3.2.&nbsp;Small&nbsp;size,&nbsp;low&nbsp;weight&nbsp;<br />Many&nbsp;embedded&nbsp;computers&nbsp;are&nbsp;physically&nbsp;located&nbsp;within&nbsp;some&nbsp;larger&nbsp;artifact.&nbsp;&nbsp;<br />Therefore,&nbsp;their&nbsp;form&nbsp;factor&nbsp;may&nbsp;be&nbsp;dictated&nbsp;by&nbsp;aesthetics,&nbsp;form&nbsp;factors&nbsp;exi&nbsp;<br />sting&nbsp;in&nbsp;pre-electronic&nbsp;versions,&nbsp;or&nbsp;having&nbsp;to&nbsp;fit&nbsp;into&nbsp;interstices&nbsp;among&nbsp;me&nbsp;<br />chanical&nbsp;components.&nbsp;In&nbsp;transportation&nbsp;and&nbsp;portable&nbsp;systems,&nbsp;weight&nbsp;may&nbsp;be&nbsp;c&nbsp;<br />ritical&nbsp;for&nbsp;fuel&nbsp;economy&nbsp;or&nbsp;human&nbsp;endurance.&nbsp;Among&nbsp;the&nbsp;examples,&nbsp;the&nbsp;Mission&nbsp;<br />&nbsp;Critical&nbsp;system&nbsp;has&nbsp;much&nbsp;more&nbsp;stringent&nbsp;size&nbsp;and&nbsp;weight&nbsp;requirements&nbsp;than&nbsp;t&nbsp;<br />he&nbsp;others&nbsp;because&nbsp;of&nbsp;its&nbsp;use&nbsp;in&nbsp;a&nbsp;flight&nbsp;vehicle,&nbsp;although&nbsp;all&nbsp;examples&nbsp;have&nbsp;<br />&nbsp;restrictions&nbsp;of&nbsp;this&nbsp;type.&nbsp;<br />Design&nbsp;challenges:&nbsp;<br />·&nbsp;Non-rectangular,&nbsp;non-planar&nbsp;geometries.&nbsp;<br />·&nbsp;Packaging&nbsp;and&nbsp;integration&nbsp;of&nbsp;digital,&nbsp;analog,&nbsp;and&nbsp;power&nbsp;circuits&nbsp;to&nbsp;reduc&nbsp;<br />e&nbsp;size.&nbsp;<br />3.3.&nbsp;Safe&nbsp;and&nbsp;reliable&nbsp;<br />Some&nbsp;systems&nbsp;have&nbsp;obvious&nbsp;risks&nbsp;associated&nbsp;with&nbsp;failure.&nbsp;In&nbsp;mission-critical&nbsp;<br />&nbsp;applications&nbsp;such&nbsp;as&nbsp;aircraft&nbsp;flight&nbsp;control,&nbsp;severe&nbsp;personal&nbsp;injury&nbsp;or&nbsp;equ&nbsp;<br />ipment&nbsp;damage&nbsp;could&nbsp;result&nbsp;from&nbsp;a&nbsp;failure&nbsp;of&nbsp;the&nbsp;embedded&nbsp;computer.&nbsp;Traditio&nbsp;<br />nally,&nbsp;such&nbsp;systems&nbsp;have&nbsp;employed&nbsp;multiply-redundant&nbsp;computers&nbsp;or&nbsp;distribute&nbsp;<br />d&nbsp;consensus&nbsp;protocols&nbsp;in&nbsp;order&nbsp;to&nbsp;ensure&nbsp;continued&nbsp;operation&nbsp;after&nbsp;an&nbsp;equipm&nbsp;<br />ent&nbsp;failure&nbsp;(e.g.,&nbsp;[10],&nbsp;[11])&nbsp;<br />However,&nbsp;many&nbsp;embedded&nbsp;systems&nbsp;that&nbsp;could&nbsp;cause&nbsp;personal&nbsp;or&nbsp;property&nbsp;damage&nbsp;&nbsp;<br />cannot&nbsp;tolerate&nbsp;the&nbsp;added&nbsp;cost&nbsp;of&nbsp;redundancy&nbsp;in&nbsp;hardware&nbsp;or&nbsp;processing&nbsp;capac&nbsp;<br />ity&nbsp;needed&nbsp;for&nbsp;traditional&nbsp;fault&nbsp;tolerance&nbsp;techniques.