Atmel QTouch, QTouchADC, Qmatrix Application Note
Atme l-42094C-QTouch -Schematic-and-Layout-Checklist-ApplicationNote_072014
APPLICATION NOTE
AT02259: QTouch Schematic and Layout Checklist
Atmel QTouch
Introduction
Designing a capacitive touch interface is an involved process that requires myriad
factors to be considered. These interfaces frequently need to co-exist with other
complex systems, which may affect its performance.
Since there are several considerations to be made while designing Atmel
QTouch
design. The purpose of this document is to provide a checklist that can be used to
review the schematic and PCB layout of these designs. This includes separate
checklists for designs using Atmel QTouch, QTouchADC and QMatrix. This
document only highlights the most important aspects and should not be
considered as a substitute for QTAN0079 Buttons, Sliders and Wheels – Sensor
Design Guide.
®
solutions it is important to know these for overall improvement of the
®
Features
• Checklist for reviewing Atmel QTouch schematics and PCB layouts
2
1 Abbreviations and Definitions
The following is a list of terms, which will be used throughout the document.
• Acquisition: A single capacitive measurement process
• Atmel QTouch Library: The set of libraries for the touch sensing technologies offered by Atmel (QTouch,
QTouchADC, and QMatrix)
®
• AVR
• Button: It is a zero dimensional sensor used to implement On/Off digital sensors, and is composed of a
• Channel: A channel is a logical group of pins used to perform the touch acquisition measurement. It can be
• Charge Cycle Period: It is the width of the charging pulse applied to the channel sampling capacitor
• Charge Share Delay: It is the duration when charge is shared between Cs and Cs in QTouchADC
• Ct: The capacitance caused by a finger touch over the Sense Electrode
• Cx: The self-capacitance or mutual-capacitance of the Sense Electrode
• Dwell Cycle: In a QMatrix acquisition method, the duration in which charge coupled from X to Y is
• Port Pair: A combination of SNS port and SNSK port to which sensors are connected in QTouch
• QMatrix: A type of capacitive touch sensing technology that uses the mutual capacitance between two
• QTouch: A type of capacitive touch sensing technology that uses the self capacitance of an electrode
• QTouchADC: A type of capacitive touch sensing technology that uses the self capacitance of an electrode
• Rs: Series resistor on the sense line. This should be in the charging path of Cs
• Sense Component: The set of components connected to the MCU, which are used to perform a touch
• Sense Electrode: Electrodes are typically areas of copper on a printed circuit board. An electrode or a pair
• Sense Line: Any track that connects the Sense Electrode to Sense Components
• Sensor: A channel or group of channels used to form a touch sensor. The three types of sensors are
• Slider: It is a one dimensional sensor used to implement linear position sensors. A group of channels
• SNS Pin: ‘Sense’ pin connected to the sampling capacitor (Cs) in QTouch technology
• SNSK Pin: ‘Sense Key’ pin connected to the electrode through a series resistor (Rs) in QTouch technology
• Wheel: It is a one dimensional sensor used to implement angular position controls. A QTouch Wheel is
• X Line: The Sense Line connected to the X Electrode in QMatrix Technology
• Y Line: The Sense Line connected to the Y Electrode used in QMatrix Technology
• Intra-Port: A configuration for QTouch acquisition method libraries, when the sensor SNS and SNSK pins
• Inter-Port: A configuration for QTouch acquisition method libraries, when the sensor SNS and SNSK pins
: Refers to a device in the Atmel tinyAVR®, megaAVR®, XMEGA®, and UC3 microcontroller (MCU)
family
single channel. It is also known as a Key
composed of a single pin (QTouchADC), a pair of pins (QTouch) or a matrix of pins (QMatrix)
captured
technology. The SNS and SNSK ports used in a port pair can be located in the same AVR Port (eight pins
for four sensors), or they may be on two different AVR Ports (8+8 pins for eight sensors)
electrodes. Each channel has a drive electrode (X Electrode) and a receive electrode (Y Electrode)
connected to a single pin (ADC input)
measurement
of electrodes used to detect a finger touch
Buttons, Sliders, and Wheels
forms a Slider, which is used to detect the linear position of touch. A QTouch Slider is composed of three
channels. A QMatrix Slider can be composed of three to eight channels
composed of three channels. A QMatrix Wheel can be composed of three to eight channels. It is also know
as a Rotor
are available on the same port
are available on distinct ports
QTouch Schematic and Layout Checklist [APPLICATION NOTE]
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2 Schematic Review
This chapter consists of the checklists used for reviewing the schematics of Atmel QTouch, QTouchADC and
QMatrix implementations. There are sections corresponding to each individual technology. Section 2.4 General
is applicable to all technologies.
