Atmel QTouch Instructions d'installation et d'utilisation

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®solutions it is important to know these for overall improvement of the

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.

Features

•Checklist for reviewing Atmel QTouch schematics and PCB layouts

Atmel-42094C-QTouch-Schematic-and-Layout-Checklist-ApplicationNote_072014

QTouch Schematic and Layout Checklist [APPLICATION NOTE]

Atmel-42094C-QTouch-Schematic-and-Layout-Checklist-ApplicationNote_072014

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®: Refers to a device in the Atmel tinyAVR®, megaAVR®, XMEGA®, and UC3 microcontroller (MCU)

family

•Button: It is a zero dimensional sensor used to implement On/Off digital sensors, and is composed of a

single channel. It is also known as a Key

•Channel: A channel is a logical group of pins used to perform the touch acquisition measurement. It can be

composed of a single pin (QTouchADC), a pair of pins (QTouch) or a matrix of pins (QMatrix)

•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

captured

•Port Pair: A combination of SNS port and SNSK port to which sensors are connected in QTouch

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)

•QMatrix: A type of capacitive touch sensing technology that uses the mutual capacitance between two

electrodes. Each channel has a drive electrode (X Electrode) and a receive electrode (Y Electrode)

•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

connected to a single pin (ADC input)

•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

measurement

•Sense Electrode: Electrodes are typically areas of copper on a printed circuit board. An electrode or a pair

of electrodes used to detect a finger touch

•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

Buttons, Sliders, and Wheels

•Slider: It is a one dimensional sensor used to implement linear position sensors. A group of channels

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

•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

composed of three channels. A QMatrix Wheel can be composed of three to eight channels. It is also know

as a Rotor

•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

are available on the same port

•Inter-Port: A configuration for QTouch acquisition method libraries, when the sensor SNS and SNSK pins

are available on distinct ports

QTouch Schematic and Layout Checklist [APPLICATION NOTE]

Atmel-42094C-QTouch-Schematic-and-Layout-Checklist-ApplicationNote_072014

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.

QTouch Schematic and Layout Checklist [APPLICATION NOTE]

Atmel-42094C-QTouch-Schematic-and-Layout-Checklist-ApplicationNote_072014

•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

QTouch Schematic and Layout Checklist [APPLICATION NOTE]

Atmel-42094C-QTouch-Schematic-and-Layout-Checklist-ApplicationNote_072014

•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.

QTouch Schematic and Layout Checklist [APPLICATION NOTE]

Atmel-42094C-QTouch-Schematic-and-Layout-Checklist-ApplicationNote_072014

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

cases higher values of Rs, going up to 1MΩ may be required.

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.

The AIN0 pin is not user configurable.

Rs1

PA0 (ADC0)

PA1(ADC1)

Rs2

Rs3

PA2 (ADC2)

PA3 (ADC3)

Rs4

Key 0

Key 1

Key 2

Key 3

QTouch Schematic and Layout Checklist [APPLICATION NOTE]

Atmel-42094C-QTouch-Schematic-and-Layout-Checklist-ApplicationNote_072014

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.

QTouch Schematic and Layout Checklist [APPLICATION NOTE]

Atmel-42094C-QTouch-Schematic-and-Layout-Checklist-ApplicationNote_072014

•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

QTouch Schematic and Layout Checklist [APPLICATION NOTE]

Atmel-42094C-QTouch-Schematic-and-Layout-Checklist-ApplicationNote_072014

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]

Atmel-42094C-QTouch-Schematic-and-Layout-Checklist-ApplicationNote_072014

For lower cost products

•Any load on GPIO pins does not source or sink more than ~1mA

If there is any load on a discrete out pin of the microcontroller, ensure that the load does not sink or source too

much current directly from chip. This can cause ground bounce which would in turn affect QTouch

measurements. Typically the current should be limited to ~1mA, but this can vary depending on the part and

sensitivity of channels. If required use transistors etc. to lower load current.

•No additional circuitry on sense lines

Placing ESD protection circuitry, MUX, etc are not recommended since they add parasitic capacitance and may

introduce noise. Placing option resistors on the sense lines to connect the pins to different circuitry is acceptable.

But the sensitivity of the channel needs to be re-tuned if options change.

•Nearby LEDs have a bypass capacitor

The changing capacitance from switching LEDs can cause detection instability and stuck-on state in nearby

sensors. This is particularly true if LEDs are pulled down or up to switch on, but are allowed to float when off.

If such LEDs are less than 4mm away from capacitive sensors, they must be bypassed with a capacitor that has

a typical value of 1nF.

Bypass capacitor does not need to be physically close to the LED.

Figure 2-8. Bypass Capacitors on Nearby LEDs

Ce manuel convient aux modèles suivants

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