&nbsp;This&nbsp;vulnerability&nbsp;is&nbsp;<br />&nbsp;often&nbsp;resolved&nbsp;at&nbsp;the&nbsp;system&nbsp;level&nbsp;as&nbsp;discussed&nbsp;later.&nbsp;<br />Design&nbsp;challenge:&nbsp;<br />·&nbsp;Low-cost&nbsp;reliability&nbsp;with&nbsp;minimal&nbsp;redundancy.&nbsp;<br />3.4.&nbsp;Harsh&nbsp;environment&nbsp;<br />Many&nbsp;embedded&nbsp;systems&nbsp;do&nbsp;not&nbsp;operate&nbsp;in&nbsp;a&nbsp;controlled&nbsp;environment.&nbsp;Excessive&nbsp;&nbsp;<br />heat&nbsp;is&nbsp;often&nbsp;a&nbsp;problem,&nbsp;especially&nbsp;in&nbsp;applications&nbsp;involving&nbsp;combustion&nbsp;(e.&nbsp;<br />g.,&nbsp;many&nbsp;transportation&nbsp;applications).&nbsp;Additional&nbsp;problems&nbsp;can&nbsp;be&nbsp;caused&nbsp;for&nbsp;<br />&nbsp;embedded&nbsp;computing&nbsp;by&nbsp;a&nbsp;need&nbsp;for&nbsp;protection&nbsp;from&nbsp;vibration,&nbsp;shock,&nbsp;lightnin&nbsp;<br />g,&nbsp;power&nbsp;supply&nbsp;fluctuations,&nbsp;water,&nbsp;corrosion,&nbsp;fire,&nbsp;and&nbsp;general&nbsp;physical&nbsp;a&nbsp;<br />buse.&nbsp;For&nbsp;example,&nbsp;in&nbsp;the&nbsp;Mission&nbsp;Critical&nbsp;example&nbsp;application&nbsp;the&nbsp;computer&nbsp;&nbsp;<br />must&nbsp;function&nbsp;for&nbsp;a&nbsp;guaranteed,&nbsp;but&nbsp;brief,&nbsp;period&nbsp;of&nbsp;time&nbsp;even&nbsp;under&nbsp;non-sur&nbsp;<br />vivable&nbsp;fire&nbsp;conditions.&nbsp;<br />Design&nbsp;challenges:&nbsp;<br />·&nbsp;Accurate&nbsp;thermal&nbsp;modelling.&nbsp;<br />·&nbsp;De-rating&nbsp;components&nbsp;differently&nbsp;for&nbsp;each&nbsp;design,&nbsp;depending&nbsp;on&nbsp;operating&nbsp;&nbsp;<br />environment.&nbsp;<br />3.5.&nbsp;Cost&nbsp;sensitivity&nbsp;<br />Even&nbsp;though&nbsp;embedded&nbsp;computers&nbsp;have&nbsp;stringent&nbsp;requirements,&nbsp;cost&nbsp;is&nbsp;almost&nbsp;a&nbsp;<br />lways&nbsp;an&nbsp;issue&nbsp;(even&nbsp;increasingly&nbsp;for&nbsp;military&nbsp;systems).&nbsp;Although&nbsp;designers&nbsp;&nbsp;<br />of&nbsp;systems&nbsp;large&nbsp;and&nbsp;small&nbsp;may&nbsp;talk&nbsp;about&nbsp;the&nbsp;importance&nbsp;of&nbsp;cost&nbsp;with&nbsp;equal&nbsp;&nbsp;<br />urgency,&nbsp;their&nbsp;sensitivity&nbsp;to&nbsp;cost&nbsp;changes&nbsp;can&nbsp;vary&nbsp;dramatically.