ST STM32F0 Series Istruzioni per l'installazione e il funzionamento

Introduction

This document helps customers to quickly locate information regarding touch sensing on STM32 microcontrollers.

It is applicable to STM32F0, STM32F3, STM32L0, STM32L1 and STM32L4 Series products. It lists all the existing application

notes and user manuals covering touch sensing. It indicates where the key aspects of touch sensing are documented.

It also explains how to build touch sensing applications on STM32L0538-DISCO and STM32F072B-DISCO discovery boards

using the STM32CubeMx graphical interface.

Getting started with touch sensing control on STM32 microcontrollers

AN5105

Application note

AN5105 - Rev 1 - September 2018

For further information contact your local STMicroelectronics sales office.

www.st.com

1General information

This document applies to Arm®-based devices.

Note: Arm is a registered trademark of Arm Limited (or its subsidiaries) in the US and/or elsewhere.

AN5105

General information

AN5105 - Rev 1 page 2/52

2Terminology and principle

2.1 Terminology

The touch sensing most relevant acronyms are described below:

• Acquisition mode

– CT: Charge-Transfer acquisition principle. This mode is used on STM32 microcontrollers.

• Touch sensing STM32 peripheral

– TSC: touch sensing controller peripheral

– Bank: set of channels acquired simultaneously

– Channel: elementary acquisition item

– Group: set of 1..3 channels plus 1 sampling capacitor (Cs)

• Sensors

– Touchkey or TKey: single channel sensor

– Linear sensor: multi-channels sensor with the electrodes positioned in a linear way

– Rotary sensor: multi-channels sensor with the electrodes positioned in a circular way

– Active shield: track running along or copper plane surrounding the sensor track and/or sensor itself.

Active shield is driven similarly to the sensor. Improve noise robustness without decreasing the

sensitivity.

• STM32 software

– TSL: touch sensing library

– Delta: difference between the measure and the reference

– Measure or meas: current signal measured on a channel

– Reference or ref: reference signal based on the average of a sample of measures

– DTO: detection time out. Time out is defined by TSLPRM_DTO. See TSLPRM_DTO in tsl_conf.h file.

– DXS: detection exclusion system. Exclusion system is defined by TSLPRM_USE_DXS. See

TSLPRM_USE_DXS in tsl_conf.h file.

– ECS: environment change system. See TSLPRM_ECS_DELAY in tsl_conf.h file.

• Hardware Involved

– Cx: sensor capacitance (typical value is few pF)

– Cp: parasitic capacitance (typical value few pF)

– Ct: equivalent touch capacitance

– Cs/Cskey/Csshield: sampling capacitor (typical value from 2.2 to 100nF)

– Rs/Rskey/Rsshield: serial resistor, ESD protection (typical value from 100Ohms to 10K)

2.2 Principle

The STM32 touch sensing feature is based on charge transfer.

The surface charge transfer acquisition principle consists in charging a sensor capacitance (Cx) and in

transferring the accumulated charge into a sampling capacitor (Cs).

This sequence is repeated until the voltage across Cs reaches VIH.

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

electrode capacitance. When the sensor is touched, the sensor capacitance to the earth is increased. This mean

the C voltage reaches VIH with less count and the measurement value decreases. When this measurement goes

below a threshold, a detection is reported by the TSL. The schematic below do not take into account the parasitic

capacitor.

AN5105

Terminology and principle

AN5105 - Rev 1 page 3/52

Figure 1. Change transfer principle

Human

body

coupling

to earth

touchkey

sensor

eletrode

User finger

Sampling

capacitor

VDD

VSS

STM32

MCU

Sensor capacitor

Table 1. Change transfer principle documentation gives a list of documents containing information about the

change transfer principle.

