Hello, and welcome to this presentation of the STM32 Touch

Hello, and welcome to this presentation of the STM32 Touch
Sensing Controller (TSC) which enables the designer to
simply add touch sensing functionality to any application.
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Over recent years, Touch Sensing has become quite
common in many applications such as mobile phones,
induction cooktops and ovens, coffee machines, etc. This
type of interface is more flexible and reliable compared to
standard push buttons because mechanical parts are no
longer needed.
The Touch Sensing Controller (TSC) embedded in STM32L4
devices offers a simple way to manage such interfaces. The
TSC supports a robust charge transfer acquisition principle
with up to 24 capacitive sensing channels. It is fully
configurable and only a few external components are
required to design a user-friendly interface.
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The key features of the Touch Sensing Controller are:
• Proven and robust charge transfer acquisition principle
which is available on several STM32 MCU series
(STM32F0, STM32F3, STM32L0 and STM32L4).
• Supports up to 24 capacitive sensing channels which are
split over 8 analog I/O groups. The number of channels
and I/O groups depends on the selected MCU.
• For optimum performance, up to 8 capacitive sensing
channels can be acquired in parallel. This offers a very
good response time.
• Only one sampling capacitor is needed to manage up to
3 capacitive sensing channels. This ensures a reduced
BOM.
• The charge transfer acquisition is fully managed by
hardware to reduce CPU overhead. A spread spectrum
feature is available to improve system robustness in
noisy environments.
• Finally, the Touch Sensing Controller is designed to
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operate with the free STM32Cube touch sensing library
available in the corresponding STM32Cube package. This
library offers all the processing required to develop a
robust capacitive sensing solution and supports proximity,
touchkey, linear and rotary touch sensors.
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To offer sufficient clock granularity, the TSC is directly
clocked using the AHB clock. This clock is used by the
spread spectrum block while the clock feeding the pulse
generator is reduced using a prescaler. GPIOs supporting
touch sensing must be configured in an alternate mode in
order to connect them to the Touch Sensing Controller. The
SYNC input pin is used to synchronize the capacitive
sensing acquisition with an external stimulus without the
need for CPU interaction. One counter per analog I/O group
is used to store the result of the acquisition. An interrupt can
be generated upon the end of acquisition of all the enabled
analog I/O groups or when an error is detected. This
interrupt helps limit CPU overhead.
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The charge transfer acquisition technique works using the
electrical properties of the capacitor. It consists in charging
the sensor capacitor (CX) to VDD. Once this capacitor is fully
charged, a part of the accumulated charge is transferred into
a sampling capacitor (CS). The number of charges
transferred to the sampling capacitor depends on the factor
CX/CS. The charge transfer cycle is repeated N times until
the voltage on the sampling capacitor reaches a threshold
(VIH in our case). The number N represents the size of CX.
When there is a touch, the sensor capacitor is increased and
thus the amount of charge transferred from the sensor
capacitor to the sampling capacitor is higher leading to a
decrease in the number of charge transfer cycles.
The charge transfer is performed through analog switches
embedded in the GPIO.
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The measuring circuit is simple. Let’s consider one analog
I/O group composed of four I/Os. One of these I/Os is the
sampling capacitor I/O. It is connected to an external
capacitor (CS) commonly called the sampling capacitor.
There is a single sampling capacitor per analog I/O group.
The sampling capacitor value depends on the channel
sensitivity. The higher the CS, the higher the sensitivity and
the longer the acquisition time.
The three other I/Os are dedicated to channels. Each of
them is connected to a sensor electrode through a serial
resistor (RS). RS is used to improve the ESD robustness of
the application. Within one analog I/O group, only one
channel is acquired at a time. This means that if three
channels are implemented, three consecutive acquisitions
will be required to get the image of the three sensors. For
optimum performance, the sensor capacitance should be as
low as possible. We often consider this capacitor value to be
in the tens of picofarads. A touch leads to an increase in the
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sensor capacitance by a few picofarads, for example 5
picofarads.
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The charge transfer acquisition sequence is composed of 7
steps.
First, the sampling capacitor and the sensor capacitor are
discharged to get a stable starting point by closing the
analog switch S1 and enabling S3. Between each major
step, an intermediate step is inserted to avoid an acquisition
artifact. This step, called dead time, consists of opening all
active analog switches and disabling all active transistors.
Next, the sensor capacitor (CX) is charged to VDD by closing
S2.
After the dead time, a portion of the charge accumulated in
CX is transferred into the sampling capacitor CS by closing
the analog switch S1.