&nbsp;A&nbsp;reason&nbsp;f&nbsp;<br />or&nbsp;this&nbsp;may&nbsp;be&nbsp;that&nbsp;the&nbsp;effect&nbsp;of&nbsp;computer&nbsp;costs&nbsp;on&nbsp;profitability&nbsp;is&nbsp;more&nbsp;a&nbsp;&nbsp;<br />function&nbsp;of&nbsp;the&nbsp;proportion&nbsp;of&nbsp;cost&nbsp;changes&nbsp;compared&nbsp;to&nbsp;the&nbsp;total&nbsp;system&nbsp;cost&nbsp;<br />,&nbsp;rather&nbsp;than&nbsp;compared&nbsp;to&nbsp;the&nbsp;digital&nbsp;electronics&nbsp;cost&nbsp;alone.&nbsp;For&nbsp;example,&nbsp;i&nbsp;<br />n&nbsp;the&nbsp;Signal&nbsp;Processing&nbsp;system&nbsp;cost&nbsp;sensitivity&nbsp;can&nbsp;be&nbsp;estimated&nbsp;at&nbsp;approxim&nbsp;<br />ately&nbsp;$1000&nbsp;(i.e.,&nbsp;a&nbsp;designer&nbsp;can&nbsp;make&nbsp;decisions&nbsp;at&nbsp;the&nbsp;$1000&nbsp;level&nbsp;without&nbsp;&nbsp;<br />undue&nbsp;management&nbsp;scrutiny).&nbsp;However,&nbsp;with&nbsp;in&nbsp;the&nbsp;Small&nbsp;system&nbsp;decisions&nbsp;incr&nbsp;<br />easing&nbsp;costs&nbsp;by&nbsp;even&nbsp;a&nbsp;few&nbsp;cents&nbsp;attract&nbsp;management&nbsp;attention&nbsp;due&nbsp;to&nbsp;the&nbsp;hug&nbsp;<br />e&nbsp;multiplier&nbsp;of&nbsp;production&nbsp;quantity&nbsp;combined&nbsp;with&nbsp;the&nbsp;higher&nbsp;percentage&nbsp;of&nbsp;t&nbsp;<br />otal&nbsp;system&nbsp;cost&nbsp;it&nbsp;represents.&nbsp;<br />Design&nbsp;challenge:&nbsp;<br />·&nbsp;Variable&nbsp;&quot;design&nbsp;margin&quot;&nbsp;to&nbsp;permit&nbsp;tradeoff&nbsp;between&nbsp;product&nbsp;robustness&nbsp;an&nbsp;<br />d&nbsp;aggressive&nbsp;cost&nbsp;optimization.&nbsp;<br />4.&nbsp;System-level&nbsp;requirements&nbsp;<br />In&nbsp;order&nbsp;to&nbsp;be&nbsp;competitive&nbsp;in&nbsp;the&nbsp;marketplace,&nbsp;embedded&nbsp;systems&nbsp;require&nbsp;that&nbsp;<br />&nbsp;the&nbsp;designers&nbsp;take&nbsp;into&nbsp;account&nbsp;the&nbsp;entire&nbsp;system&nbsp;when&nbsp;making&nbsp;design&nbsp;decisi&nbsp;<br />ons.&nbsp;<br />4.1.&nbsp;End-product&nbsp;utility&nbsp;<br />The&nbsp;utility&nbsp;of&nbsp;the&nbsp;end&nbsp;product&nbsp;is&nbsp;the&nbsp;goal&nbsp;when&nbsp;designing&nbsp;an&nbsp;embedded&nbsp;system&nbsp;<br />,&nbsp;not&nbsp;the&nbsp;capability&nbsp;of&nbsp;the&nbsp;embedded&nbsp;computer&nbsp;itself.&nbsp;Embedded&nbsp;products&nbsp;are&nbsp;&nbsp;<br />typically&nbsp;sold&nbsp;on&nbsp;the&nbsp;basis&nbsp;of&nbsp;capabilities,&nbsp;features,&nbsp;and&nbsp;system&nbsp;cost&nbsp;rathe&nbsp;<br />r&nbsp;than&nbsp;which&nbsp;CPU&nbsp;is&nbsp;used&nbsp;in&nbsp;them&nbsp;or&nbsp;cost/performance&nbsp;of&nbsp;that&nbsp;CPU.