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Whitepaper Easily Add Capacitive Sense to Anything FINAL .pdf


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Touch and Go:
Easily Add Capacitive Sense To
Just About Anything
By Mark Beecham

www.silabs.com | Smart. Connected. Energy-Friendly.

Table!of!Contents!
Adding!Capacitive!Sense!Screens!to!Just!About!Everything!....................................................................................!2!
Why!Add!Touch!Sense?!............................................................................................................................................!2!
Capacitive!Sensing!Basics!........................................................................................................................................!3!
Sensitivity)...............................................................................................................................................................)3)
Power).....................................................................................................................................................................)3)
Response)Time)......................................................................................................................................................)5)
Silicon!Labs!Capacitive!Sense!Solutions!..................................................................................................................!6!
Silicon)Labs’)“Charge)timing”)Capacitive)Sense)Technology)................................................................................)6)
TouchXpress)–)FixedGFunction)Capacitive)Sense)Family).....................................................................................)6)
Sleepy)Bee)–)EFM8)Microcontroller)with)Integrated)Capacitive)Sense).................................................................)8)
Conclusion!.................................................................................................................................................................!9!
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Adding!Capacitive!Sense!Screens!to!Just!About!Everything!
From!light!switches!to!washing!machines,!developers!are!adding!capacitive!sense!controls!to!hundreds!of!
devices!that!benefit!from!improved!longMterm!reliability!and!a!sleek,!lowMprofile,!customized!interface.!But!the!
capacitive!sensing!technology!behind!this!growing!trend!can!be!a!design!challenge!for!even!senior!developers.!
So!why!add!it?!

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Why!Add!Touch!Sense?!
Touch!sense!screens!are!sexy,!but!they’re!more!than!just!a!pretty!interface.!!
Because!there!are!no!moving!buttons,!rockers,!or!knobs!in!a!touch!sense!screen!there!are!no!moving!parts!to!
break,!and!therefore!there!are!lower!maintenance!costs.!This!increased!reliability!and!lower!cost!drives!almost!
instant!benefits!since!maintenance!calls!can!be!extremely!expensive.!This!is!pushing!touch!control!into!many!
devices,!from!white!goods!to!industrial!controls!to!home!automation.!!
Additionally,!the!cost!for!touch!sensing!solutions!has!decreased!over!time!as!the!underlying!technology!has!
become!far!more!cost!effective!due!to!the!enormous!volumes!of!the!touchMscreen!handset!revolution.!What!was!
once!a!highMpriced!oddity!in!the!marketplace!and!perceived!as!“very!high!end,”!is!now!commonplace!and!very!
inexpensive!to!manufacture.!!
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Furthermore,!technology!innovation!has!driven!costs!down.!Touch!sense!solutions!have!gone!from!dedicated!
modules!with!dozens!of!components!to!single!CMOSMbased!ICs!that!are!connected!to!lowMcost!touch!screens,!
printed!on!plastic,!or!even!metal,!as!part!of!the!overall!device!enclosure.!This!innovation!has!not!only!driven!cost!
down,!it!has!also!decreased!complexity!in!touch!design,!making!it!easier!to!add!to!almost!any!device.!
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Example)Microwave)Oven)with)Capacitive)Touch)Sense)Controls)in)the)Casing)
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Mechanical!buttons!and!sliders!are!passé!by!today’s!standards.!Users!want!to!buy!the!stateMofMtheMart!devices!
with!a!touch!control!interface,!and!as!a!result!of!cost!reductions!and!innovation,!manufacturers!want!to!provide!
them.!!