2.1 Atmel QTouch
This section consists of the checklist for reviewing a QTouch schematic.
• Connect SNSK to the sensor electrode
The sense electrode should be connected to the SNSK pin and not to the SNS pin. If the electrode is connected
incorrectly, in some cases it may appear to work. This can happen due to parasitic capacitances providing an
alternate path for charge transfer to occur. But the channel will have very poor sensitivity and will be very
sensitive to temperature and humidity. Therefore it is important to verify the connections before attempting to
improve sensitivity.
Figure 2-1. Typical QTouch Circuit
• Rs = 1kΩ to 10kΩ
The value of Rs should typically be within the range of 1kΩ to 10kΩ. If Rs is increased the value of Charge Cycle
Period will need to be increased appropriately to ensure complete charging of Cx.
Increase Rs in steps up to 100kΩ to improve performance in noisy environments. In extreme
cases higher values of Rs, going up to 1MΩ may be required.
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• Cs = 1nF to 47nF
The value of Cs should typically be within the range of 1nF to 47nF (not pF or µF). It is best to start with a nominal
value of 10nF and fix the Cs values after tuning is done on a prototype.
Cs can be larger than 47nF for proximity sensors.
• Sense pin selection for Inter-Port / Intra-Port
In Inter-Port connection, sense pins (SNS/SNSK) selected should be in incremental order as channel number of
the sensor.
Figure 2-2. Pin Selection for Inter-Port Design
In Intra-Port connection, sense pins selected should be in incremental order as channel number of the sensor.
Always SNS pin should be the lower numbered pin, in a channel.
Figure 2-3. Pin Selection for Intra-Port Design
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• Slider/Wheel uses only three channels
A QTouch Slider/Wheel always uses only three channels. It is important to use channels with similar sensitivity.
Using mismatched channels with significantly different sensitivities will lead to an unbalanced Slider/Wheel.
Always split third channel (highest numbered channel) in a QTouch Slider.
• Sense pins selected for Slider/Wheel are not unbalanced
Ensure that the sense pins selected for Slider/Wheel are not unbalanced. If loaded pins are combined with
regular GPIOs, the linearity of the Slider/Wheel will be poor.
• Multiplexing sense lines with Programming lines
SNSK sense line can be multiplexed with ISP lines. It is better to not use SNS lines for multiplexing with
programming lines, as the extended track lengths might cause loading and interference effects.
For lesser pin count devices which mandate multiplexing, SNS lines can be used with shorter trace length
possible.
Do not use same channel sense (SNS and SNSK) lines for multiplexing programming lines.
Since the sampling capacitor (Cs) used across would affect device programming.
2.2 Atmel QTouchADC
This section consists of the checklist for reviewing a QTouchADC schematic.
• Sense Electrode is connected to an ADC input pin
The QTouchADC acquisition method uses the ADC module. Hence the sense channel needs to be configured on
a port which has the ADC input pins.
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Rs1
PA0 (ADC0)
PA1(ADC1)
Rs2
Rs3
PA2 (ADC2)
PA3 (ADC3)
Rs4
Key 0
Key 1
Key 2
Key 3
Figure 2-4. Typical QTouchADC Circuit
• Rs = 1kΩ to 10kΩ
The value of Rs should typically be within the range of 1kΩ to 10kΩ. If Rs is increased the value of Charge Share
Delay (CSD) will need to be increased appropriately to ensure complete charging of Cx.
Increase Rs in steps up to 100kΩ to improve performance in noisy environments. In extreme
2.3 Atmel QMatrix
This section consists of the checklist for reviewing a QMatrix schematic.
