Features
•Number of Keys:
– Comms Mode: 1 to 12 keys (1 to 9 if wheel or slider option enabled)
– Standalone Mode: 1 to 5 keys
•Technology:
– Patented spread-spectrum QTouchADC charge-transfer
•Number of Output Lines:
– Comms Mode: Up to 10 channels can be configured as outputs (but they will
replace the keys)
– Standalone Mode: 1 to 5 channels can be configured as outputs
•Key Outline Sizes:
– 5 mm x 5 mm or larger (panel thickness dependent)
•Key Spacings:
– 6 mm or wider, center to center (panel thickness, human factors dependent)
•Key Design:
– Single solid or ring shaped electrodes; widely different sizes and shapes possible
•Proximity Electrode Design:
– Single solid electrodes; Key Design, Loop, PCB Trace - different sizes and shapes
possible
•Wheel Size:
– Typically 30 mm – 50 mm diameter
•Wheel Electrode Design:
– Spatially interpolated wheel up to 80 mm diameter
– Typical width of segments 12 mm
•Slider Electrode Design:
– Spatially interpolated, resistorless design
– Typical length 50 mm – 100 mm, typical width 12 mm
– Can be an arc or other irregular shape
•Substrates:
– FR-4, low cost CEM-1 or FR-2 PCB materials; polyamide FPCB; PET films, glass
•Adjacent Metal:
– Compatible with grounded metal immediately next to keys
•Layers Required:
– One; electrodes and components can be on same side
•Electrode Materials:
– Etched copper, silver, carbon, indium tin oxide (ITO), PEDOT
•Panel Materials:
– Plastic, glass, composites, painted surfaces (nonconductive paints)
•Key Panel Thickness:
– Up to 15 mm glass (key size dependent)
– Up to 10 mm plastic (key size dependent)
•Wheel/slider Panel Thickness:
– Up to 4 mm glass
– Up to 3 mm plastic
•Key Sensitivity:
– Comms Mode – individually settable via simple commands over I2C interface
– Standalone mode – settings are fixed
•Interface:
–I2C slave mode (400 kHz)
– CHANGE status indication pin
•Signal Processing:
– Self-calibration, Auto drift compensation, Noise filtering, Adjacent Key
Suppression® (AKS®)
•Power:
–1.8V to 5.5V
•Packages:
– 20-pin SOIC/TSSOP RoHS compliant IC
– 20-pad VQFN RoHS compliant IC
QTouch
12-channel
Touch Sensor
IC
AT42QT2120
9634E–AT42–06/12
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AT42QT2120
1. Pinouts and Schematics
1.1 Pinouts
1.1.1 20-pin SOIC/TSSOP – Comms Mode
1.1.2 20-pin SOIC/TSSOP – Standalone Mode
KEY8/GPO6
KEY7/GPO5
KEY6/GPO4
KEY5/GPO3
KEY4/GPO2
KEY3/GPO1
KEY2/GPO0
KEY1
KEY0
VSS
RESET
CHANGE
SDA
N/C
KEY11/GPO9
KEY9/GPO7
KEY10/GPO8
VDD
MODE
SCL
1
2
3
4
5
6
7
8
9
10 11
12
13
14
15
16
17
18
19
20
QT2120
OUT3
OUT2
KEY6
KEY5
KEY4
KEY3
KEY2
GUARD
PROX
VSS
RESET
N/C
PXOUT
OUT6
OUT4
OUT5
VDD
MODE
N/C
N/C
1
2
3
4
5
6
7
8
9
10 11
12
13
14
15
16
17
18
19
20
QT2120
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AT42QT2120
1.1.3 20-pin VQFN – Comms Mode
1.1.4 20-pin VQFN – Standalone Mode
VSS
RESET
CHANGE
SDA
SCL
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
QT2120
KEY8/GPO6
KEY7/GPO5
KEY6/GPO4
KEY5/GPO3
KEY4/GPO2
KEY3/GPO1
KEY2/GPO0
KEY1
KEY0
N/C
KEY11/GPO9
KEY9/GPO7
KEY10/GPO8
VDD
MODE
VSS
RESET
N/C
N/C
N/C
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
QT2120
OUT3
OUT2
KEY6
KEY5
KEY4
KEY3
KEY2
GUARD
PROX
PXOUT
OUT6
OUT4
OUT5
VDD
MODE
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AT42QT2120
1.2 Pin Descriptions
1.2.1 20-pin SOIC/TSSOP
Table 1-1. Pin Listings (20-pin SOIC/TSSOP)
Pin
Name
(Comms)
Name
(Standalone) Type Description If Unused...
1KEY8/
GPO6 OUT3 I/O Comms mode: Key 8 / General-purpose output
Standalone mode: push-pull output for key 3 Leave open
2KEY7/
GPO5 OUT2 I/O Comms mode: Key 7 / General-purpose output
Standalone mode: push-pull output for key 2 Leave open
3KEY6/
GPO4 KEY6 I/O Comms mode: Key 6 / General-purpose output
Standalone mode: Key 6 Leave open
4KEY5/
GPO3 KEY5 I/O Comms mode: Key 5 / General-purpose output
Standalone mode: Key 5 Leave open
5KEY4/
GPO2 KEY4 I/O Comms mode: Key 4 / General-purpose output
Standalone mode: Key 4 Leave open
6KEY3/
GPO1 KEY3 I/O Comms mode: Key 3 / General-purpose output
Standalone mode: Key 3 Leave open
7KEY2/
GPO0 KEY2 I/O Comms mode: Key/slider/wheel 2 / General-purpose output
Standalone mode: Key 2 Leave open
8 KEY1 GUARD I/O Comms mode: Key/slider/wheel 1
Standalone mode: Guard channel Leave open
9 KEY0 PROX I/O Comms mode: Key/slider/wheel 0
Standalone
mode: Proximity channel Leave open
10 VSS VSS P Ground –
11 VDD VDD P Power –
12 MODE MODE I
Mode selection pin
Comms mode: connect to Vss
Standalone mode: connect to Vdd
–
13 SDA N/C OD Comms mode: Serial Interface Data
Standalone
mode: Unused Pull up to Vdd
14 RESET RESET I Active low reset; has internal pull-up 60 k resistor Tie to Vdd
15 N/C PXOUT O Comms mode: no connection
Standalone mode: open drain output for proximity channel Leave open
16 SCL N/C OD Comms mode: Serial Interface Clock
Standalone
mode: Unused Pull up to Vdd
17 CHANGE N/C OD
Comms mode: Active low s
tate change interrupt (external
pull-up resistor needed)
Standalone mode: Unused
Pull up to Vdd
18 KEY11/
GPO9 OUT6 I/O Comms mode: Key 11 / General-purpose output
Standalone mode: push-pull output for key 6 Leave open
19 KEY10/
GPO8 OUT5 I/O Comms mode: Key 10 / General-purpose output
Standalone mode: push-pull output for key 5 Leave open
20 KEY9/
GPO7 OUT4 I/O Comms mode: Key 9 / General-purpose output
Standalone mode: push-pull output for key 4 Leave open
I Input only I/O Input and output OD Open drain output P Ground or power
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AT42QT2120
1.2.2 20-pin VQFN
Table 1-2. Pin Listings (20-pin VQFN)
Pin
Name
(Comms)
Name
(Standalone) Type Description If Unused...