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Capacitive!Sensing!Basics!
Most!of!today’s!touch!interfaces!work!on!the!principle!of!capacitive!sensing.!Capacitive!sensing!works!by+sensing!
the!proximity!of!an!object,!like!a!user’s!finger,!that!is!conductive!or!has!a!different!dielectric!constant!compared!to!
the!surrounding!air.!!
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When!two!conductive!plates!are!placed!in!close!proximity!to!one!another!with!an!insulator!(or!dielectric)!between!
them,!a!capacitor!is!created!when!charge!is!applied.!When!the!capacitance!of!the!plates!changes!as!a!result!of!a!
finger!coming!into!close!proximity!or!contact!with!it,!capacitive!sense!solutions!detect!the!change!in!the!
capacitance!load,!its!location,!its!direction!if!any,!its!duration,!and!so!on.!They!then!synthesize!the!gathered!
information!into!digital!signals!and!pass!the!information!to!an!associated!processor!for!corresponding!action(s).!!
There!are!different!techniques!for!sensing!the!capacitive!change!and!many!parameters!that!affect!the!system’s!
ability!to!do!so.!!The!sections!below!investigate!these!areas.!
Sensitivity!
To!most!accurately!qualify!a!user’s!intentional!touch!on!a!control!interface,!a!capacitive!touch!design!
must!be!sensitive!enough!to!accurately!detect!the!presence!of!the!user’s!finger!while!also!being!immune!
to!noise!sources.!This!provides!a!responsive!system!and!prevents!false!positives,!and!is!generally!
specified!as!the!signal!to!noise!ratio!or!SNR.!!
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SNR!indicates!the!strength!of!the!received!signal!produced!by!user’s!finger!versus!the!amount!of!noise!it!
is!receiving.!A!higher!SNR!is!advantageous!because!it!means!the!capacitive!touch!system!is!able!to!
better!differentiate!a!user’s!touch!versus!the!surrounding!noise!sources.!!
SNR!can!also!incorporate!the!noise!immunity!of!the!overall!system.!A!consistently!low!SNR!relative!to!the!
specification!shows!that!the!system!is!likely!susceptible!to!noise.!This!can!be!due!to!either!the!overall!
system!design!including!ground!traces,!batteries,!digital!routes,!etc.,!or!the!design!of!the!capacitive!touch!
system!itself.!!
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A!lower!SNR!specification!greatly!indicates!higher!chances!of!misinterpreting!noise!for!a!user!touch!or!
human!proximity,!leading!to!potential!system!issues!from!false!positive!detections.!These!false!detections!
lead!to!system!actions!that!consume!battery!life!and!can!result!in!a!poor!user!experience.!
In!summary,!solutions!with!higher!SNR!specifications!are!generally!easier!to!design!with!because!they!
allow!for!a!greater!margin!of!error!than!systems!with!lower!SNR!specifications.!
Power!
Power!consumption!is!an!increasingly!important!buying!consideration!for!consumers,!even!for!devices!
that!are!plugged!into!the!electrical!outlets!like!white!goods!and!home!automation!systems.!Today’s!
buyers!are!more!aware!of!power!efficiency!ratings!like!“Energy!Star,”!and!are!making!buying!decisions!
based!on!power!efficiency!claims.!That!said,!in!a!wallMpowered!system!like!a!microwave!oven!or!