Table 1. Change transfer principle documentation

Id Title Chapters

AN4299 Guidelines to improve conducted noise robustness on

STM32F0/F3/L0/L4 series touch sensing applications Surface charge transfert acquisition principle overview

AN4310 Sampling capacitor selection guide for MCU based

touch sensing applications Charge transfert acquisition principle overview

AN4312 Guidelines for designing touch sensing applications

with surface sensors Capacitive sensing technology in ST

AN4316 Tuning a STMTouch-based application Charge transfer periode tuning

OLT STM32L4 On Line Training Touch sensing controller (TSC)

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Principle

AN5105 - Rev 1 page 4/52

3Document reference

Figure 2. Main documentation tree shows the main documentation tree related to TSC and TSL.

Figure 2. Main documentation tree

Application Note

overview

(AN5105)

Tuning

(AN4316)

Getting Started

with TSL

(UM1913)

STM32 On-line

Training

Sampling

Capacitor

(AN4310)

Conducted Noise

(AN4299)

ESD

(AN3960)

Sensors

(AN4312)

TSC

Table 2. References documentation

Document name Document title

UM1913 Developing applications on STM32Cube with STMTouch® touch sensing library

AN3960 ESD considerations for touch sensing applications

AN4299 Guidelines to improve conducted noise robustness on STM32F0/F3/L0/L4 series touch sensing

applications

AN4310 Sampling capacitor selection guide for MCU based touch sensing applications

AN4312 Guidelines for designing touch sensing applications with surface sensors

AN4316 Tuning a STMTouch-based application

AN5105

Document reference

AN5105 - Rev 1 page 5/52

4STM32L4 touch sensing controller online presentation

An online training is available under our website www.st.com.

Insert the STM32L4 Online Training” string in the "Search" fuction and press enter.

To find it use the function "Search" and insert the strings “STM32L4 Online Training”. Figure 4. STM32L4 Touch

Sensing Controller online training shows the online page available .

Figure 3. STM32L4 online training

Figure 4. STM32L4 Touch Sensing Controller online training

AN5105

STM32L4 touch sensing controller online presentation

AN5105 - Rev 1 page 6/52

5Main characteristics

5.1 Description

The following Figure 5. TSC characteristics shows all touch sensing controller (TSC) characteristics and their

correlation.

The TSC main characteristics are described in the following pages.

Figure 5. TSC characteristics

TSC

Guard ringShieldRotaryLinearKeys Conducted

Noise

immunity

False

detection

Power

supply

Noise

PCBSensorSensitivity

Charge

transfer

Signal

threshold

5.2 Signal threshold

To tune the detection thresholds, it must determine the sensitivity of each touchkey. For each touchkey, can be

used few parameters to adjust these signal thresholds.

For debug purpose, it can get touchkey parameters using printf or STMStudio tool:

for (Index = 0;Index < NUMBER_OF_TOUCHKEYS;Index++)

{

printf("K%1d [%2d][%4d %3d %3d %4d] %d %d %d %d %d"

,Index

,MyTKeys[Index].p_Data->StateId

,MyTKeys[Index].p_ChD->Ref

,MyTKeys[Index].p_ChD->RefRest

,MyTKeys[Index].p_ChD->Delta

,MyTKeys[Index].p_ChD->Meas

,MyTKeys[Index].p_Param->ProxInTh

,MyTKeys[Index].p_Param->ProxOutTh

,MyTKeys[Index].p_Param->DetectInTh

,MyTKeys[Index].p_Param->DetectOutTh

,MyTKeys[Index].p_Param->CalibTh

);

}

Note: ProxInTh and ProxOutTh are defined for proximity detection feature only, when TSLPRM_USE_PROX = 1.

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Main characteristics

AN5105 - Rev 1 page 7/52

Figure 6. STMStudio outputs

• On software side:

– Relevant information are located in tsl_conf.h and tscl_user.c files.

