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AN2869: DescriptionGuidelines for designing touch sensing applications; File Size923KB，43 Pages; ManufacturerSTMicroelectronics; Websitehttp://www.st.com/

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Guidelines for designing touch sensing applications

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AN2869

Application note

Guidelines for designing touch sensing applications

Introduction

This application note describes the layout and mechanical design guidelines used for touch

sensing applications.

Capacitive sensing interfaces provide many advantages compared to mechanical user

interfaces. They:

●

offer a modern look and feel

are easy to clean

are waterproof

are robust

Capacitive sensing interfaces are more and more used in a wide range of applications.

The main difficulty designing such interfaces is to ensure that none of the items interfere

with each other.

This document provides simple guidelines covering three main aspects:

Printed circuit board (PCB)

Overlay and panel materials

All other items in the capacitive sensor environment

Depending on which application you are designing, you may not need to refer to all of the

contents of this document. You can go to the appropriate section after reading the common

part which contains the main capacitive sensing guidelines. For example, if you are

developing an application with only projected electrode, you should first read the main

capacitive sensing guidelines and then go through the sections giving specific

recommendations for projected electrode designs.

Table 1

lists the microcontrollers concerned by this application note.

Table 1.

Applicable products

Type

Applicable products

Proximity and touchkey microcontrollers

STM8A automotive microcontrollers

STM8L ultra-low-power microcontrollers

STM8S mainstream microcontrollers

STM8T touch-sensing microcontrollers

STM32 F0 entry-level Cortex™-M0 microcontrollers

STM32 L1 ultra-low-power ARM Cortex™-M3 based microcontrollers

Product family

May 2012

Doc ID 15298 Rev 6

1/43

www.st.com

Contents

AN2869

Contents

Capacitive sensing technology in ST . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.1

1.2

1.3

1.4

1.5

1.6

RC acquisition principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Charge transfer acquisition principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Surface ProxSense

acquisition principle . . . . . . . . . . . . . . . . . . . . . . . . 5

Projected ProxSense

acquisition principle . . . . . . . . . . . . . . . . . . . . . . . 5

Surface capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Projected capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Main capacitive sensing guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1

2.2

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.1

2.2.2

2.2.3

2.2.4

2.2.5

2.2.6

2.2.7

Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Electrode and interconnection materials . . . . . . . . . . . . . . . . . . . . . . . . . 9

Panel materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Mechanical construction and PCB to panel bonding . . . . . . . . . . . . . . . 13

Metal chassis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Air gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Transfer of an electrode from PCB to the front panel . . . . . . . . . . . . . . . 14

2.3

2.4

Placing of LEDs close to sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Surface electrode design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.1

3.2

3.3

Touchkey sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Touchkey matrix sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Linear sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.3.1

3.3.2

Normal patterned linear sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Interlaced linear sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.4

Rotary sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.4.1

3.4.2

3.4.3

Normal patterned rotary sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Interlaced patterned rotary sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Rotary sensor with central touchkey . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.5

Specific recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.5.1

LEDs and sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2/43

Doc ID 15298 Rev 6

List of figures

AN2869

List of figures

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.

Figure 11.

Figure 12.

Figure 13.

Figure 14.

Figure 15.

Figure 16.

Figure 17.

Figure 18.

Figure 19.

Figure 20.

Figure 21.

Figure 22.

Figure 23.

Figure 24.

Figure 25.

Figure 26.

Figure 27.

Figure 28.

Figure 29.

Figure 30.

Figure 31.

Figure 32.

Figure 33.

Figure 34.

Figure 35.

Equivalent touch sensing capacitances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Example of capacitive sensor construction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Clear ITO on PET with silver connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Silver printing on PET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Flexible PCB (FPC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

FR4 (2-sided epoxy-fiberglass). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Typical panel stack-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Examples of cases where a LED bypass capacitor is required . . . . . . . . . . . . . . . . . . . . . 15

Typical power supply schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Sensor size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Recommended electrode size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Simple matrix implementation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Normal patterned linear sensor with five electrodes (20-50 mm long) . . . . . . . . . . . . . . . . 20

Interlaced linear touch sensor with three elements (up to 60 mm long) . . . . . . . . . . . . . . . 21

Normal patterned rotary sensor (three electrodes) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Interlaced patterned rotary sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Back-lighting touchkey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

STM8T141 driven shield solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Simple driven shield using RC acquisition principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

STM8L1xx driven shield example using the charge-transfer acquisition principle . . . . . . . 26

STM32L driven shield example using the charge-transfer acquisition principle . . . . . . . . . 27

Printed electrode method showing several connection methods . . . . . . . . . . . . . . . . . . . . 28

Spring and foam picture (both are not compressed) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Track routing recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Ground plane example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Hatched ground and signal tracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Electric field between 2 surface electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Diamond implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

H sensor (single layer) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Two-layer implementation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Normal linear sensor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Small linear sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Rotary sensor made of 5 parcels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Ground floods around Tx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Potential false key detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

4/43

Doc ID 15298 Rev 6

AN2869

Capacitive sensing technology in ST

STMicroelectronics offers different capacitive sensing technologies for STM8 and STM32

family products. These technologies are based on:

●

The RC acquisition principle for STM8A, STM8S and STM8L microcontrollers.

The charge transfer acquisition principle for STM8L, STM32 F0 and STM32 L1

microcontrollers.

The surface ProxSense

acquisition principle for STM8T14x microcontrollers.

The projected ProxSense

acquisition principle for STM8TL5x microcontrollers.

Note:

ProxSense™ is a trademark of Azoteq.

1.1

RC acquisition principle

The RC acquisition principle is based on the charging/discharging time measurement of an

electrode capacitance through a resistor. When the electrode is touched, the

charging/discharging time increases and the variation is used to detect the finger proximity.

The RC acquisition principle is detailed in AN2927.

1.2

Charge transfer acquisition principle

The charge transfer acquisition principle uses the electrical properties of the capacitor

charge (Q). The electrode capacitance is repeatedly charged and then discharged in a

sampling capacitor until the voltage on the sampling capacitor reaches a given threshold.

The number of transfers required to reach the threshold is a representation of the size of the

electrode’s capacitance. When the electrode is “touched”, the charge stored on the

electrode is higher and the number of cycles needed to charge the sampling capacitor

decreases.

1.3

Surface ProxSense

acquisition principle

The surface ProxSense

acquisition principle is similar to the charge transfer one, except

that the acquisition is fully managed by a dedicated hardware IP providing improved

performance. For more information, please refer to the application note AN2970.

1.4

Projected ProxSense

acquisition principle

The projected ProxSense

acquisition principle is a measurement of a charge transferred

by a driven electrode to another one. Like the charge transfer, there is also a sampling

capacitor which stores the charges coming from the electrodes which form a coupling

capacitor with less capacitance than the sample one. When a finger approaches, the

dielectric (between the two electrodes) is modified and so the capacitance decreases. As a

consequence, the time taken to load the sample capacitor will increase and this difference is

used to detect if a finger is present or not.

Doc ID 15298 Rev 6

5/43

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