&nbsp;<br />One&nbsp;way&nbsp;of&nbsp;looking&nbsp;at&nbsp;an&nbsp;embedded&nbsp;system&nbsp;is&nbsp;that&nbsp;the&nbsp;mechanisms&nbsp;and&nbsp;their&nbsp;as&nbsp;<br />sociated&nbsp;I/O&nbsp;are&nbsp;largely&nbsp;defined&nbsp;by&nbsp;the&nbsp;application.&nbsp;Then,&nbsp;software&nbsp;is&nbsp;used&nbsp;&nbsp;<br />to&nbsp;coordinate&nbsp;the&nbsp;mechanisms&nbsp;and&nbsp;define&nbsp;their&nbsp;functionality,&nbsp;often&nbsp;at&nbsp;the&nbsp;le&nbsp;<br />vel&nbsp;of&nbsp;control&nbsp;system&nbsp;equations&nbsp;or&nbsp;finite&nbsp;state&nbsp;machines.&nbsp;Finally,&nbsp;computer&nbsp;&nbsp;<br />hardware&nbsp;is&nbsp;made&nbsp;available&nbsp;as&nbsp;infrastructure&nbsp;to&nbsp;execute&nbsp;the&nbsp;software&nbsp;and&nbsp;int&nbsp;<br />erface&nbsp;it&nbsp;to&nbsp;the&nbsp;external&nbsp;world.&nbsp;While&nbsp;this&nbsp;may&nbsp;not&nbsp;be&nbsp;an&nbsp;exciting&nbsp;way&nbsp;for&nbsp;a&nbsp;<br />&nbsp;hardware&nbsp;engineer&nbsp;to&nbsp;look&nbsp;at&nbsp;things,&nbsp;it&nbsp;does&nbsp;emphasize&nbsp;that&nbsp;the&nbsp;total&nbsp;funct&nbsp;<br />ionality&nbsp;delivered&nbsp;by&nbsp;the&nbsp;system&nbsp;is&nbsp;what&nbsp;is&nbsp;paramount.&nbsp;<br />Design&nbsp;challenge:&nbsp;<br />·&nbsp;Software-&nbsp;and&nbsp;I/O-driven&nbsp;hardware&nbsp;synthesis&nbsp;(as&nbsp;opposed&nbsp;to&nbsp;hardware-drive&nbsp;<br />n&nbsp;software&nbsp;compilation/synthesis).&nbsp;<br />4.2.&nbsp;System&nbsp;safety&nbsp;&amp;&nbsp;reliability&nbsp;<br />An&nbsp;earlier&nbsp;section&nbsp;discussed&nbsp;the&nbsp;safety&nbsp;and&nbsp;reliability&nbsp;of&nbsp;the&nbsp;computing&nbsp;har&nbsp;<br />dware&nbsp;itself.&nbsp;But,&nbsp;it&nbsp;is&nbsp;the&nbsp;safety&nbsp;and&nbsp;reliability&nbsp;of&nbsp;the&nbsp;total&nbsp;embedded&nbsp;sy&nbsp;<br />stem&nbsp;that&nbsp;really&nbsp;matters.&nbsp;The&nbsp;Distributed&nbsp;system&nbsp;example&nbsp;is&nbsp;mission&nbsp;critical&nbsp;<br />,&nbsp;but&nbsp;does&nbsp;not&nbsp;employ&nbsp;computer&nbsp;redundancy.&nbsp;Instead,&nbsp;mechanical&nbsp;safety&nbsp;backup&nbsp;<br />s&nbsp;are&nbsp;activated&nbsp;when&nbsp;the&nbsp;computer&nbsp;system&nbsp;loses&nbsp;control&nbsp;in&nbsp;order&nbsp;to&nbsp;safely&nbsp;sh&nbsp;<br />ut&nbsp;down&nbsp;system&nbsp;operation.&nbsp;<br />A&nbsp;bigger&nbsp;and&nbsp;more&nbsp;difficult&nbsp;issue&nbsp;at&nbsp;the&nbsp;system&nbsp;level&nbsp;is&nbsp;software&nbsp;safety&nbsp;and&nbsp;<br />&nbsp;reliability.