– Threshold (xx_TH) can be adjust in tsl_conf_tsc.h file.:

See below an example:

#define TSLPRM_TKEY_DETECT_IN_TH (64)

#define TSLPRM_TKEY_DETECT_OUT_TH (60)

#define TSLPRM_TKEY_CALIB_TH (56)

#define TSLPRM_LINROT_DETECT_IN_TH (50)

#define TSLPRM_LINROT_DETECT_OUT_TH (40)

• The TSL api, tsl_user_SetThresholds, located in tsl_user.c allows to adjust each channel independently. See

below an example:

void tsl_user_SetThresholds(void)

{

/* USER CODE BEGIN Tsl_user_SetThresholds */

/* Example: Decrease the Detect thresholds for the TKEY 0*/

MyTKeys_Param[0].DetectInTh -= 10;

MyTKeys_Param[0].DetectOutTh -= 10;

/* USER CODE END Tsl_user_SetThresholds */

}

Table 3. Signal threshold usage documentation gives a list of documents containing information about the signal

threshold usage.

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Signal threshold

AN5105 - Rev 1 page 8/52

Table 3. Signal threshold usage documentation

Id Title Chapters

UM1913 Developing applications on STM32Cube with

STMTouch® touch sensing library Debug with STMStudio

AN4316 Tuning a STMTouch-based application

Monitoring STMTouch driver variables using

STMStudio

Tuning of the Thresholds

Touchkeys thersholds

Linear and Rotary touch sensors thresholds

5.3 Charge transfer

The acquisition is based on the measurement of the sensor channel capacitance.

To ensure that the Cx capacitance is correctly charged, it is necessary to monitor the pin connected to the sensor.

On sensors and shield sides, it must observe a complete Charge/Discharge cycle.

Figure 7. Incomplete and complete charge transfert cycle

In this example , to complete the charge transfer cycles, the following parameter must be modified as below:

• INCREASE:

– htsc.Init.PulseGeneratorPrescaler

– htsc.Init.CTPulseHighLength

– htsc.Init.CTPulseLowLength

• DECREASE:

– Sysclk

Table 4. Charge transfer documentation gives a list of documents containing information about the charge

transfer.

Table 4. Charge transfer documentation

Id Title Chapters

AN4299 Guidelines to improve conducted noise robustness on

STM32F0/F3/L0/L4 series touch sensing applications Active shield

AN4316 Tuning a STMTouch-based application Charge transfer period tuning

AN5105

Charge transfer

AN5105 - Rev 1 page 9/52

5.4 Sensitivity

Sensitivity is a key point in touch sensing applications. The sensitivity can be improved by:

• Reduce air gap

• Reduce panel thickness

• Choose dielectric with higher εR

• GND plane must not too close from shield and sensors

• Avoid metallic paint near shield and sensors

Table 5. Sensitivity documentation gives a list of documents containing information about the sensitivity.

Table 5. Sensitivity documentation

Id Title Chapter

AN1913 Developing applications on STM32Cube with

STMTouch® touch sensing library GPIO mode (table)

AN4312 Guidelines for designing touch sensing applications

with surface sensors

Air Gap:

• Reduce air gap

Changing the Panel material:

• Reduce Panel thickness

• Choose dielectric with higher εR

Metal chassis:

• GND not too closed from Shield and Sensors

• Avoid Metallic paint near Shield and Sensors

Mechanical construction and PCB to panel bonding.

Surface sensor design

AN4316 Tuning a STMTouch-based application All chapter

Dielectric example

Table 6. Dielectric constants of common materials used in a panel construction

Material εR

Air 1.00059

Glass 4 to 10

Sapphire glass 9 to 11

Mica 4 to 8

Nylon 3

Plexiglass 3.4

Polyethylene 2.2

Polystyrene 2.56

Polyethylene terephthalate (PET) 3.7

FR4 (fiberglass + epoxy 4.2

PMMA (Poly methyl methacrylate) 2.6 to 4

Typical PSA 2.0 - 3.0 (approximately)

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Sensitivity

AN5105 - Rev 1 page 10/52

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