Once the charge is transferred, the voltage on CS (VCS) is
read. If the voltage is lower than VIH, a logical ‘0’ is returned.
If it is greater than VIH, a logical ‘1’ is read. If the returned
logical value is ‘0’, Steps 3 to 7 are repeated. After each
charge transfer loop, a counter is incremented, representing
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the capacitance of the sensor.
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In order for a Touch Sensing GPIO to be controlled by the
TSC:
- A sampling capacitor I/O must be configured in alternate
output open-drain mode. In addition, to avoid artifacts, the
Schmidt trigger hysteresis must be disabled.
- A channel I/O must be configured in alternate output pushpull mode.
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To reduce the CPU load, two acquisition modes are
supported:
• Normal acquisition mode where the acquisition starts
by setting the START bit of the TSC_CR register.
• Synchronized acquisition mode where the acquisition
only starts upon the detection of a falling or rising
edge and a high level on the SYNC input pin. This
mode is useful to limit the effect of noise in some
applications such as an induction cooktop.
In both modes, the end of acquisition and/or max count
error can be managed either by polling or interrupt.
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In addition to managing the charge transfer capacitive
acquisition principle, the TSC peripheral allows the designer
to individually control the analog switch and Schmidt trigger
hysteresis of I/Os belonging to analog I/O groups. This
capability could be useful to implement a different capacitive
sensing acquisition principle or for other purposes such as
an analog multiplexer.
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The TSC peripheral offers two interrupt sources:
- End of acquisition which notifies the CPU when all the
active channels are acquired.
- Max count error which is set when the acquisition fails on
one or several channels. It is useful in preventing an infinite
acquisition which can occur in the event of a hardware
failure.
The touch sensing controller is active in Run, Sleep, Lowpower run and Low-power sleep modes. This means that
charge transfer acquisition can only be performed in these
modes. In all others modes (Stop 1, Stop 2, Standby and
Shutdown), the touch sensing controller is not operational.
In Stop modes, the peripheral is frozen but the registers
content is kept. In Standby and Shutdown modes, the
registers content is lost and the peripheral must be
reinitialized.
This table lists the number of capacitive sensing channels
depending on the STM32L4 device.
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This example details a solution with 6 touchkeys and one
linear touch sensor. It is important to note that a dedicated
voltage regulator is used and that the solution also uses the
active shield for optimum conducted noise robustness. A
COG type capacitor is used for the sampling capacitors as
they offer a good stability over the temperature range and no
memory effect.
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A voltage regulator used to power the touch controller is
strongly recommended. It will minimize the measurement
noise induced by power supply variations.
For optimum sensitivity, the parasitic capacitance to ground
must be minimized. This implies short and thin sensing
tracks. Serial resistors (Rs) and sampling capacitors (Cs)
must be placed as close as possible to the MCU. The
sensing tracks driving sensors which are acquired at the
same time should be grouped together (bank) and kept
separated from others banks. Finally, a bypass capacitor
should be used in case of high impedance drive (i.e. LED
driven through an open drain circuitry) to ensure a low
impedance path.
For optimum conducted noise performance, we recommend
to use an active shield around tracks and sensor pads
combined with spread spectrum.
Finally, conductive paint must be avoided and a stable
mechanical assembly is required to avoid false or spurious
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touch detections.
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As previously indicated, the TSC peripheral is designed to
operate with the touch sensing library. This free C library
supports proximity, touchkey, linear and rotary touch
sensors. It allows combining capacitive sensing functions
with traditional MCU features such as LCD drive,
communication with a host device, … This library offers all
the processing required to get optimum sensitivity and to
design a robust application. Some of the features include
power-on calibration, environment control system (ECS),
debounce filtering and a detection exclusion system (DxS).
This library offers a simple API to configure the channels,
sensors and to get the state of the sensors. It is MISRA
compliant and it supports all STM32 C compilers.
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The STM32Cube touch sensing library is composed of
several modules. The library relies on the corresponding
STM32 series HAL and it is configured through a dedicated
configuration file. Once included into your project, the
STM32Cube touch sensing library is part of the overall
application and each C function can be launched to get the
appropriate behavior.
For further details on the STM32Cube touch sensing library,
please refer to the corresponding user manual.
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This is a list of peripherals related to the STM32 Touch
Sensing Controller. Please refer to these peripheral trainings
for more information if needed.
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For more details, please refer to application notes AN4299,
AN4310, AN4312 and AN4316.
Thank you.
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