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washer/dryer!set,!the!power!consumption!of!the!capacitive!touch!interface!can!be!a!very!low!priority!since!
it!will!likely!make!very!little!difference!to!the!overall!system!power!consumption.!!
However,!in!batteryMpowered!applications!like!remote!controls,!saving!a!few!milliamps!can!directly!affect!
the!user!experience.!A!powerMhungry!device!can!lead!to!a!very!short!battery!life,!and!users!do!not!want!to!
change!batteries,!ever.!!
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Generally,!capacitive!touch!solutions!spend!a!majority!of!their!time!in!a!lowMpower!polling!mode,!
constantly!going!through!a!sleepMwakeMsenseMsleep!cycle!to!detect!user!proximity!events.!Some!solutions!
use!capacitive!thresholds!to!reduce!polling!current,!and!remain!in!sleep!mode!until!a!certain!capacitive!
charge!threshold!is!met.!!
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When!an!event!does!occur,!capacitive!sense!solutions!go!into!a!highMpower!scan!mode,!sensing!the!
changes!in!capacitance,!converting!that!information!to!digital!signals!and!passing!it!to!an!accompanying!
processor!for!postMprocessing!and!system!actions.!Upon!completion!of!the!user’s!action!and!associated!
capacitive!sense!scan,!the!device!returns!to!its!low!power!mode.!!
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Capacitive!touch!solutions!consume!their!peak!current!when!performing!the!scan!function.!Smart!designs!
are!optimized!to!consume!as!little!current!as!possible!during!scan!by!performing!the!scan!as!quickly!as!
possible!with!innovative!techniques.!!
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Processing!the!signals!from!the!capacitive!touch!solution!in!the!accompanying!MCU!core!can!also!
consume!large!amounts!of!power.!For!this!reason,!integrated!capacitive!sensing!peripherals!on!a!multiM
function!MCU!may!consume!less!current!if!they!operate!in!an!autonomous!mode.!In!other!words,!
capacitive!sense!peripherals!that!can!poll/scan/sleep/poll!without!waking!the!core!will!likely!consume!less!
power!than!capacitive!sense!peripherals!that!require!the!core’s!interaction.!!
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Likewise,!dedicated!capacitive!sensing!solutions!can!operate!very!power!efficiently!by!allowing!the!host!
MCU!to!remain!in!deep!sleep!mode!while!it!monitors!for!user!interactions.!As!a!dedicated!capacitive!
sense!device,!it!can!be!optimized!for!very!low!power.!
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Some!devices!allow!for!programmable!thresholds!and!current!consumption!during!scan!mode!through!
programmable!drive!strength.!This!programmability!facilitates!fineMtuning!a!design!to!meet!users’!
expectations!while!optimizing!for!the!lowest!possible!current!consumption.!The!key!is!to!prototype!the!
design!and!perform!user!testing!to!be!certain!that!the!settings!are!truly!optimized!for!a!broad!set!of!users!
and!their!interaction!styles!versus!current!consumption.!!
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Response!Time!
Response!time!is!the!time!it!takes!for!the!capacitive!sense!system!to!qualify!and!process!a!touch!event—
from!the!user’s!initial!interaction!with!the!control!screen!to!the!completion!and!/!or!confirmation!of!the!
user’s!desired!action.!!
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This!can!be!a!complex!series!of!events!that!occurs!between!the!capacitive!sense!system!and!the!overall!
product.!Careful!system!design!can!minimize!the!user’s!perceived!response!time!from!their!input!to!
system!confirmation.!Some!tactics!include!lighting!confirmation!indicator!on!the!touch!screen!or!changing!
the!color!of!an!LED!to!indicate!action!receipt.!!
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One!example!is!a!touchMcontrol!microwave!where!a!user!first!touches!a!capacitive!sense!enabled!“ON”!
button.!The!capacitive!sense!system!needs!to!wake!up,!confirm!the!touch!of!the!button,!pass!the!
command!to!the!MCU!core,!which!then!processes!it!and!perhaps!lights!up!an!LCD!screen!or!“READY”!
indicator!to!confirm!the!oven!is!ready!for!the!next!inputs.!This!allows!the!user!to!quickly!see!confirmation!
of!their!action.!!
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Another!example!could!be!a!capacitive!sense!button!to!start!a!pool!pump.!In!this!case,!the!user!touches!
an!LCD!screen!to!activate!the!pump,!and!the!system!passes!the!command!to!the!engine!while!also!
passing!a!command!to!light!an!LED!confirmation.!In!this!way!the!user!sees!the!confirmation!quickly!and!
does!not!need!to!wait!for!the!pool!pump!to!come!on.!!
As!the!first!contact!point!in!the!user’s!input!to!the!overall!product,!the!touch!controller’s!response!time!
needs!to!be!very!fast!and!very!accurate.!This!allows!the!input!to!be!passed!to!the!processor!quickly,!
subsequent!actions!to!be!taken,!and!user!confirmation!provided.!
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Silicon!Labs!Capacitive!Sense!Solutions!
Designers!adding!capacitive!sense!functionality!to!their!products!need!to!consider!whether!to!use!a!dedicated!
capacitive!sense!device!or!a!general!purpose!MCU!with!integrated!capacitive!sense!functionality.!Each!solution!
provides!benefits!that!scale!in!importance!depending!on!the!target!product,!its!price!point,!the!product!size,!and!
the!urgency!of!time!to!market.!
Silicon!Labs’!“Charge!timing”!Capacitive!Sense!Technology!
There!are!various!types!of!capacitive!sense!technology.!Silicon!Labs!pioneered!an!innovative!capacitive!
sense!technology!called!“charged!timing”!in!the!early!2000s.!Charge!timing!uses!an!external!capacitive!
plate!that!is!charged!and!discharged!periodically!and!compared!to!a!reference!capacitance!value.!Charge!
timing!is!very!power!efficient!and!generates!a!very!high!SNR.!!
With!its!low!power!and!high!SNR,!charge!timing!is!able!detect!very!small!changes!in!capacitance!while!
consuming!very!little!power.!Silicon!Labs’!charge!timing!technology!is!incorporated!in!its!fixedMfunction,!
dedicated!capacitive!sense!products!and!its!multiMfunction!MCUs!with!integrated!capacitive!sense!
peripherals.!
TouchXpress!–!FixedMFunction!Capacitive!Sense!Family!
The!TouchXpress!family!of!fixed!function!capacitive!sense!devices!make!adding!capacitive!sense!to!an!
existing!or!new!product!fast!and!easy.!The!family!offers!lowMpower,!standalone!capacitive!sense!solutions!
that!easily!hook!up!to!capacitive!plates!and!translate!user!interactions!to!GPIO!outputs!or!I2C!outputs.!
The!family!is!designed!for!very!low!power!autonomous!operation,!allowing!higher!power!host!MCUs!to!
remain!in!deep!sleep!mode!until!users!interact!with!the!capacitive!sense!touch!interface.!!
TouchXpress!devices!are!a!good!fit!for!designs!that!need!to!add!capacitive!touch!but!must!leave!the!
existing!MCU!and!code!untouched.!They!are!small,!autonomous,!cost!effective,!and!very!low!power.!!
TouchXpress!devices!are!preloaded!with!productionMquality!capacitive!sense!firmware.!The!firmware!
incorporates!decades!of!experience!in!capacitive!sense!technology!and!is!thoroughly!tested!and!
guaranteed!in!each!device!across!temperature!and!voltage.!This!is!important!because!touch!control!
firmware!is!generally!easy!to!for!engineers!to!conceptualize!and!prototype!on!evaluation!kits,!but!can!be!
more!temperamental!to!get!into!production.!!
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However,!the!preloaded!firmware!is!also!customizable!along!several!key!parameters!through!the!Xpress!
Configurator!included!in!Simplicity!Studio.!Xpress!Configurator!allows!designers!to!customize!activation!
thresholds,!gain,!active!mode!and!sleep!mode!scan!periods,!operating!frequency,!and!drive!strength.!
These!customizable!parameters!facilitate!design!optimization,!but!do!not!affect!the!firmware’s!production!
quality!readiness.!!
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Example)Xpress)Configurator)Interface)for)TouchXpress)Customization)
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The!family!also!uses!Simplicity!Studio’s!Capacitive!Sense!Profiler!tool!to!visualize!the!capacitive!device’s!
performance!and!output!information!such!as!detection!time,!touch!thresholds,!charge!levels,!etc.!
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Example)of)Capacitive)Sense)Profiler)Interface)
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These!Simplicity!Studio!tools!allow!designers!to!easily!configure!their!systems!to!meet!their!users’!needs!
and!use!cases.!Each!tool!also!provides!helpful!guidance!on!each!capacitive!sense!setting!and!what!it!
does!for!users!that!are!still!learning!about!capacitive!sense!solutions.!!
TouchXpress!devices!are!a!good!fit!for!adding!capacitive!touch!to!any!new!or!existing!product.!They!are!
standalone,!small,!and!cost!effective,!and!allow!capacitive!touch!to!run!autonomously!to!the!host!MCU,!
saving!power!and!speeding!time!to!market.!