1KEY6/
GPO4 KEY6 I/O Comms mode: Key 6 / General-purpose output
Standalone mode: Key 6 Leave open
2KEY5/
GPO3 KEY5 I/O Comms mode: Key 5 / General-purpose output
Standalone mode: Key 5 Leave open
3KEY4/
GPO2 KEY4 I/O Comms mode: Key 4 / General-purpose output
Standalone mode: Key 4 Leave open
4KEY3/
GPO1 KEY3 I/O Comms mode: Key 3 / General-purpose output
Standalone mode: Key 3 Leave open
5KEY2/
GPO0 KEY2 I/O Comms mode: Key/slider/wheel 2 / General-purpose output
Standalone mode: Key 2 Leave open
6 KEY1 GUARD I/O Comms mode: Key/slider/wheel 1
Standalone mode: Guard channel Leave open
7 KEY0 PROX I/O Comms mode: Key/slider/wheel 0
Standalone
mode: Proximity channel Leave open
8 VSS VSS P Ground –
9 VDD VDD P Power –
10 MODE MODE I
Mode selection pin
Comms mode: connect to Vss
Standalone mode: connect to Vdd
–
11 SDA N/C OD Comms mode: Serial Interface Data
Standalone
mode: Unused Pull up to Vdd
12 RESET RESET I Active low reset; has internal pull-up 60 k resistor Tie to Vdd
13 N/C PXOUT OD Comms mode: no connection
Standalone mode: open drain output for proximity channel Leave open
14 SCL N/C OD Comms mode: Serial Interface Clock
Standalone
mode: Unused Pull up to Vdd
15 CHANGE N/C OD
Comms mode: Active low s
tate change interrupt (external
pull-up resistor needed)
Standalone mode:
Unused
Pull up to Vdd
16 KEY11/
GPO9 OUT6 I/O Comms mode: Key 11 / General-purpose output
Standalone mode: push-pull output for key 6 Leave open
17 KEY10/
GPO8 OUT5 I/O Comms mode: Key 10 / General-purpose output
Standalone mode: push-pull output for key 5 Leave open
18 KEY9/
GPO7 OUT4 I/O Comms mode: Key 9 / General-purpose output
Standalone mode: push-pull output for key 4 Leave open
19 KEY8/
GPO6 OUT3 I/O Comms mode: Key 8 / General-purpose output
Standalone mode: push-pull output for key 3 Leave open
20 KEY7/
GPO5 OUT2 I/O Comms mode: Key 7 / General-purpose output
Standalone mode: push-pull output for key 2 Leave open
I Input only I/O Input and output OD Open drain output P Ground or power O Output only
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AT42QT2120
1.3 Schematics
Figure 1-1. 20-pin SOIC/TSSOP – Comms Mode
Figure 1-2. 20-pin SOIC/TSSOP – Standalone Mode
VSS
VDD
R12
VSS
VDD
SCL
CHANGE
R11
R10
R9
R8
R7
R6
R5
R4
R3
R2
R1
R0
KEY8/GPO6 1
KEY7GPO5 2
KEY6/GPO4 3
KEY5/GPO3 4
KEY4/GPO2 5
KEY3/GPO1 6
KEY2/GPO0
7
KEY1 8
KEY0 9
VSS
10 VDD 11
MODE
12
SDA
13
RESET
14
N/C
15
SCL
16
CHANGE
17
KEY11/GPO9 18
KEY10/GPO8 19
KEY9/GPO7 20
SDA
R13
R14
R15
VDD
KEY 11
KEY 9
KEY 7
KEY 5
KEY 3
KEY 1
KEY 10
KEY 8
KEY 6
KEY 4
KEY 2
KEY 0
May be used for
wheel or slider
VSS
VDD
R12
VDD
R6
R5
R4
R3
R2
R1
R0
OUT3 1
OUT2 2
KEY6 3
KEY5 4
KEY4 5
KEY3 6
KEY2 7
GUARD 8
PROX 9
VSS
1VDD
MODE
12
N/C
13
RESET
14
PXOUT
15
N/C
16
N/C
17
OUT6 18
OUT5 19
OUT4 20
R13
D
KEY 5
KEY 3
GUARD KEY
KEY 6
KEY 4
KEY 2
PROXIMITY
VDD
SENSOR
011
VSS
1
OUTPUTS
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AT42QT2120
Figure 1-3. 20-pin VQFN – Comms Mode
Figure 1-4. 20-pin VQFN – Standalone Mode
VSS
VDD
R12
VSS
VDD
SCL
CHANGE
VSS
8VDD 9
MODE
10
SDA
11
RESET
12
N/C
13
SCL
14
CHANGE
15
SDA
R13 R14 R15
VDD
R11
R10
R9
R8
R7
R6
R5
R4
R3
R2
R1
R0
KEY8/GPO6 19
KEY7/GPO5 20
KEY6/GPO4 1
KEY5/GPO3 2
KEY4/GPO2 3
KEY3/GPO1 4
KEY2/GPO0 5
KEY1 6
KEY0 7
KEY11/GPO9 16
KEY10/GPO8 17
KEY9/GPO7 18 KEY 11
KEY 9
KEY 7
KEY 5
KEY 3
KEY 1
KEY 10
KEY 8
KEY 6
KEY 4
KEY 2
KEY 0
May be used for
wheel or slider
VSS
VDD
R6
R5
R4
R3
R2
R1
R0
OUT3 19
OUT2 20
KEY6 1
KEY5 2
KEY4 3
KEY3 4
KEY2 5
GUARD 6
PROX 7
VSS
8VDD 9
OUT6 16
OUT5 17
OUT4 18
KEY 5
KEY 3
GUARD KEY
KEY 6
KEY 4
KEY 2
PROXIMITY
SENSOR
R12
VDD
MODE
10
N/C
14
RESET
12
PXOUT
13
N/C
15
N/C
11
R13
VDD
VSS
D
1
OUTPUTS
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9634E–AT42–06/12
AT42QT2120
2. Overview
2.1 Introduction
The AT42QT2120 (QT2120) is a QTouchADC sensor driver. The device can sense from one to
12 keys, dependent on mode. Three of the keys can be used as sense channels for a slider or
wheel, leaving a maximum of nine standard touch keys. The device also supports the use of
proximity sensors and a guard channel.
