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QTouch Schematic and Layout Checklist [APPLICATION NOTE]   

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

 

3 

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

. 

 

Summary of Contents for Qmatrix

Page 1: ... 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 B...

Page 2: ... 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 componen...

Page 3: ...ot 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 Figur...

Page 4: ... 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 Al...

Page 5: ...nearity 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...

Page 6: ...uired 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 ...

Page 7: ...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 per...

Page 8: ... 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 ...

Page 9: ...ll 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 applic...

Page 10: ...onal 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 switchi...

Page 11: ...e same port as the sense lines since switching LEDs can cause the touch measurement to be noisy If they are required to be on the same port it is recommended to increase the value of the LED series resistor to minimize the impact Use multiple grounds in case of isolated sensors When sensor board is isolated from touch MCU board prefer using multiple ground pins in the connector for effective retur...

Page 12: ...se components Cs and Rs for QTouch only Rs for QTouchADC should be placed close to the microcontroller sense pins If they are placed too far from the pin there will be increased noise pick up from nearby sources 3 1 2 Sensors Electrode size is slightly larger than a normal finger It is recommended to have a key size that is slightly larger than the item to be sensed finger thumb etc In general lar...

Page 13: ...les have diameter less than 4mm LED holes that are too large can lead to poor sensitivity or cause the key to have a dead spot in the middle Figure 3 2 Hole for Backlighting LED 3 1 3 Routing Sense Lines are routed on the bottom layer Sense tracks are sensitive to touch and wherever possible they must be routed on the bottom layer to prevent false keys on the touch surface Spacing between adjacent...

Page 14: ...d Figure 3 4 Typical QTouch Button Layout Sense Lines are separated from GND tracks by at least T 2 GND tracks should not be placed near Sense Lines This will load the Sense Lines and reduce the sensitivity of the channel To reduce loading the Sense Lines and GND track should cross at 90 on separate layers Figure 3 5 Typical QTouch Button Layout Sense Lines are as short as possible Having long Sen...

Page 15: ...ing from noise sources is necessary a thin meshed ground may be used behind electrodes 40 copper Meshed ground can also be helpful in increasing SNR and stability when conducted noise is present Figure 3 6 No Ground Flood Under Sense Components Tracks and Electrodes Figure 3 7 Meshed Ground to Provide Shielding from Noise Sources Behind the Board 3 1 4 QTouch Slider or Wheel The checklist in this ...

Page 16: ...ed to the same channel This needs to be ensured for both spatially and resistively interpolated QTouch Sliders The track connecting end segments is placed away from the Slider The track needs to be placed away from the Slider to prevent accidentally touching it while sliding the finger This can lead to incorrect position being reported Medium large spatially interpolated Sliders have segment width...

Page 17: ...r the Slider Wheel Ground planes under the Slider Wheel will load the sensor and make it less sensitive Ground planes that unevenly load some of the channels are worse than all the channels being equally loaded Although GND loading decreases sensitivity it increases SNR and Signal stability which can be useful in noisy environments Medium large spatially interpolated Wheels have segment width of 4...

Page 18: ...be placed closer to Y sense pin It is OK to place the Rsmp resistor further from SMP pin since it is always driven 3 2 2 Sensors There are no ground planes directly under co planar Buttons These will load the sensor and decrease the sensitivity But the effect is not as dramatic as in the case of QTouch Sensors Y Electrodes are located on the top layer The Y electrodes should be located on the laye...

Page 19: ...hen the X Y gap should be at least T 2 There should be no other tracks running below the X Y gap as this can desensitize the key In flooded X designs X Y separation is determined by the PCB thickness Therefore the acceptable front panel thickness is constrained by the thickness of the PCB used Figure 3 14 X Y Separation and Front Panel Thickness Single Layer Planar Electrode Layout Double Layer Fl...

Page 20: ...ng the sensitivity of the sensor X Electrode on the bottom layer can provide shielding from noise sources behind the board Figure 3 16 QMatrix Flooded X Two Layer Design 3 2 3 Routing Y Lines are as thin and short as possible Having long Sense Lines will worsen any loading and interference from nearby tracks and components The thickness of the Y electrode should be kept as low as possible 0 1mm to...

Page 21: ... the Y Lines if they are placed too close X Lines and X Electrodes are not sensitive to loading from ground planes and foreign tracks components Y Lines are routed on the bottom layer Y Lines are sensitive to touch and wherever possible they must be routed on the bottom layer to prevent false keys on the touch surface Figure 3 18 Cross Section of a Multi Layer PCB Spacing between adjacent Y Lines ...

Page 22: ...tra care needs to be taken to avoid false keys Some routing suggestions are illustrated in Figure 3 20 Figure 3 19 X and Y Lines Placed Close to Each Other Can Behave Like a Button Figure 3 20 Tips for Minimizing Interaction Between X and Y Routing a 90 o crossing b Separating ground trace c X under Ground trace d Y under X X Lines and Y Lines Interconnection using Flex cables When using Flex cabl...

Page 23: ...ouch Segments that are too large will lead to poor linearity In case of Wheels the outer arc of the segment should be 6mm to 8mm long In single layer Slider Wheel isolated X regions are connected using vias Since the Slider segments are on the same layer the Y Electrode cuts through the X Electrode This causes the X Electrodes to be split These isolated regions need to be connected on the bottom l...

Page 24: ...TAN0079 Buttons Sliders and Wheels Sensor Design Guide for more details on Front panel materials and its usage Floating metal nearby touch sensors to be grounded Floating conductive items nearby touch sensors would re radiate the sensor s electric field and might cause false detections It is essential to ground the conductive items nearby touch sensors for normal touch operation Proximity sensor d...

Page 25: ...l QTouch Library User Guide http www atmel com Images doc8207 pdf 3 AVR3000 QTouch Conducted Immunity http www atmel com Images doc8425 pdf 4 QTAN0087 Proximity Design Guide http www atmel com Images doc10760 pdf 5 QTAN0062 QTouch and QMatrix Sensitivity Tuning for Keys Sliders and Wheels http www atmel com Images QTAN0062 pdf 6 AVR042 AVR Hardware Design Considerations http www atmel com images a...

Page 26: ...pin for PWM generation Sharing YA YB Ports Multiplexing sense line with programming line 4 Additional Schematic checklist items added under General section Using multiple ground pins 5 Added below checklist items under General Layout section Effects of conductive coating over front panel materials Effects of floating metal near touch sensors Proximity sensor design 6 Added References section 7 Mod...

Page 27: ...FITS BUSINESS INTERRUPTION OR LOSS OF INFORMATION ARISING OUT OF THE USE OR INABILITY TO USE THIS DOCUMENT EVEN IF ATMEL HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES Atmel makes no representations or warranties with respect to the accuracy or completeness of the contents of this document and reserves the right to make changes to specifications and products descriptions at any time without n...

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