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Sleepy!Bee!–!EFM8!Microcontroller!with!Integrated!Capacitive!Sense!
Some!designs!may!need!the!capacitive!sense!to!be!integrated!within!the!host!MCU’s!functionality!and!
footprint.!This!may!save!both!cost!and!size,!but!may!also!bring!complexity!with!code!development!as!the!
capacitive!sense!functionality!will!share!resources!with!other!MCU!requirements.!!
The!low!power!Silicon!Labs!Sleepy!Bee!EFM8SB!Microcontroller!(MCU)!family!provides!integrated!
capacitive!sense!functionality!as!a!lowMpower,!autonomous!peripheral.!In!other!words,!the!capacitive!
sense!functionality!runs!without!interrupting!or!waking!the!MCU!core!or!other!peripherals.!!
Sleepy!Bee!MCUs!are!optimized!for!low!power!designs,!and!targeted!to!battery!powered!systems.!They!
use!the!same!“charge!timing”!technology!used!in!the!TouchXpress!family!to!offer!up!to!14!capacitive!
sense!channels,!in!addition!to!a!comprehensive!set!of!targeted!functionality!for!portable!devices.!!!
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Silicon)Labs)Sleepy)Bee)MCU)Feature)Set)

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Since!the!capacitive!sensing!functionality!is!integrated!in!the!Sleepy!Bee!MCUs,!the!overall!solution!
requires!a!smaller!footprint!and!may!be!a!lower!cost!solution!than!using!two!separate!devices.!!
Like!the!TouchXpress!solutions,!Sleepy!Bee!MCUs!provide!capacitive!sense!example!firmware!code,!
although!it!must!be!more!customized!than!the!TouchXpress!code!to!accommodate!other!MCU!
functionality.!Sleepy!Bee!MCUs!are!highly!configurable!using!Simplicity!Studio’s!Configurator,!Capacitive!
Sense!Profiler,!and!Power!Profiler!tools.!
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www.silabs.com!!|!Configurable!Logic!in!Microcontrollers!Revolutionizes!Small!Applications!!
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