The QT2120 includes all signal processing functions necessary to provide stable sensing under
a wide variety of changing conditions, and the outputs are fully debounced. Only a few external
parts are required for operation and no external Cs capacitors are required.
The QT2120 modulates its bursts in a spread-spectrum fashion in order to heavily suppress the
effects of external noise, and to suppress RF emissions. The QT2120 uses a QTouchADC
method of acquisition. This provides greater noise immunity and eliminates the need for external
sampling capacitors, allowing touch sensing using a single pin.
The QT2120 can operate in two ways; comms and standalone.
2.2 Modes
2.2.1 Comms Mode
The QT2120 can operate in comms mode where a host can communicate with the device via an
I2C bus. This allows the user to configure settings such as Threshold, Adjacent Key Suppression
(AKS), Detect Integrator, Low Power (LP) Mode, Guard Channel and Max Time On for keys.
2.2.2 Standalone Mode
The QT2120 can operate in a standalone mode where an I2C-compatible interface is not
required. To enter standalone mode, connect the Mode pin to Vdd before powering up the
QT2120.
In standalone mode, the start-up values are hard coded in firmware and cannot be changed.
The default start-up values are used. This means that key detection is reported via its respective
input/output.
The Guard channel feature is automatically implemented on key 1 in standalone mode. This
means that this channel has a higher sensitivity and is used to protect against false triggering,
perhaps by a hand covering all keys.
A proximity sensor is also available on channel (key) 0 in standalone mode.
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AT42QT2120
2.3 Keys
Dependent on mode, the QT2120 can have a minimum of one key and a maximum of 12 keys.
These can be constructed in different shapes and sizes. See “Features” on page 1 for the
recommended dimensions.
The possible combinations of keys are:
Comms mode:
• 1 to 12 keys
or
• 1 to 9 keys plus 1 slider/wheel
• Key channels 2 to 11 can be reassigned as general outputs, if required
Note: Any number of keys can be configured as proximity channels.
Standalone mode:
• 1 to 5 keys plus corresponding discrete outputs
• 1 Guard Channel
• 1 Proximity sensor
Unused keys should be disabled by setting bit 0 of their control bytes to 1 (see Section 5.17 on
page 26).
The Key Status register (see Section 5.5 on page 21) can be read to determine the touch status
of the corresponding key. It is recommended using the open-drain CHANGE line to detect when
a change of status has occurred.
2.4 Output Lines
In comms mode some pins, normally used for touch keys, can be used as output pins. If the Key
Control bit 0 (EN) is set to 1, the pin can be used as an output. The state of the pin is then
controlled by Key Control bit 1 (GPO).
In Standalone mode pins OUT2 to OUT6 are driven by KEY2 to KEY6 respectively. The OUT
pins drive high during touch and can be used to drive, for example, LEDs.
2.5 Acquisition/Low Power Mode (LP)
There are 255 different acquisition times possible. These are controlled via the LP Mode byte
(see Section 5.9 on page 22) which can be written to via I2C-compatible communication.
LP mode controls the intervals between acquisition measurements. Longer intervals consume
lower power but have an increased response time. During calibration, touch and during the
detect integrator (DI) period, the LP mode is temporarily set to LP mode 1 for a faster response.
The QT2120 operation is based on a fixed cycle time of approximately 16 ms. The LP mode
setting indicates how many of these periods exist per measurement cycle. For example, If LP
mode = 1, there is an acquisition every cycle (16 ms). If LP mode = 3, there is an acquisition
every 3 cycles (48 ms). If a high PULSE setting is selected then the acquisition time may exceed
16 ms.
An LP setting of 0 will send the device into Power-down mode. To wake the device from this
mode a nonzero LP setting should be written to the LP address at location 8.
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AT42QT2120
2.6 Adjacent Key Suppression (AKS) Technology
The device includes the Atmel patented Adjacent Key Suppression (AKS) technology, to allow
the use of tightly spaced keys on a keypad with no loss of selectability by the user.
There can be up to three AKS groups, implemented so that only one key in the group may be
reported as being touched at any one time. Once a key in a particular AKS group is in detect no
other key in that group can go into detect. Only when the key in detect goes out of detection can
another key go into detect state.
Keys which are members of the AKS groups can be set in the Key Control register (see
Section 5.17 on page 26). Keys outside the group may be in detect simultaneously.
Note: To use a key as a guard channel, its AKS group should be set to be the same as that of
the keys it is to protect.
2.7 CHANGE Line (Comms Mode Only)
The CHANGE line is active low and signals when there is a change of state in the Detection
Status and/or Key Status bytes. It is cleared (allowed to float high) when the host reads the
status bytes.
