AN4316 - STMicroelectronics

AN4316 - STMicroelectronics
AN4316
Application note
Tuning a STMTouch-based application
1
Introduction
This document is intended for touch sensing application designers and provides guidelines
on how to tune their system. STM Studio tool is introduced and information is provided on
how to use it in order to monitor the variables. A particular emphasis will be placed on
providing a methodology to configure the STMTouch library parameters.
This document shows how to trim firmware parameters and adjust hardware components to
optimize the performance of your application.
This document is not intended to replace product documentation and library user manual
All values given in this document are for guidance only. Please, refer to the related
datasheet to get guaranteed values.
Note:
STMicroelectronics is providing free STMTouch touch sensing firmware libraries which are
available either as standalone packages (STM8L-TOUCH-LIB) or directly integrated into the
corresponding STM32Cube package (STM32CubeL0, STM32CubeF0, …).
Table 1. Applicable products
Type
Microcontrollers
October 2015
Applicable products
STM32F0 series, STM32F3 series, STM32L0 series, STM32L1 series,
STM32L4 series, STM8L series, STM8AL series.
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www.st.com
Contents
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Contents
1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2
STM Studio overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
Monitoring STMTouch driver variables using STM Studio . . . . . . . . . . 6
4
Tuning of the thresholds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1
Use of a standard test finger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
4.2
Threshold definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
4.3
4.2.1
Touchkeys thresholds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2.2
Linear and rotary touch sensors thresholds . . . . . . . . . . . . . . . . . . . . . . 14
4.2.3
Proximity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Debounce settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5
Charge transfer period tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
6
Hardware trimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.1
Cs trimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.2
Shield adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7
Performance comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Appendix A
9
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
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List of tables
List of tables
Table 1.
Table 2.
Applicable products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
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List of figures
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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.
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STM Studio variable selection window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
VarViewers with variable name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Data log setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Standard 8mm diameter finger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Threshold position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Rotary sensor log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Sensor log before balancing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Debouncing example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Metallic coin probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Ideal charge transfers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Non-ideal charge transfers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Active shield Cs trimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Active shield Rs trimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
SNR computation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Recommended standard finger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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STM Studio overview
STM Studio overview
STM Studio is a free software tools that helps to debug and diagnose STM8 and STM32
applications while they are running by reading and displaying their variables in real-time.
Running on a PC, STM Studio interfaces with STM8 and STM32 MCUs via standard
development tools, such as the low cost ST-LINK and RLink along with the high-end STM8
STice emulation system.
STM Studio is a non-intrusive tool, preserving the real-time behavior of applications.
STM Studio perfectly complements traditional debugging tools to fine tune applications. It is
well suited for debugging applications which cannot be stopped, such as motor control
applications.
Different graphic views are available to match the needs of debugging and diagnosis or to
demonstrate application behavior. This tool works with STM8 microcontrollers through
SWIM (single wire interface module) and with STM32 microcontrollers through JTAG or
SWD (serial wire debug) interface.
It is a graphical user interface for probing and visualizing in real time application's variables
while it is running. It is designed to run on a computer with Microsoft® Windows operating
systems.
Please refer to STM Studio release notes to know the host PC system requirements and
supported hardware.
For advanced information on how to use STM Studio, please refer to its user manual
(UM1025; Getting started with STM Studio).
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Monitoring STMTouch driver variables using STM Studio
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Monitoring STMTouch driver variables using STM
Studio
The main parameters to trim a touch sensing application are:

The channel references, the “Ref” element of an array of TSL_ChannelData_T
structure

The channel deltas, the “Delta” element of an array of TSL_ChannelData_T structure

