ST AN2734 Application note

AN2734

Application note

S-Touch™ design procedure

Introduction

The purpose of this application note is to provide the system/hardware engineers enough
ground knowledge to start the design of capacitive touch inferface solutions with the
S-Touch

The document highlights the recommended step-by-step procedures to be followed for a
successful S-Touch
production and production tests.

Each topic covered in this document is explained in detail in other application notes.

™ capacitive controller devices.

™ design-in: design flow, PCB layout, fabrication, assembly pre-

January 2010 Doc ID 14573 Rev 2 1/14

www.st.com

Contents AN2734

Contents

1 Touch module design flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1 Touch module specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 PCB layout, fabrication and assembly . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1 Pre-production tuning/test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1.1 Hardware tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1.2 Software tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2 Production test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2/14 Doc ID 14573 Rev 2

AN2734 Touch module design flow

1 Touch module design flow

Figure 1 shows a simplified flowchart which describes the complete touch module design

flow starting from the specification to the mass production test of the PCB and the overall
system.

Figure 1. Touch module design flow

Spec ification

Dev ice

selection

Schematic

Specification review

layout, fab & ass embly

System integration test

PCB

Hardware

tuning

(s oftware tuning)

Mass production
(produc tion test)

PCB revision

GC25419

Doc ID 14573 Rev 2 3/14

Touch module design flow AN2734

1.1 Touch module specifications

Before starting the design of a capacitive touch module, a clear specification should be
defined to establish:

● Touchkeys number and functions

● Slider/rotator implementation

● LED backlighting - white LEDs or red LEDs, 5 V system or 3.6 V system etc

● Signal connections to the host system

● Module form factor

Figure 2. Specification definitions to schematic implementation

Keys

Slider/rotator

Specifications:

Backlighting led

Connection to host

Form factor

PCB

The next step to be performed after finalizing the module specification is to choose the most
appropriate device from the S-Touch product family, based on the requirements of the target
application.

4/14 Doc ID 14573 Rev 2

AN2734 Touch module design flow

The S-Touch devices have several sensor channels and various features. The STMPE821,
for example, consists of 8 I/Os that can be used as either normal input/outputs or as touch
sensor inputs. On a particular application, out of the eight I/Os, four I/Os can be used as
capacitive touch key input and the other four can be used for the backlighting LED control. If
the I/O is used for LED control, it can be configured as PWM output to create elaborate LED
effects.

Figure 3. STMPE821 block diagram

INT

RST

SCLK

SDAT

GPIO

controller

interface

GND VCC VIO

PWM

controller

I2 C

Impedance

Touchkey

controller

sensor

GPIO 0 - 7
/Touch 0 - 7

ARef

CS00046

For further details on S-Touch working principles and functional features, please refer to the
application note AN2693: "S-Touch

TM

devices: system considerations".

Doc ID 14573 Rev 2 5/14