If the status bytes change back to their original state before the host has read the status bytes
(for example, a touch followed by a release), the CHANGE line will be held low. In this case, a
read to any memory location will clear the CHANGE line.
The CHANGE line is open-drain and should be connected via a 47 k resistor to Vdd. It is
necessary for minimum power operation as it ensures that the QT2120 can sleep for as long as
possible. Communications wake up the QT2120 from sleep causing a higher power
consumption if the part is randomly polled.
Note that the CHANGE line is pulled low 85 ms after power-up or reset. The CHANGE line is
pulled low approximately for another 16 ms before any bursting on the touch pins will occur. If
any of the pins are required to be outputs then the relevant Key Control settings should be
written within this 16 ms time to prevent bursting on pins required as outputs. Also note that the
CHANGE line is cleared during a read of the Detection Status bytes when all bytes differing from
the previous read have been read.
2.8 Types of Reset
2.8.1 External Reset
An external reset logic line can be used if desired, fed into the RESET pin. However, under most
conditions it is acceptable to tie RESET to Vdd. The minimum reset pulse width is 2 µs.
2.8.2 Soft Reset
The host can cause a device reset by writing a nonzero value to the Reset byte. This soft reset
triggers the internal watchdog timer on a 125 ms interval. After 125 ms the device executes a full
reset.
The device NACKs any attempts to communicate with it for approximately 200 ms after the soft
reset command. Communication can begin as soon the CHANGE line is first asserted.
Note: The device can process a Soft Reset command while in Power Down (LPM = 0) mode,
causing a chip reset.
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AT42QT2120
2.9 Calibration
Writing a nonzero value to the calibration byte can force a recalibration at any time. This can be
useful to clear out a stuck key condition after a prolonged period of uninterrupted detection. A
calibration command executes 15 burst cycles at LPM 1 and sets the CALIBRATE bit of the
Detection Status register during the calibration sequence.
Note: A calibration command should be sent whenever Key Control bit 0 (EN) is changed.
This changes the use of the key from a standard touch key to an output pin and
vice-versa.
2.10 Guard Channel
A guard channel to help prevent false detection is available in both modes. This is fixed on key 1
for standalone mode and programmable for comms mode by setting Key Control bit 4 (GUARD)
(see Section 5.17 on page 26).
Guard channel keys should be more sensitive than the other keys and physically bigger.
Because the guard channel key is physically bigger it becomes more susceptible to noise so it
should have a higher Oversampling (see Section 5.18 on page 27) than the other keys. In
standalone mode it is assigned to key 1 and cannot be changed.
In comms mode any key can be selected to be a guard key by setting Key Control bit 4
(GUARD).
The guard channel is connected to a sensor pad which detects the presence of touch. Because
of its larger size and sensitivity it goes into touch before the keys it surrounds (if, for example, a
hand covers all the keys).
Figure 2-1. Guard Channel Example
2.11 Signal Processing
2.11.1 Detect Threshold
The device detects a touch when the signal has crossed a threshold level and remained there
for a specified number of counts (see Section 5.11 on page 24). This can be altered on a
key-by-key basis using the key Detect Threshold I2C-compatible commands.
This detect threshold is based on the reference value of the particular key. The delta of the key
is obtained by subtracting the reference value from the signal value (the signal value rises when
touch is present).
Guard channel
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AT42QT2120
In standalone mode the detect threshold is set to a fixed value of 10 counts of change with
respect to the internal reference level for the guard channel and 10 counts for the other six keys
(including proximity channel). The reference level has the ability to adjust itself slowly in
accordance with the drift compensation mechanism.
The drift mechanism will drift toward touch at a rate of 160 ms x 20 = 3.2 seconds (Towards
Touch Drift (TTD) register) and away from touch at a rate of 160 ms x 5 = 0.8 seconds (Away
from Touch Drift (ATD) register). These values are fixed in standalone mode but can be
configured in comms mode see Section 5.10 on page 22.
2.11.2 Detect Integrator
The device features a fast detection integrator counter (DI filter), which acts to filter out noise at
the small expense of a slower response time. The DI filter requires a programmable number of
consecutive samples confirmed in detection before the key is declared to be touched. The
minimum number for the DI filter is 1. A setting of 0 for the DI also defaults to 1. The DI has a
maximum usable value of 32. Values above this will prevent a key from entering touch.
The DI is also implemented when a touch is removed.
2.11.3 Cx Limitations
The recommended range for key capacitance Cx is 1 pF – 30 pF. Larger values of Cx will give
reduced sensitivity.
2.11.4 Touch Recalibration Delay
If an object or material obstructs the sense pad the signal may rise enough to create a detection,
preventing further operation. To prevent this, the sensor includes a timer which monitors
detections. If a detection exceeds the timer setting the sensor performs a key recalibration. This
is known as the Touch Recalibration Delay (TRD) and is set to approximately 30 s in standalone
mode.
In comms mode this feature can be changed by setting a value in the range 1 – 255 (160 ms –
40,800 ms) in steps of 160 ms. A setting of 0 disables the TRD.
TRD is a global setting and applies to all keys.
2.11.5 Away from Touch Recalibration
If a keys signal jumps in the negative direction (with respect to its reference) by more than the
Away from Touch Recalibration setting (25% of detect threshold), then a recalibration of that key
takes place.
Note: The minimum Away from Touch Recalibration is hard limited to 4 counts.
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AT42QT2120
2.11.6 Drift Hold Time
Drift Hold Time (DHT) is used to restrict drift on all keys while one or more keys are activated.
DHT restricts the drifting on all keys until approximately four seconds after all touches have been
removed.
This feature is particularly useful in cases of high-density keypads where touching a key or
hovering a finger over the keypad would cause untouched keys to drift, and therefore create a
sensitivity shift, and ultimately inhibit touch detection. In Comms mode this value is settable, see
Section 5.13 on page 24.
The QT2120 will remain in fast mode (LP = 1) for the duration of the DHT counter. The total DHT
time is 160 ms × DHT value. The default setting for DHT is 25, so 160 ms × 25 = ~4 seconds.