• For QMatrix designs VCC is 4.5V or below
The supply voltage for QMatrix designs should be kept at 4.5V or below even if the device is capable of operating
above 4.5V. This is required for reliable operation of the QMatrix sensors.
• Pin selection is appropriate
X pins, YA pins and SMP pin are configured on GPIOs. YB pins are configured on the ADC port. The AIN0 pin is
fixed for the device and can be easily identified from the QTouch Library Selection Guide and the device
datasheet. The AIN0 pin needs to be grounded.
cases higher values of Rs, going up to 1MΩ may be required.
The AIN0 pin is not user configurable.
QTouch Schematic and Layout Checklist [APPLICATION NOTE]
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Figure 2-5. Typical QMatrix Circuit
• Cs = 1nF to 47nF
The value of Cs should typically be within the range of 1nF to 47nF. In QMatrix acquisition technique a dual-slope
approach is used to detect a touch. Hence Cs does not affect sensitivity. But if extreme values are used it can
hinder the acquisition process. If the value of Cs is too low it is possible that the reverse charge build-up (VCS)
gets saturated. If it is set too high the VCS may be too small.
Typical value of Cs is 4.7nF. Increase it to 10nF if saturated.
• Rx/Ry = 1kΩ to 10kΩ
The value of Rx/Ry should typically be within the range of 1kΩ to 10kΩ. If Rx/Ry is increased the value of Dwell
Time will need to be increased appropriately to ensure complete charging of Cx.
Increase Ry in steps up to 100kΩ to improve performance in noisy environments. In extreme
cases higher values of Ry, going up to 1MΩ may be required.
• Rsmp = 220kΩ to 1MΩ
Rsmp can be any value within the range of 220kΩ to 1MΩ. This value is used to tune the sensitivity of the
channels sharing the corresponding Y-line.
Typically a 470kΩ resistor is used.
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• Do not use AIN1 pin for PWM generation
QMatrix method uses Analog comparator for touch measurement and the AIN1 pin is connected to negative
terminal of Analog comparator. So, the switching signals in AIN1 pin would affect touch measurement.
• Using same Port for YA/YB Lines
Prefer using different Port for YA and YB lines, in case of designs that does not have pin count constraints.
• Multiplexing sense lines with Programming lines
X lines can be multiplexed with ISP lines. It is preferable to not share Y lines (both YA and YB) with programming
lines.
For lesser pin count devices which mandate multiplexing, Y lines can be used with shorter trace length possible.
Do not share same channel Y lines (YA and YB) with programming lines.
• Slider/Wheel uses four to eight channels
A QMatrix Slider/Wheel always uses a minimum of four channels. This can be increased up to eight channels for
a bigger Slider/Wheel. It is important to use channels with similar sensitivity. Using mismatched channels with
significantly different sensitivities will lead to an unbalanced Slider/Wheel.
• Slider/Wheel resistive interpolation resistors are in range
Total resistance between channels is 2kΩ to 10kΩ. The resistance between segments will depend on the
number of segments used.
There are no end resistors or split channels for a QMatrix Slider.
Figure 2-6. Resistive Interpolated QMatrix Slider and Wheel
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2.4 General
This section consists of the general checklist for reviewing a schematic. This is applicable to all implementations.
• Sense pins are not loaded
Ensure that the sense pins are not heavily multiplexed as this can cause increased loading. XTAL/OSC/RESET
pins usually have high parasitic capacitance values and are not suitable. If loaded pins are used, the sensitivity of
the channel will be poor.
• Sampling Capacitor is X7R or better
Stability of the Cs capacitor is important in achieving a consistent and repeatable measurement. A capacitor with
X7R dielectric or better has a low temperature co-efficient and will be more stable.
Quality of series resistor Rs is not critical.
• Dedicated voltage regulator is used
It is recommended to use a dedicated voltage regulator for a QTouch application. If the voltage regulator is
shared with other components it is important to ensure that the power supply is clean.
The ripple on the supply voltage needs to be within ±10mV.
Figure 2-7. Regulated Supply Circuit Configurations (Optional Components are Highlighted in Red)
Typical regulated supply circuit
With other devices
QTouch Schematic and Layout Checklist [APPLICATION NOTE]
Atme l-42094C-QTouch -Schematic-and-Layout-Checklist-ApplicationNote_072014
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