The object states, the “StateId” element of an array of TSL_TouchKeyData_T structure
or a TSL_LinRotData_T structure
This list is not exhaustive and will depends on the application.
The following procedure provides an easy way to import such variables:
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1.
Open STM Studio
2.
Right-click in the “Display Variables” tab and select “Import...” or select the “File/Import
Variables” menu
3.
In the “Import variables from executable” window,
a)
select your application Elf file (.elf, .out or .axf) Through the “Executable file” field
using the browse button
b)
check the “Expand table elements” check-box
c)
Select the “Store executable path relatively to the user setting file” check box to
use relative path.
d)
enter “Ref” in the “Show symbol containing...” text box
e)
select “Add variables to the display variables table” in the Variables list box
f)
select the “.Ref” ended variables and click on “Import” button or Ctrl+Click to
operate an uncontinuous multi-selection
g)
repeat step d) and f) with Delta
h)
repeat step d) and f) with StateId
i)
click “Close” button
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Monitoring STMTouch driver variables using STM Studio
Figure 1. STM Studio variable selection window
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Monitoring STMTouch driver variables using STM Studio
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Once imported, the variables must be assigned to Viewer in order to be displayed:
1.
In the “Display Variables settings” table, select all the “Ref” items (you can use
Shift+Click to operate a continuous multi-selection or Ctrl+Click to operate an
uncontinuous multi-selection)
2.
Right-click in the table and select “Send To → VarViewer1”, or drag them directly to the
right viewer.
3.
In the “Viewers settings” window dock, right-click in the greyed part and select “New
VarViewer”. A “VarViewer2” tab appears.
4.
Repeat step 4, 5 and 6 for the “Delta” items
5.
Repeat step 4 and 5 for the “StateId” items
To ease the navigation, you can rename the VarViewer windows with the name of the
monitored variables by right click and rename.
Variables can be displayed as a curve, as a bar graph or in a table. Table display is
recommended for variables with very slow variation. Curve and bar graph suit for variables
with quick variation.
Figure 2. VarViewers with variable name
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Monitoring STMTouch driver variables using STM Studio
Then, adjust the value range for each varviewer:

The Delta depends on the application sensitivity and can be positive or negative

The State varies from 0 to 19, for value meanings refer to TSL_StateId_enum_T in
tsl_types.h.

The reference depends on Cx/Cs
At this point, you have to connect the PC to your application with the selected binary code
downloaded in the microcontroller, through a USB cable and the appropriate hardware tool
(such as a ST-Link).
To start monitoring, click on the green arrow
or select the “Run/Start” menu.
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Monitoring STMTouch driver variables using STM Studio
The data can be stored in a file:
1.
Open “Options” and “Acquisition Settings” window.
2.
Select “Log to file” check box and set you log file path.
Figure 3. Data log setting
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Tuning of the thresholds
Tuning of the thresholds
This section provides recommendations on how to select reliable thresholds. Depending on
application use cases, some recommendations will have to be adapted.
Capacitive touch sensing applications are sensitive to earth coupling. The parameter
tunning must be done in the same environment as the final application. The hardware tool
connected to the application may change the earth coupling. This is especially true for
example in battery power applications. ST is providing a galvanic insulated hardware tool
(ST-Link/ISOL) to minimize this effect.
4.1
Use of a standard test finger
In order to build an application working with the widest range of people finger
characteristics, we propose to use a standard test finger which will give a worst case but
also will allow to repeat the test without human dependency, such as finger size, pressure
and contact area, skin conductivity, etc... To perform repeatable test, it is recommended to
use an electrically conductive pen-shape tool with a flat rubber end. The flat end made of
conductive rubber, allows to touch the touchkey with a constant contact surface.
The operator performing the validation will take care to center the contact area on the
touchkey.
Additionally, it makes sense to plan a final validation with a panel of users.
A picture of a standard finger is provided below.
Figure 4. Standard 8mm diameter finger
069
Refer to Appendix A for a recommended standard test finger.
4.2
Threshold definitions
4.2.1
Touchkeys thresholds
To tune the detection thresholds, it is first necessary to determine the sensitivity of each
touchkey. In order to do so:
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Tuning of the thresholds
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1.
Connect the final hardware to a PC through the ST-Link and power the application
2.
Download the firmware which will be used in the final application with the final
parameters of the STMTouch driver.The default detection thresholds can be set to a
low value but keeping it higher than the noise Level.
3.
Launch STM Studio and configure it as explained in Section 2: STM Studio overview on
page 5.
4.
Use the standard finger as described in Section 4.1: Use of a standard test finger on
page 11.
5.
Touch a touchkey and move the finger in order to find the maximum delta and write
down this value, then repeat it for each touchkey. If around the maximum delta there is
still significant jittering on the measure, then compute the average and use it as
baseline.
These values will be the baseline of all the thresholds. If a significant variation exists
between the baseline of the application touchkeys, it is recommended to set a specific
threshold for each touchkey.
The detection threshold, which must be exceeded in order to report the touchkey as
detected, must be set between 55 % and 65 % of the baseline. The end-of-detection
threshold, below which the key is not detected anymore, must be set between 35 % and
45 % of the baseline.
Figure 5. Threshold position
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An example of threshold firmware adjustment, with a baseline measured at 80 and a
threshold adjusted between ~65 % and ~55 % of the baseline is shown below:
MyTKeys[0].p_Param->DetectInTh = 50; // Key 1 detection threshold
MyTKeys[0].p_Param->DetectOutTh = 30; // Key 1 end of detection threshold
The calibration threshold (TSLPRM_TKEY_CALIB_TH) can be common to all keys and set
to 60 % of the maximum baseline.
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Tuning of the thresholds
If one of these thresholds must be greater than 255, the TSLPRM_COEFF_TH must be set
in order to bring the value in the correct range [0;255]. This is obtained by dividing all
thresholds, except the calibration threshold, by 2 to the power of TSLPRM_COEFF_TH. In
that case, the divided coefficients values (except for the calibration threshold) are the values
that need to be configured in the firmware. A compensation factor of 2 to the power of
TSLPRM_COEFF_TH will then be applied to all coefficients (except the calibration
threshold) by the firmware.
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Tuning of the thresholds
4.2.2
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Linear and rotary touch sensors thresholds
For such type of sensors, the approach is different in the sense that these sensors are
composed of several channels. The standard test finger must be moved along the whole
sensor and a log of the delta must be recorded using STM Studio.
Detection thresholds adjustment
A rotary sensor log example is provided below:
Figure 6. Rotary sensor log
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The worst case delta as shown in the figure above must be considered as the baseline to
compute the threshold for this sensor. This threshold must be reached in order to trig the
computation of the position and to report a detection on this sensor.
The same ratio used for a touchkey can be applied to this baseline, so between 65 % and
55 % to enter in detection and between 35 % and 45 % to stop reporting a detection.
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Tuning of the thresholds
Balancing between channels
While a significant difference of sensitivity appear on the channels, the STMTouch driver
provide a way to balance the delta in order to minimize the error on the position
computation.
The sensitivity is determined with the maximum delta on each channel.
The figure below shows a log from a rotary sensor with an excessive difference of
sensitivity.
Figure 7. Sensor log before balancing
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To get well-balanced channels, a coefficient will be applied to each channel delta. This
coefficient is the ratio between the maximum deltas of each channel. The reference channel
is the one with the highest delta (A for channel 1), the others channels have their maximum
delta in B for channel 0, and C for channel 2. The coefficient of channel 0 will be set to A/B
and the one of channel 2 set to A/C. Channel 1 is not changed and gets its coefficient at 1.
These coefficients must be multiplied by 255 as they are used as a fraction of 255.
For instance:

A/B = 1.75 the coefficient is 488 = 0x1C0

A/C = 3 the coefficient is 768 = 0x300 then the code will be:
CONST uint16_t MyLinRot0_DeltaCoeff[LINROT_CHANNELS] =
{
0x1C0, 0x100, 0x300,// CH0, CH1, CH2
};
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MyLinRot0_DeltaCoeff will be pointed by the p_DeltaCoeff item in the declaration of the
TSL_LinRot_T or TSL_LinRotB_T structure.
CONST TSL_LinRotB_T MyLinRots[TSLPRM_TOTAL_LNRTS] =
{
{
&MyLinRots_Data[0],
&MyLinRots_Params[0],
&MyChannels_Data[CHANNEL_16_DEST],
(TSL_tNb_T)LINROT_CHANNELS,
MyLinRot0_DeltaCoeff,
(TSL_tsignPosition_T *)TSL_POSOFF_3CH_LIN_INTERLACED,
(TSL_tIndex_T) TSL_SCTCOMP_3CH_LIN_INTERLACED,
(TSL_tIndex_T) TSL_POSCORR_3CH_LIN_INTERLACED
}
};
4.2.3
Proximity
To define the proximity thresholds, the designer must consider the noise sensitivity and the
expected detection distance, but also the minimum surface to detect.
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4.3
Tuning of the thresholds
Debounce settings
In order to improve the robustness of the application, the STMTouch driver provides the
debounce feature. To validate a touch detection, the delta must have exceeded the
threshold during a certain number of consecutive samples as shown in Figure 8. This is to
avoid a false-detection due to a noise peak.
Figure 8. Debouncing example
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Due to the use of a down-counter, the DEBOUNCE preprocessor constant must be set to n1 while n consecutive sample must be measured below the detection threshold before
triggering a detection.
#define TSLPRM_DEBOUNCE_DETECT (2) //3 consecutive samples needed to enter
in Detection.
The debounce feature is configurable for each state transition:

TSLPRM_DEBOUNCE_PROX: while switching from release state to proximity state

TSLPRM_DEBOUNCE_DETECT: while switching from release state or proximity to
touch detection state

TSLPRM_DEBOUNCE_RELEASE: while switching from touch or proximity state to
release state

TSLPRM_DEBOUNCE_CALIB: while switching from release state to calibration state
(compute again the reference)