The QT2120 will not drift or re-enter slow LP mode during this time.
2.11.7 Hysteresis
Hysteresis is fixed at 12.5% of the Detect Threshold. When a key enters a detect state once the
DI count has been reached, the Detect threshold (DTHR) value is changed by a small amount
(12.5% of DTHR) in the direction away from touch. This is done to effect hysteresis and so
makes it less likely a key will dither in and out of detect. DTHR is restored once the key drops out
of detect.
Note: The minimum value for hysteresis is 2 counts.
3. Wiring and Parts
3.1 Rs Resistors
Series resistors Rs (Rs0 – Rs11 for comms mode and Rs0 – Rs6 for standalone mode) are
in line with the electrode connections and should be used to limit electrostatic discharge (ESD)
currents and to suppress radio frequency interference (RFI). Series resistors are recommended
for noise reduction. They should be approximately 4.7 k to 20 k each. For maximum noise
rejection the value may be up to 100 k. Care should be taken in this case that the sensor keys
are fully charged. The Charge Time may need to be increased (see Section 5.15 on page 26).
Each count increase will extend the charge pulse by approximately 1 µs.
3.2 LED Traces and Other Switching Signals
Digital switching signals near the sense lines induce transients into the acquired signals,
deteriorating the signal-to-noise (SNR) performance of the device. Such signals should be
routed away from the sensing traces and electrodes, or the design should be such that these
lines are not switched during the course of signal acquisition (bursts).
LED terminals which are multiplexed or switched into a floating state, and which are within, or
physically very near, a key (even if on another nearby PCB) should be bypassed to either Vss or
Vdd with at least a 10 nF capacitor. This is to suppress capacitive coupling effects which can
induce false signal shifts. The bypass capacitor does not need to be next to the LED, in fact it
can be quite distant. The bypass capacitor is noncritical and can be of any type.
LED terminals which are constantly connected to Vss or Vdd do not need further bypassing.
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3.3 PCB Cleanliness
Modern no-clean flux is generally compatible with capacitive sensing circuits.
If a PCB is reworked in any way, clean it thoroughly to remove all traces of the flux residue
around the capacitive sensor components. Dry it thoroughly before any further testing is
conducted.
3.4 Power Supply
See Section6.2 on page30 for the power supply range. If the power supply fluctuates slowly
with temperature, the device tracks and compensates for these changes automatically with only
minor changes in sensitivity. If the supply voltage drifts or shifts quickly, the drift compensation
mechanism is not able to keep up, causing sensitivity anomalies or false detections.
The power should be clean and come from a separate regulator if possible. However, this device
is designed to minimize the effects of unstable power, and except in extreme conditions should
not require a separate Low Dropout (LDO) regulator.
It is assumed that a larger bypass capacitor (like 1 µF) is somewhere else in the power circuit;
for example, near the regulator.
CAUTION: If a PCB is reworked to correct soldering faults relating to the device, or
to any associated traces or components, be sure that you fully understand the nature
of the flux used during the rework process. Leakage currents from hygroscopic ionic
residues can stop capacitive sensors from functioning. If you have any doubts, a
thorough cleaning after rework may be the only safe option.
!
CAUTION: A regulator IC shared with other logic can result in erratic operation and is
not advised.
A single ceramic 0.1 µF bypass capacitor, with short traces, should be placed very
close to the power pins of the IC. Failure to do so can result in device oscillation,
high current consumption and erratic operation.
!
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4. I2C-compatible Communications (Comms Mode Only)
4.1 I2C-compatible Protocol
4.1.1 Protocol
The I2C-compatible protocol is based around access to an address table (see Table5-1 on
page18) and supports multibyte reads and writes. The maximum clock rate is 400 kHz.
See Section A on page 41 for an overview of I2C bus operation.
4.1.2 Signals
The I2C-compatible interface requires two signals to operate:
•SDA – Serial Data
•SCL – Serial Clock
A third line, CHANGE, is used to signal when the device has seen a change in the status byte:
•CHANGE: Open-drain, active low when the device status has changed since the last I2C
read. After reading the four status bytes (1) (or all the status bytes which have changed since
the previous read), this pin floats (high) again if it is pulled up with an external resistor. If the
status bytes change back to their original state before the host has read the status bytes (for
example, a touch followed by a release), the CHANGE line is held low. In this case, a read to
any memory location clears the CHANGE line.
4.2 I2C-compatible Address
There is one preset I2C-compatible address of 0x1C (28). This is not changeable.
4.3 Data Read/Write
4.3.1 Address Pointer
The internal address pointer is initialized to address 0.
4.3.2 Writing Data to the Device
The sequence of events required to write data to the device is shown next.
1. Detection Status Byte, Key Status Byte[0], Key Status Byte[1], Slider Position
Table 4-1. Description of Write Data Bits
Key Description
S START condition
SLA+W Slave address plus write bit
A Acknowledge bit
MemAddress Target memory address within device
Data Data to be written
P Stop condition
SLA+W MemAddress
AASData A P
Host to Device Device Tx to Host
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1. The host initiates the transfer by sending the START condition
2. The host follows this by sending the slave address of the device together with the
WRITE bit.
3. The device sends an ACK.
4. The host then sends the memory address within the device it wishes to write to.
5. The device sends an ACK.
6. The host transmits one or more data bytes; each is acknowledged by the device
(unless trying to write to an invalid address). Valid write address are 5 – 51.
7. If the host sends more than one data byte, they are written to consecutive memory
addresses.
8. The device automatically increments the target memory address after writing each data
byte.
9. After writing the last data byte, the host should send the STOP condition.
Note: the host should not try to write to addresses outside the range 0x06 to 0x33 (6 – 51)
because this is the limit of the device’s internal memory address.
4.3.3 Reading Data From the Device
The sequence of events required to read data from the device is shown next.
1. The host initiates the transfer by sending the START condition
2. The host follows this by sending the slave address of the device together with the
WRITE bit.
3. The device sends an ACK.
4. The host then sends the memory address within the device it wishes to read from.
5. The device sends an ACK if the address to be read from is less than 0x63 otherwise it
sends a NACK).