&nbsp;While&nbsp;software&nbsp;doesn't&nbsp;normally&nbsp;&quot;break&quot;&nbsp;in&nbsp;the&nbsp;sense&nbsp;of&nbsp;hardwa&nbsp;<br />re,&nbsp;it&nbsp;may&nbsp;be&nbsp;so&nbsp;complex&nbsp;that&nbsp;a&nbsp;set&nbsp;of&nbsp;unexpected&nbsp;circumstances&nbsp;can&nbsp;cause&nbsp;so&nbsp;<br />ftware&nbsp;failures&nbsp;leading&nbsp;to&nbsp;unsafe&nbsp;situations.&nbsp;This&nbsp;is&nbsp;a&nbsp;difficult&nbsp;problem&nbsp;th&nbsp;<br />at&nbsp;will&nbsp;take&nbsp;many&nbsp;years&nbsp;to&nbsp;address,&nbsp;and&nbsp;may&nbsp;not&nbsp;be&nbsp;properly&nbsp;appreciated&nbsp;by&nbsp;n&nbsp;<br />on-computer&nbsp;engineers&nbsp;and&nbsp;managers&nbsp;involved&nbsp;in&nbsp;system&nbsp;design&nbsp;decisions&nbsp;([12]&nbsp;<br />&nbsp;discusses&nbsp;the&nbsp;role&nbsp;of&nbsp;computers&nbsp;in&nbsp;system&nbsp;safety).&nbsp;<br />Design&nbsp;challenges:&nbsp;<br />·&nbsp;Reliable&nbsp;software.&nbsp;<br />·&nbsp;Cheap,&nbsp;available&nbsp;systems&nbsp;using&nbsp;unreliable&nbsp;components.&nbsp;<br />·&nbsp;Electronic&nbsp;vs.&nbsp;non-electronic&nbsp;design&nbsp;tradeoffs.&nbsp;<br />4.3.&nbsp;Controlling&nbsp;physical&nbsp;systems&nbsp;<br />The&nbsp;usual&nbsp;reason&nbsp;for&nbsp;embedding&nbsp;a&nbsp;computer&nbsp;is&nbsp;to&nbsp;interact&nbsp;with&nbsp;the&nbsp;environmen&nbsp;<br />t,&nbsp;often&nbsp;by&nbsp;monitoring&nbsp;and&nbsp;controlling&nbsp;external&nbsp;machinery.&nbsp;In&nbsp;order&nbsp;to&nbsp;do&nbsp;th&nbsp;<br />is,&nbsp;analog&nbsp;inputs&nbsp;and&nbsp;outputs&nbsp;must&nbsp;be&nbsp;transformed&nbsp;to&nbsp;and&nbsp;from&nbsp;digital&nbsp;signal&nbsp;<br />&nbsp;levels.&nbsp;Additionally,&nbsp;significant&nbsp;current&nbsp;loads&nbsp;may&nbsp;need&nbsp;to&nbsp;be&nbsp;switched&nbsp;in&nbsp;&nbsp;<br />order&nbsp;to&nbsp;operate&nbsp;motors,&nbsp;light&nbsp;fixtures,&nbsp;and&nbsp;other&nbsp;actuators.&nbsp;All&nbsp;these&nbsp;requ&nbsp;<br />irements&nbsp;can&nbsp;lead&nbsp;to&nbsp;a&nbsp;large&nbsp;computer&nbsp;circuit&nbsp;board&nbsp;dominated&nbsp;by&nbsp;non-digital&nbsp;<br />&nbsp;components.&nbsp;<br />In&nbsp;some&nbsp;systems&nbsp;&quot;smart&quot;&nbsp;sensors&nbsp;and&nbsp;actuators&nbsp;(that&nbsp;contain&nbsp;their&nbsp;own&nbsp;analog&nbsp;<br />&nbsp;interfaces,&nbsp;power&nbsp;switches,&nbsp;and&nbsp;small&nbsp;CPUS)&nbsp;may&nbsp;be&nbsp;used&nbsp;to&nbsp;off-load&nbsp;interfa&nbsp;<br />ce&nbsp;hardware&nbsp;from&nbsp;the&nbsp;central&nbsp;embedded&nbsp;computer.