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™ capacitive controller devices.

The document highlights the recommended step-by-step procedures to be followed for a successful S-Touch™ design-in: design flow, PCB layout, fabrication, assembly preproduction and production tests.

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

January 2010

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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

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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

Specification

Device

selection

Specification review

Schematic

PCB

layout, fab & assembly

Hardware	PCB revision
tuning

System integration test (software tuning)

Mass production (production test)

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Touch module design flow	AN2734

1.1Touch 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

Backlighting led

Specifications:

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.

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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

				GPIO 0 - 7
	GPIO			/Touch 0 - 7
	controller
	PWM		Impedance
	controller
			sensor
INT	I2C
RST			Touchkey	ARef
	interface
			controller
SCLK
SDAT
GND	VCC	VIO

CS00046

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

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