6. The host must then send a STOP and a START condition followed by the slave
address again but this time accompanied by the READ bit.
Note: Alternatively, instead of step 6, a repeated START can be sent so the host does not
need to relinquish control of the bus.
7. The device returns an ACK, followed by a data byte.
8. The host must return either an ACK or NACK.
a. If the host returns an ACK, the device subsequently transmits the data byte from
the next address. Each time a data byte is transmitted, the device automatically
increments the internal address. The device continues to return data bytes until the
host responds with a NACK.
b. If the host returns a NACK, it should then terminate the transfer by issuing the
STOP condition.
9. The device resets the internal address to the location indicated by the memory address
sent to it previously. Therefore, there is no need to send the memory address again
when reading from the same location.
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Note: Reading the 16-bit reference and signal values is not an automatic operation; reading
the first byte of a 16-bit value does not lock the other byte. As a result glitches in the
reported value may be seen as values increase from 255 to 256, or decrease from 256
to 255. This device also supports the use of a repeated START condition as an
alternative to the Stop condition.
4.4 SDA, SCL
The I2C-compatible bus transmits data and clock with SDA and SCL respectively. They are
open-drain; that is I2C-compatible master and slave devices can only drive these lines low or
leave them open. The termination resistors pull the line up to Vdd if no I2C-compatible device is
pulling it down.
The termination resistors commonly range from 1 k to 10 k and should be chosen so that the
rise times on SDA and SCL meet the I2C-compatible specifications (300 ns maximum for
400 kHz operation).
4.5 Standalone Mode
If I2C-compatible communications are not required, then standalone mode can be enabled by
connecting the MODE pin to Vdd. See Section 2.4 on page 9 for more information.
In Standalone mode (Mode pin connected to Vdd at start-up) the chip is configured to specific
settings:
• Key0 is configured as a proximity channel. If this key goes into detect then PXOUT is
asserted high.
• Key1 is configured as a guard channel and should have a PCB layout which reflects this.
• Keys 2 – 6 are standard QTouchADC keys and have pins OUT 2 – 6 configured to reflect
their respective touch status.
• Keys1 – 6 are configured to have the same AKS group setting.
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5. Setups
5.1 Introduction
The device calibrates and processes signals using a number of algorithms specifically designed
to provide for high survivability in the face of adverse environmental challenges. User-defined
Setups are employed to alter these algorithms to suit each application. These Setups are loaded
into the device over the I2C-compatible serial interfaces. In standalone mode these settings are
fixed to predetermined values.
Table 5-1. Internal Register Address Allocation
Address Use Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 R/W
0 Chip ID Chip Id = 0x3E (62) R
1 Firmware Version Major version Minor version R
2 Detection Status CALIBRATE OVERFLOW – – – – SDET TDET R
3
Key Status
KEY7 KEY6 KEY5 KEY4 KEY3 KEY2 KEY1 KEY0 R
4Reserved KEY11 KEY10 KEY9 KEY8 R
5 Slider Position Slider position R
6 Calibrate Calibrate Command R/W
7 Reset Reset Command R/W
8 LP Low Power (LP) Mode R/W
9TTD 0 Towards Touch Drift compensation R/W
10 ATD 0 Away from Touch Drift compensation R/W
11 DI Detection integrator R/W
12 TRD Touch Recal Delay R/W
13 DHT Drift Hold Time R/W
14 Slider Options EN WHEEL Reserved R/W
15 Charge Time Reserved Charge Time –
16 Key 0 Detect Threshold Detect Threshold level for key 0 R/W
17 Key 1 Detect Threshold Detect Threshold level for key 1 R/W
18 Key 2 Detect Threshold Detect Threshold level for key 2 R/W
19 Key 3 Detect Threshold Detect Threshold level for key 3 R/W
20 Key 4 Detect Threshold Detect Threshold level for key 4 R/W
21 Key 5 Detect Threshold Detect Threshold level for key 5 R/W
22 Key 6 Detect Threshold Detect Threshold level for key 6 R/W
23 Key 7 Detect Threshold Detect Threshold level for key 7 R/W
24 Key 8 Detect Threshold Detect Threshold level for key 8 R/W
25 Key 9 Detect Threshold Detect Threshold level for key 9 R/W
26 Key 10 Detect Threshold Detect Threshold level for key 10 R/W
27 Key 11 Detect Threshold Detect Threshold level for key 11 R/W
28 Key 0 Control Reserved GUARD AKS GPO EN R/W
29 Key 1 Control Reserved GUARD AKS GPO EN R/W
30 Key 2 Control Reserved GUARD AKS GPO EN R/W
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31 Key 3 Control Reserved GUARD AKS GPO EN R/W
32 Key 4 Control Reserved GUARD AKS GPO EN R/W
33 Key 5 Control Reserved GUARD AKS GPO EN R/W
34 Key 6 Control Reserved GUARD AKS GPO EN R/W
35 Key 7 Control Reserved GUARD AKS GPO EN R/W
36 Key 8 Control Reserved GUARD AKS GPO EN R/W
37 Key 9 Control Reserved GUARD AKS GPO EN R/W
38 Key 10 Control Reserved GUARD AKS GPO EN R/W
39 Key 11 Control Reserved GUARD AKS GPO EN R/W