&nbsp;This&nbsp;brings&nbsp;the&nbsp;additional&nbsp;a&nbsp;<br />dvantage&nbsp;of&nbsp;reducing&nbsp;the&nbsp;amount&nbsp;of&nbsp;system&nbsp;wiring&nbsp;and&nbsp;number&nbsp;of&nbsp;connector&nbsp;con&nbsp;<br />tacts&nbsp;by&nbsp;employing&nbsp;an&nbsp;embedded&nbsp;network&nbsp;rather&nbsp;than&nbsp;a&nbsp;bundle&nbsp;of&nbsp;analog&nbsp;wires.&nbsp;<br />&nbsp;However,&nbsp;this&nbsp;change&nbsp;brings&nbsp;with&nbsp;it&nbsp;an&nbsp;additional&nbsp;computer&nbsp;design&nbsp;problem&nbsp;o&nbsp;<br />f&nbsp;partitioning&nbsp;the&nbsp;computations&nbsp;among&nbsp;distributed&nbsp;computers&nbsp;in&nbsp;the&nbsp;face&nbsp;of&nbsp;a&nbsp;<br />n&nbsp;inexpensive&nbsp;network&nbsp;with&nbsp;modest&nbsp;bandwidth&nbsp;capabilities.&nbsp;<br />Design&nbsp;challenge:&nbsp;<br />·&nbsp;Distributed&nbsp;system&nbsp;tradeoffs&nbsp;among&nbsp;analog,&nbsp;power,&nbsp;mechanical,&nbsp;network,&nbsp;an&nbsp;<br />d&nbsp;digital&nbsp;hardware&nbsp;plus&nbsp;software.&nbsp;<br />4.4.&nbsp;Power&nbsp;management&nbsp;<br />A&nbsp;less&nbsp;pervasive&nbsp;system-level&nbsp;issue,&nbsp;but&nbsp;one&nbsp;that&nbsp;is&nbsp;still&nbsp;common,&nbsp;is&nbsp;a&nbsp;need&nbsp;<br />&nbsp;for&nbsp;power&nbsp;management&nbsp;to&nbsp;either&nbsp;minimize&nbsp;heat&nbsp;production&nbsp;or&nbsp;conserve&nbsp;battery&nbsp;<br />&nbsp;power.&nbsp;While&nbsp;the&nbsp;push&nbsp;to&nbsp;laptop&nbsp;computing&nbsp;has&nbsp;produced&nbsp;&quot;low-power&quot;&nbsp;variants&nbsp;<br />&nbsp;of&nbsp;popular&nbsp;CPUs,&nbsp;significantly&nbsp;lower&nbsp;power&nbsp;is&nbsp;needed&nbsp;in&nbsp;order&nbsp;to&nbsp;run&nbsp;from&nbsp;i&nbsp;<br />nexpensive&nbsp;batteries&nbsp;for&nbsp;30&nbsp;days&nbsp;in&nbsp;some&nbsp;applications,&nbsp;and&nbsp;up&nbsp;to&nbsp;5&nbsp;years&nbsp;in&nbsp;&nbsp;<br />others.&nbsp;<br />Design&nbsp;challenge:&nbsp;<br />·&nbsp;Ultra-low&nbsp;power&nbsp;design&nbsp;for&nbsp;long-term&nbsp;battery&nbsp;operation.&nbsp;<br />5.&nbsp;Life-cycle&nbsp;support&nbsp;<br />Figure&nbsp;2&nbsp;shows&nbsp;one&nbsp;view&nbsp;of&nbsp;a&nbsp;product&nbsp;life-cycle&nbsp;(a&nbsp;simplified&nbsp;version&nbsp;of&nbsp;the&nbsp;<br />&nbsp;view&nbsp;taken&nbsp;by&nbsp;[13]).&nbsp;<br />Figure&nbsp;2.&nbsp;An&nbsp;embedded&nbsp;system&nbsp;lifecycle.&nbsp;<br />