ST STM32F0 Series Installation and operating instructions
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
Cs
VDD
VSS
STM32
MCU
Sensor capacitor
Cx
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
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
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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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