40 Key 0 Pulse Scale PULSE3 PULSE2 PULSE1 PULSE0 SCALE3 SCALE2 SCALE1 SCALE0 R/W
41 Key 1 Pulse Scale PULSE3 PULSE2 PULSE1 PULSE0 SCALE3 SCALE2 SCALE1 SCALE0 R/W
42 Key 2 Pulse Scale PULSE3 PULSE2 PULSE1 PULSE0 SCALE3 SCALE2 SCALE1 SCALE0 R/W
43 Key 3 Pulse Scale PULSE3 PULSE2 PULSE1 PULSE0 SCALE3 SCALE2 SCALE1 SCALE0 R/W
44 Key 4 Pulse Scale PULSE3 PULSE2 PULSE1 PULSE0 SCALE3 SCALE2 SCALE1 SCALE0 R/W
45 Key 5 Pulse Scale PULSE3 PULSE2 PULSE1 PULSE0 SCALE3 SCALE2 SCALE1 SCALE0 R/W
46 Key 6 Pulse Scale PULSE3 PULSE2 PULSE1 PULSE0 SCALE3 SCALE2 SCALE1 SCALE0 R/W
47 Key 7 Pulse Scale PULSE3 PULSE2 PULSE1 PULSE0 SCALE3 SCALE2 SCALE1 SCALE0 R/W
48 Key 8 Pulse Scale PULSE3 PULSE2 PULSE1 PULSE0 SCALE3 SCALE2 SCALE1 SCALE0 R/W
49 Key 9 Pulse Scale PULSE3 PULSE2 PULSE1 PULSE0 SCALE3 SCALE2 SCALE1 SCALE0 R/W
50 Key 10 Pulse Scale PULSE3 PULSE2 PULSE1 PULSE0 SCALE3 SCALE2 SCALE1 SCALE0 R/W
51 Key 11 Pulse Scale PULSE3 PULSE2 PULSE1 PULSE0 SCALE3 SCALE2 SCALE1 SCALE0 R/W
52–53 Key Signal 0 Key signal 0 (MSByte) – Key signal 0 (LSByte) R
54–55 Key Signal 1 Key signal 1 (MSByte) – Key signal 1 (LSByte) R
56–57 Key Signal 2 Key signal 2 (MSByte) – Key signal 2 (LSByte) R
58–59 Key Signal 3 Key signal 3 (MSByte) – Key signal 3 (LSByte) R
60–61 Key Signal 4 Key signal 4 (MSByte) – Key signal 4 (LSByte) R
62–63 Key Signal 5 Key signal 5 (MSByte) – Key signal 5 (LSByte) R
64–65 Key Signal 6 Key signal 6 (MSByte) – Key signal 6 (LSByte) R
66–67 Key Signal 7 Key signal 7 (MSByte) – Key signal 7 (LSByte) R
68–69 Key Signal 8 Key signal 8 (MSByte) – Key signal 8 (LSByte) R
70–71 Key Signal 9 Key signal 9 (MSByte) – Key signal 9 (LSByte) R
72–73 Key Signal 10 Key signal 10 (MSByte) – Key signal 10 (LSByte) R
74–75 Key Signal 11 Key signal 11 (MSByte) – Key signal 11 (LSByte) R
76–77 Reference Data 0 Reference data 0 (MSByte) – Reference data 0 (LSByte) R
78–79 Reference Data 1 Reference data 1 (MSByte) – Reference data 1 (LSByte) R
80–81 Reference Data 2 Reference data 2 (MSByte) – Reference data 2 (LSByte) R
82–83 Reference Data 3 Reference data 3 (MSByte) – Reference data 3 (LSByte) R
84–85 Reference Data 4 Reference data 4 (MSByte) – Reference data 4 (LSByte) R
Table 5-1. Internal Register Address Allocation (Continued)
Address Use Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 R/W
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5.2 Address 0: Chip ID
CHIP ID: Holds the chip ID; always 0x3E.
5.3 Address 1: Firmware Version
MAJOR VERSION: Holds the major firmware version (for example revision 1.5).
MINOR VERSION: Holds the minor firmware version (for example revision 1.5).
5.4 Address 2: Detection Status
CALIBRATE: This bit is set during a calibration sequence.
OVERFLOW: This bit is set if the time to acquire all key signals exceeds 16 ms.
SDET: This bit is set if any of the slider/wheel channels are in detect.
TDET: This bit is set if any of the keys are in detect.
Note: If the slider or wheel is enabled then the SDET bit will be set when it is in detect. Also
the relevant Key Status bit (0 – 2) and TDET will be set. These bits can be ignored if the
SDET bit is set as the slider/wheel takes priority.
A change in these bytes will cause the CHANGE line to trigger.
86–87 Reference Data 5 Reference data 5 (MSByte) – Reference data 5 (LSByte) R
88–89 Reference Data 6 Reference data 6 (MSByte) – Reference data 6 (LSByte) R
90–91 Reference Data 7 Reference data 7 (MSByte) – Reference data 7 (LSByte) R
92–93 Reference Data 8 Reference data 8 (MSByte) – Reference data 8 (LSByte) R
94–95 Reference Data 9 Reference data 9 (MSByte) – Reference data 9 (LSByte) R
96–97 Reference Data 10 Reference data 10 (MSByte) – Reference data 10 (LSByte) R
98–99 Reference Data 11 Reference data 11 (MSByte) – Reference data 11 (LSByte) R
Table 5-2. Chip ID
Address Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
0CHIP ID
Table 5-1. Internal Register Address Allocation (Continued)
Address Use Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 R/W
Table 5-3. Firmware Version
Address Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
1 MAJOR VERSION MINOR VERSION
Table 5-4. Detection Status
AddressBit 7Bit 6Bit 5Bit 4Bit 3Bit 2Bit 1Bit 0
2
CALIBRATE OVERFLO
W
– – – – SDET TDET
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5.5 Addresses 3 – 4: Key Status
KEY0 – KEY11: These bits indicate which keys are in detection, if any. Touched keys report as
1, untouched or disabled keys report as 0. A change in these bytes will cause the CHANGE line
to trigger.
5.6 Address 5: Slider Position
SLIDER POSITION: Reports the slider/wheel position. This value is only valid when the SDET
bit in the Detection Status byte is set. A change in this value will cause the CHANGE line to
assert low.
5.7 Address 6: Calibrate
CALIBRATE COMMAND: Writing any nonzero value into this address triggers the device to
start a calibration cycle. The CALIBRATE flag in the detection status register is set when the
calibration begins and clears when the calibration has finished.
5.8 Address 7: Reset
RESET COMMAND: Writing any nonzero value to this address triggers the device to reset.