TSLPRM_DEBOUNCE_ERROR: while switching from any state to error state
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Charge transfer period tuning
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Charge transfer period tuning
The acquisition is based on the measurement of the sensor channel capacitance (or a set of
sensors in the case of linear and rotary sensors). The more charged this sensor
capacitance, the more accurate the measure and the better the noise immunity.
To ensure that the capacitance is correctly charged, it is necessary to monitor the pin
connected to the sensor plate or through a metallic coin put on the sensor.
Figure 9. Metallic coin probe
069
The signal must show a square wave which indicates a fully loaded capacitance.
Figure 10 shows an example of ideal charge transfers and Figure 11 a non-ideal ones.
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Charge transfer period tuning
Figure 10. Ideal charge transfers
Figure 11. Non-ideal charge transfers
In case of an uncompleted charge, the user must increase the charge transfer period.
Depending on the product and type of acquisition, different adjustments are necessary. The
involved parameters are in the MCU PARAMETERS section of the tsl_conf_stmxxxx.h file:

For STM32F0, STM32F3 and STM32L0 products embedding the TSC peripheral, the
trimming is done by increasing TSLPRM_TSC_CTPH for the charge period and/or
TSLPRM_TSC_CTPL for the transfer period, and optionally TSLPRM_TSC_PGPSC to
divide the pulse generator frequency by a power of 2.

For STM32L1 product, the charge transfer period is set through
TSLPRM_CT_PERIOD and TSLPRM_TIMER_FREQ parameters.
For STM8L and STM8AL products, the delay is expressed in number of NOP instructions,
executed in one cycle at the CPU frequency (max 16 MHz so in 62.5 ns). Two parameters
are provided: one for the charge period TSLPRM_DELAY_CHARGE and one for the
transfer period TSLPRM_DELAY_TRANSFER.
Please refer to the product related firmware documentation for more details.
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6
Hardware trimming
6.1
Cs trimming
The Cs capacitance is a key parameter for sensitivity. For touchkey sensors, the Cs value is
usually comprised between 8.7nF to 22nF. For linear and rotary touch sensors, the value is
usually comprised between 47nF and 100nF. These values are given as reference for an
electrode fitting a human finger tip size across a few millimeters dielectric panel.
The signal delta for a touchkey is usually above 20 while it’s around 100 for linear and rotary
touch sensors. These values are given for a normalized test finger.
6.2
Shield adjustment
The efficiency of the shield depends on the waveforms matching between the shielded
channel and the shield burst pulses. The parameters to adapt the shield waveform are Cs
and Rs. These parameters adjustment is performed in 2 steps:
1.
Active shield Cs trimming
The burst envelop of the channels belonging to a same bank should end at the same
voltage level (VIH):
Figure 12. Active shield Cs trimming
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Hardware trimming
2.
Active shield Rs trimming
Rs should be trimmed to ensure shield capacitance is fully charged.
Figure 13. Active shield Rs trimming
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Performance comparison
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Performance comparison
In order to compare the performance of a whole touch sensing application, the usual
parameter is the Signal to Noise Ratio, well-known as SNR. But each company, each team
and even each engineer as its own method to compute it.
Here below we provide a way to compute it, but the designer must keep in mind that to get
correct results, he must compare apples to apples. So the test conditions and the method of
computation must be under control and reproductible else it is meaningless to compare two
results.
SNR = Signal
-----------------Noise
In the above equation:

“Signal” is the average of the delta measurements during a touch

“Noise” is the amplitude (delta max - delta min) without touch.
Figure 14. SNR computation
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In order to make meaningful comparisons, the various SNR measurements must be
performed under similar test conditions.
The following parameters affect the results of the SNR:
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
hardware application i.e. layout, panel (dielectric, thickness, glue, ...), capacitance
value and quality, etc...

firmware application i.e. acquisition configuration (frequency, reference, ..), threshold
settings, etc...

test conditions i.e. object used to touch (standard test finger or genuine finger), way the
panel is touched (pressure, slope, ...), applied noise if any, etc...
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Conclusion
Conclusion
In order to get the best performance for his STMTouch-based application, the designer must
tune it correctly. STMicroelectronics provides STM Studio, a free and easy tool to help
performing this task. It is important to have the thresholds and the debounce values set
according to the application environment. To get the best performance, the charge transfer
must be operated completely.
If you need to compare performances, keep in mind you must compare apples to apples and
this can only be done by performing your own tests according to your application
requirements.
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Appendix A
Figure 15. Recommended standard finger
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Revision history
Revision history
Table 2. Document revision history
Date
Revision
Changes
04-Mar-2014
1
Initial release.
11-Jun-2014
2
Added support for STM32L0 series and STM8AL series.
15-Oct-2015
3
Added support for STM32L4 series.
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