AN2970
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Sensing electrode
Application note
Principles of capacitive touch and proximity sensing technology
The objective of this document is to present the principles of capacitive sensing and charge
transfer used in STMicroelectronics STM8T and STM8TS capacitive sensors. All devices
can be configured in touch or proximity sensing.
1 Capacitive sensing overview
1.1 Sensing electrode capacitance
A capacitance exists between any reference point relative to ground as long as they are
electrically isolated. If this reference point is a sensing electrode, it helps to think of it as a
capacitor. The positive electrode of the capacitor is the sensing electrode, and the negative
electrode is formed by the surrounding area (virtual ground reference, labelled 1 in
Figure 1).
When a conductive object is brought into proximity of the sensing electrode, the coupling
between the object and the electrode increases, together with the capacitance of the
sensing electrode relative to ground. For example, a human hand will increase the sensing
electrode capacitance as it approaches it. Touching the dielectric panel that protects the
electrode increases its capacitance significantly.
The sensing electrode can be made of any electrically conductive material, such as copper
on PCBs, or transparent conductive material like Indium Tin Oxide (ITO) deposited on glass
or Plexiglas.
Figure 1. Coupling with hand increases the capacitance of the sensing electrode
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Capacitive sensing overview AN2970
1.2 Principles of charge transfer
The sensing electrode is connected to the CX pin of the STM8T or STM8TS device. The
equivalent capacitance of the sensing electrode is referred to as C
C
is fully charged with a stable reference voltage VDD. The charge on CX is transferred to a
X
reference capacitor (C
C
. The process is repeated until the voltage on CS reaches a threshold (approximately
X
20% of V
). This threshold is referred to as V
DD
to reach the threshold represents the size of C
). CS capacitance is typically from 1000 to 100,000 times bigger than
S
. The number of transfer cycles required
TRIP
. Refer to Figure 3 and Ta bl e 1 for a
X
representation of the charge-transfer equivalent hardware and charge-transfer sequence for
a given channel.
Figure 2. Depiction of channel charge-transfer hardware
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Table 1. Charge transfer sequence
(1)
Step Switch S3 Switch S2 Switch S1 Description
11 0 1 C
2 0 0 0 Deadtime
BJ
discharge
S
3 0 1 0 Charge cycle (CX charge)
4 0 0 0 Deadtime
5 0 0 1 Transfer cycle (charge transferred to C
6 0 0 0 Deadtime
71 0 1 C
1. Step 2 to 7 are repeated until the voltage across C
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reaches V
S
TRIP
threshold.
discharge
X
)
S
AN2970 Capacitive sensing overview
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Figure 3 and Figure 4 show the evolution of the CX voltage during one and five charge
transfer sequences, respectively. The transfer cycle refers to the charging of C
transfer of the charge to the C
C
to V
S
using a sequence of transfer cycles. The charge cycle duration refers to the time
TRIP
needed to complete one C
capacitor. The charge cycle refers to process of charging
S
charge cycle when no proximity or touch (thus the longest
S
and the
X
duration of a charge cycle with the current system parameters). The charge cycles can be
probed from the CS pin. It is graphically illustrated in Figure 5. Refer to Section 1.4: C
capacitor for details on how to select C
capacitors.
S
S
In addition, the devices can compensate to environmental changes by tracking the average
capacitance of the sensing electrode. This average value is compared to the latest charge
cycle to determine whether a proximity or touch occurred.
Figure 3. Voltage across C
Figure 4. Voltage across C
over a full charge transfer sequence
X
over the first 5 charge transfers
X
Figure 5. Charge cycle
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Capacitive sensing overview AN2970
1.3 Transfer rate
The transfer cycles can be performed at a default rate depending on the internal oscillator
frequency. According to the device, this frequency can be modified either through OTP
option bytes or by the application software. It is recommended to modify the oscillator
frequency for advanced designs only.
The oscillator is used to determine the rate at which the charge transfers is performed. A
maximum efficiency is achieved when enough time is allowed to fully charge C
completely transfer this charge to the C
capacitor.
S
A transfer rate between 100 kHz and 150 kHz is a good choice in normal operating
conditions. The default transfer rate is in this range.
to VDD and
X
A serial resistor R
connected to the CX pin negatively influences the transfer cycle. This
X
resistor improves the conductive object electrical isolation from the sensing electrode and
provides additional ESD protection for the device. Typical R
value ranges from 1 to 2kΩ.
X
Figure 4 and Figure 6 show ideal charge cycles probed from the CX pin. In Figure 4, it can
be noted that the sensing electrode charges up to V
V
is reached.
TRIP
, and that the charge cycle halts when
DD
Figure 6. Ideal charge transfers
Figure 7 and Figure 8 shows non-ideal charge transfers and the resulting charge cycle.
It can be noted that the sensing electrode is charged up to 2.12 V instead of V
DD
comparing Figure 8 to Figure 6, it can be noted that the offset is due to the fact that a
fraction of the sensing electrode charge is not transferred to the C
corrected by decreasing either the transfer rate (oscillator frequency) or R
capacitor. This can be
S
.
X
Note: Attaching a probe to the sensing electrode increases the sensing electrode capacitance by
a few picoFarads, depending on the probe. This has an instant negative influence on the
sensitivity of the system. After a short period of time, the system automatically adjusts to
compensate this change.
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. By
AN2970 Capacitive sensing overview
Figure 7. Non-ideal charge transfers
Figure 8. Charge cycle resulting from non-ideal charge transfers
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Capacitive sensing overview AN2970
1.4 CS capacitor
The function of the CS capacitor is to collect the charge from the sensing electrode. It also
influences the sensitivity of the system. A bigger C
capacitance is computed with a higher resolution. This will increase the sensitivity of the
device.
When the STM8T and STM8TS device is powered from an AC supply voltage, the charge
cycle is always synchronized with the positive zero crossings (ZC) of the AC voltage. The
charge cycle starts after the zero crossing when the device is ready. The ZC feature is not
available on all devices.
When the device is powered from a DC supply voltage, most devices generate an internal
50 Hz sampling frequency (f
SAMPLING
) and synchronize the charge cycles with this
frequency (see Section 1.3: Transfer rate).
capacitor will ensure that the CX
S
The minimum and maximum charge cycle duration (t
SAMPLING
) is determined by the
following:
● Minimum charge cycle duration: every charge cycle must consist of at least 32 charge
transfers.
● Maximum charge cycle duration: no more than 214 charge transfers are allowed in a
charge cycle.
A larger sensing electrode surface will require a larger C
the C
capacitance is normally unknown, it is easier to design the CS capacitor using a trial-
X
and-error method. The C
capacitor typically ranges from 10 nF to 1 µF.
S
capacitor and vice versa. Since
S
It can be noted that the bigger the sensing electrode, the more likely it is for noise to couple
into the system. This could influence the sensitivity setting chosen for the device.
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AN2970 Revision history
2 Revision history
Table 2. Document revision history
Date Revision Changes
04-May-2009 1 Initial release.
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AN2970
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