Table 5-5. Key Status
Address Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
3 KEY7 KEY6 KEY5 KEY4 KEY3 KEY2 KEY1 KEY0
4Reserved KEY11 KEY10 KEY9 KEY8
Table 5-6. Slider Position
AddressBit 7Bit 6Bit 5Bit 4Bit 3Bit 2Bit 1Bit 0
5 SLIDER POSITION
Table 5-7. Calibrate
Address Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
6 CALIBRATE COMMAND
Table 5-8. Reset
Address Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
7 RESET COMMAND
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5.9 Address 8: Low Power (LP) Mode
LP MODE: This 8-bit value determines the number of 16 ms intervals between key
measurements. Longer intervals between measurements yield a lower power consumption but
at the expense of a slower response to touch.
Default: 1 (16 ms between key acquisitions)
To wake the device from Power-down mode a nonzero LP setting should be written to this
address. The QT2120 can also be reset during power-down mode by writing a nonzero value to
the reset register (address 7).
5.10 Address 9 – 10: Toward Touch and Away from Touch Drift (TTD, ATD)
TOWARD TOUCH DRIFT and AWAY FROM TOUCH DRIFT: Signals can drift because of
changes in Cx and Cs over time and temperature. It is crucial that such drift be compensated for,
else false detections and sensitivity shifts can occur.
Drift compensation (see Figure 5-1) is performed by making the reference level track the raw
signal at a slow rate, but only while there is no detection in effect. The rate of adjustment must
be performed slowly, otherwise legitimate detections could be ignored. The parameters can be
configured in increments of 0.16s.
Table 5-9. LP Mode
Address Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
8LP MODE
Setting Time
0Power Down
116 ms
232 ms
348 ms
464 ms
...254 4.064 s
255 4.08 s
Table 5-10. Toward Touch and Away from Touch Drift
Address Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
90 TOWARD TOUCH DRIFT
10 0 AWAY FROM TOUCH DRIFT
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Figure 5-1. Thresholds and Away From Touch Drift
The device drift compensates using a slew-rate limited change to the reference level; the
threshold and hysteresis values are slaved to this reference.
When a finger is sensed, the signal increases due to capacitance being added to Cx. An
isolated, untouched foreign object (a coin, or a water film) will cause the signal to drop very
slightly due to an enhancement of coupling.
Once a finger is sensed, the drift compensation mechanism ceases since the signal is
legitimately detecting an object. Drift compensation only works when the signal in question has
not crossed the negative threshold level.
The drift compensation mechanism can be asymmetric; the drift-compensation can be made to
occur in one direction faster than it does in the other simply by changing the TTD and ATD Setup
parameters. This is a global configuration.
Specifically, drift compensation should be set to compensate faster for decreasing signals than
for increasing signals. Increasing signals should not be compensated quickly, since an
approaching finger could be compensated for partially or entirely before even touching the
touchpad (Toward Touch Drift (TTD)).
However, an obstruction over the sense pad, for which the sensor has already made full
allowance, could suddenly be removed leaving the sensor with an artificially suppressed
reference level and thus become insensitive to touch. In this latter case, the sensor should
compensate for the object's removal by lowering the reference level relatively quickly (Away
from Touch Drift (ATD)).
Drift compensation and the detection time-outs work together to provide for robust, adaptive
sensing. The time-outs provide abrupt changes in reference calibration depending on the
duration of the signal 'event'.
If ATD or TTD is set to 0 then the drift compensation in the respective direction is disabled.
Note: it is recommended that the drift compensation rate be more than four times the LP mode
period. This is to prevent undersampling, which decreases the algorithm's efficiency.
Default TTD: 20 (3.2 s / reference level)
Default ATD: 5 (0.8 s / reference level)
Threshold
Signal
Hysteresis
Reference
Output
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5.11 Address 11: Detection Integrator (DI)
DI: Allows the DI level to be set for each key. This 8-bit value controls the number of consecutive
measurements that must be confirmed as having passed the key threshold before that key is
registered as being in detect. The minimum value for the DI filter is 1. A settings of 0 for the DI
also defaults to 1.
Default: 4 (maximum = 32)
5.12 Address 12: Touch Recal Delay (TRD)
If an object unintentionally contacts a key resulting in a detection for a prolonged interval it is
usually desirable to recalibrate the key in order to restore its function, perhaps after a time delay
of some seconds.
The Touch Recal Delay timer monitors such detections; if a detection event exceeds the timer's
setting, the key will be automatically recalibrated. After a recalibration has taken place, the
affected key will once again function normally even if it is still being contacted by the foreign
object. This feature is set globally.
TRD can be disabled by setting it to zero (infinite timeout) in which case the key will never
autorecalibrate during a continuous detection (but the host could still command it).
TRD is set globally, which can range in value from 1 – 255. TRD above 0 is expressed in 0.16 s
increments.
Default: 255 (Comms) 255 (Standalone)
5.13 Address 13: Drift Hold Time (DHT)
This is used to restrict drift on all keys while one or more keys are activated. DHT defines the
length of time the drift is halted after a key detection. When DHT = 0, drifting is never
suspended, even during a valid touch of another key.
Table 5-11. Detection Integrator
Address Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
11 DI
Table 5-12. Touch Recal Delay
Address Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
12 TRD
Table 5-13. Drift Hold Time
Address Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
13 DHT
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This feature is particularly useful in cases of high-density keypads where touching a key or
hovering a finger over the keypad would cause untouched keys to drift, and therefore create a
sensitivity shift, and ultimately inhibit any touch detection. It is expressed in 0.16 s increments.
DHT default value: 25
DHT range: 0 – 255
5.14 Address 14: Slider Options
EN: Setting this bit enables a Slider or Wheel to be configured. Only the first three channels (0,
1 and 2) can be used.
WHEEL: Setting this bit allows a wheel to be configured. If not set, and EN is enabled, it defaults
to a slider.
The range of both is from 0 – 255.
Figure 5-2. Slider/Wheel Settings
EN default value: 0
WHEEL default value: 0
Table 5-14. Slider Options
Address Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
14 EN WHEEL Reserved
0
255
1 to 254
Position (at 8 bits - 0 to 255
CH0
Tips of triangles should
be spaced 4 mm apart.
4 mm
4 mm
Position 0
Position 86
Position 170
CH1 CH2CH2
CH0
CH1
CH2