the CapSense controller functionality, including top-level architectural diagrams along with register and timing diagrams.

 CY8C20xx7/S Technical Reference Manual (TRM)

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Resources

9.4 Development Kits

9.4.1 CY8C20xx7/S QuietZone Starter Kit

The CY8C20xx7/S QuietZone Starter Kit shown as Figure 9-1 is a 0.75-inch squared PCB based on the CY8C20247S 16-pin QFN package.

Figure 9-1. CY8C20xx7/S QuietZone Starter Kit

The CY8C20xx7/S QuietZone Starter Kit hardware has the following features:

 Twelve capacitive sensing inputs  One 5-pin header (HD1) for easy connection to MiniProg1/3  One driven shield output  Two LEDs for visual feedback

The kit is available from our module partner ArtaFlex at the following link: www.artaflexmodules.com/quietzone

9.4.2 Universal CapSense Controller Kit

The CY8C20xx7/S family does not have on-chip debug (OCD) capability; if a debug platform is needed, then the CY8C20xx6A Universal CapSense Controller Kit is available. The Universal CapSense Controller Kits feature predefined control circuitry and plug-in hardware to make prototyping and debugging easy. Programming and I2C-toUSB Bridge hardware are included for tuning and data acquisition.

 CY3280-20xx6A Universal CapSense Controller

9.4.3 Universal CapSense Module Boards

9.4.3.1 Simple Button Module Board

The CY3280-BSM Simple Button Module consists of 10 CapSense buttons and 10 LEDs. This module connects to any CY3280 Universal CapSense Controller Board

9.4.3.2 Matrix Button Module Board

The CY3280-BMM Matrix Button Module consists of eight LEDs and eight CapSense sensors organized in a 4×4 matrix format to form 16 physical buttons. This module connects to any CY3280 Universal CapSense Controller Board.

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Resources

9.5 Sample Board Files

Cypress offers sample schematic and board files, which can be used as a reference to quickly complete your PCB design process.

 Button design with I2C header on CY8C20xx7/S  Button and slider design with I2C header on CY8C20xx7/S

Note The board files (schematic, layout, and Gerber files) will be placed in the landing page of this document.

Figure 9-2 and Figure 9-3 show the board schematics.

The following schematic is designed to support:

 Two CapSense sensors. The sensors are assigned to pins P2[5] and P1[5] of the CY8C20237-24SXI device.  Two GPIOs connected to pins P0[7] and P0[4] of CY8C20237-24SXI to drive LEDs D1 and D2.  Shield electrode is connected to pin P1[2]to drive shield plane.  Programming lines and I2C SDA/SCL signals of CY8C20237-24SXI connected to header J1.

Figure 9-2. Button Design with I2C Header on CY8C20237 - Board Schematic

The following schematic is designed to support the following:  Four CapSense sensors. The sensors are assigned to pins P0[1], P2[1], P3[3], and P1[7] of the CY8C20437-

24LQXI device.

 Guard sensor is connected to pin P4[3] of the CY8C20437-24LQXI device.  Four GPIO pins P1[4], P1[6], P3[0], and P3[2] CY8C20437-24LQXI to drive LEDs D1, D2, D3, and D4.  Programming of CY8C20437-24LQXI by way of the programming header J2.  9-V DC input can be connected to header J7.

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Resources

Figure 9-3. Liquid-Tolerant Button Design with I2C Header on CY8C20437

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Resources

9.6 PSoC Programmer

PSoC Programmer is a flexible, integrated programming application for programming PSoC devices. It can be used

with PSoC Designer and PSoC Creator™ to program any design onto a PSoC device. PSoC Programmer includes a hardware layer with APIs to design specific applications using the programmers and

bridge devices. The PSoC Programmer hardware layer is fully detailed in the COM guide documentation as well as example code across the following languages: C#, C, Perl, and Python.

9.7 CapSense Data Viewing Tools

Many times during CapSense design, you will want to monitor relevant CapSense data (raw counts, baseline, difference counts, and so on) for tuning and debugging purposes. Application note AN2397 – CapSense Data

Viewing Tools gives information to help you identify and use the right tools for CapSense data viewing and logging.

9.8 PSoC Designer

Cypress offers an exclusive integrated design environment (IDE), PSoC Designer. With PSoC Designer, you can configure analog and digital blocks, develop firmware, and tune your design. Applications are developed in a dragand-drop design environment using a library of fully characterized analog and digital functions, including CapSense. PSoC Designer comes with a built-in C compiler and an embedded programmer. A pro compiler is available for complex designs.

9.9 Code Examples

Cypress offers a large collection of code examples to get your design up and running fast.  CapSense Controller Code Examples Design Guide

9.10 Design Support

Cypress has a variety of design support channels to ensure the success of your CapSense solutions.  Knowledge Base Articles – Refer to technical articles by product family or perform a search on various

CapSense topics.  CapSense Application Notes – Refer to a wide variety of application notes built on information presented in this

document.

 White Papers – Learn about advanced capacitive-touch interface topics.  Cypress Developer Community – Connect with the Cypress technical community and exchange information.  CapSense Product Selector Guide – See the complete product offering of Cypress‟s CapSense product line.  Video Library – Quickly get up to speed with tutorial videos  Quality & Reliability – Cypress is committed to complete customer satisfaction. At our Quality website you can

find reliability and product qualification reports.  Technical Support – World class technical support is available online.

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Revision History

Document Title: AN78329 - CY8C20xx7/S CapSense® Design Guide Document Number: 001-78329

Revision

Issue Date

Origin of

Change

Description of Change

05/09/2012

DST

New Design Guide

06/28/2012

UDYG

Edits made to 8.1.3.1 Entry into I2C Sleep Mode

09/06/2012

ZINE

Updated links to external documents

11/27/2012

DST

Page 17: Added link to QuietZone Starter Kit external web Updated Section 3.4.2.4 Pg 40: Added table 4-8 Updated Section 6.1.1.1

01/21/2015

PRIA/DCHE

Added sections Switched-Capacitor Input and Sigma Delta Converter Added section GPIO Load Transient Renamed Chapter 6 to -Tolerant Design Considerations Updated template Changed the document title to CY8C20xx7/S CapSense® Design Guide – AN78329

03/24/2015

DCHE

Added section 2.2.4, 2.2.1 Updated Figure 4-3, Figure 2-1 and Figure 4-2 Added footnote in Page 8 Added information on driven shield in section 1.3 Updated recommended values in section 4.2.11 Added hyperlinks in section 9.4.2 and 9.4.3

Document Revision History

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Cypress CY8C20xx7/S CapSense Series Design Manual

Specifications and Main Features

Frequently Asked Questions

User Manual