00000010.htm

水木社区 embeded 版精华区 下载

First&nbsp;a&nbsp;need&nbsp;or&nbsp;opportunity&nbsp;to&nbsp;deploy&nbsp;new&nbsp;technology&nbsp;is&nbsp;identified.&nbsp;Then&nbsp;a&nbsp;p&nbsp;<br />
roduct&nbsp;concept&nbsp;is&nbsp;developed.&nbsp;This&nbsp;is&nbsp;followed&nbsp;by&nbsp;concurrent&nbsp;product&nbsp;and&nbsp;manu&nbsp;<br />
facturing&nbsp;process&nbsp;design,&nbsp;production,&nbsp;and&nbsp;deployment.&nbsp;But&nbsp;in&nbsp;many&nbsp;embedded&nbsp;s&nbsp;<br />
ystems,&nbsp;the&nbsp;designer&nbsp;must&nbsp;see&nbsp;past&nbsp;deployment&nbsp;and&nbsp;take&nbsp;into&nbsp;account&nbsp;support,&nbsp;<br />
&nbsp;maintenance,&nbsp;upgrades,&nbsp;and&nbsp;system&nbsp;retirement&nbsp;issues&nbsp;in&nbsp;order&nbsp;to&nbsp;actually&nbsp;cr&nbsp;<br />
eate&nbsp;a&nbsp;profitable&nbsp;design.&nbsp;Some&nbsp;of&nbsp;the&nbsp;issues&nbsp;affecting&nbsp;this&nbsp;life-cycle&nbsp;profi&nbsp;<br />
tability&nbsp;are&nbsp;discussed&nbsp;below.&nbsp;<br />
5.1.&nbsp;Component&nbsp;acquisition&nbsp;<br />
Because&nbsp;an&nbsp;embedded&nbsp;system&nbsp;may&nbsp;be&nbsp;more&nbsp;application-driven&nbsp;than&nbsp;a&nbsp;typical&nbsp;tec&nbsp;<br />
hnology-driven&nbsp;desktop&nbsp;computer&nbsp;design,&nbsp;there&nbsp;may&nbsp;be&nbsp;more&nbsp;leeway&nbsp;in&nbsp;componen&nbsp;<br />
t&nbsp;selection.&nbsp;Thus,&nbsp;component&nbsp;acquisition&nbsp;costs&nbsp;can&nbsp;be&nbsp;taken&nbsp;into&nbsp;account&nbsp;whe&nbsp;<br />
n&nbsp;optimizing&nbsp;system&nbsp;life-cycle&nbsp;cost.&nbsp;For&nbsp;example,&nbsp;the&nbsp;cost&nbsp;of&nbsp;a&nbsp;component&nbsp;ge&nbsp;<br />
nerally&nbsp;decreases&nbsp;with&nbsp;quantity,&nbsp;so&nbsp;design&nbsp;decisions&nbsp;for&nbsp;multiple&nbsp;designs&nbsp;sh&nbsp;<br />
ould&nbsp;be&nbsp;coordinated&nbsp;to&nbsp;share&nbsp;common&nbsp;components&nbsp;to&nbsp;the&nbsp;benefit&nbsp;of&nbsp;all.&nbsp;<br />
Design&nbsp;challenge:&nbsp;<br />
·&nbsp;Life-cycle,&nbsp;cross-design&nbsp;component&nbsp;cost&nbsp;models&nbsp;and&nbsp;optimization&nbsp;rather&nbsp;th&nbsp;<br />
an&nbsp;simple&nbsp;per-unit&nbsp;cost.&nbsp;<br />
5.2.&nbsp;System&nbsp;certification&nbsp;<br />
Embedded&nbsp;computers&nbsp;can&nbsp;affect&nbsp;the&nbsp;safety&nbsp;as&nbsp;well&nbsp;as&nbsp;the&nbsp;performance&nbsp;the&nbsp;syst&nbsp;<br />
em.&nbsp;Therefore,&nbsp;rigorous&nbsp;qualification&nbsp;procedures&nbsp;are&nbsp;necessary&nbsp;in&nbsp;some&nbsp;syste&nbsp;<br />
ms&nbsp;after&nbsp;any&nbsp;design&nbsp;change&nbsp;in&nbsp;order&nbsp;to&nbsp;assess&nbsp;and&nbsp;reduce&nbsp;the&nbsp;risk&nbsp;of&nbsp;malfunc&nbsp;<br />
tion&nbsp;or&nbsp;unanticipated&nbsp;system&nbsp;failure.&nbsp;This&nbsp;additional&nbsp;cost&nbsp;can&nbsp;negate&nbsp;any&nbsp;sa&nbsp;<br />
vings&nbsp;that&nbsp;might&nbsp;have&nbsp;otherwise&nbsp;been&nbsp;realized&nbsp;by&nbsp;a&nbsp;design&nbsp;improvement&nbsp;in&nbsp;the&nbsp;<br />
&nbsp;embedded&nbsp;computer&nbsp;or&nbsp;its&nbsp;software.&nbsp;This&nbsp;point&nbsp;in&nbsp;particular&nbsp;hinders&nbsp;use&nbsp;of&nbsp;&nbsp;<br />
new&nbsp;technology&nbsp;by&nbsp;resynthesizing&nbsp;hardware&nbsp;components&nbsp;--&nbsp;the&nbsp;redesigned&nbsp;compo&nbsp;<br />
nents&nbsp;cannot&nbsp;be&nbsp;used&nbsp;without&nbsp;incurring&nbsp;the&nbsp;cost&nbsp;of&nbsp;system&nbsp;recertification.&nbsp;<br />
One&nbsp;strategy&nbsp;to&nbsp;minimize&nbsp;the&nbsp;cost&nbsp;of&nbsp;system&nbsp;recertification&nbsp;is&nbsp;to&nbsp;delay&nbsp;all&nbsp;&nbsp;<br />
design&nbsp;changes&nbsp;until&nbsp;major&nbsp;system&nbsp;upgrades&nbsp;occur.&nbsp;As&nbsp;distributed&nbsp;embedded&nbsp;sy&nbsp;<br />
stems&nbsp;come&nbsp;into&nbsp;more&nbsp;widespread&nbsp;use,&nbsp;another&nbsp;likely&nbsp;strategy&nbsp;is&nbsp;to&nbsp;partition&nbsp;<br />
&nbsp;the&nbsp;system&nbsp;in&nbsp;such&nbsp;a&nbsp;way&nbsp;as&nbsp;to&nbsp;minimize&nbsp;the&nbsp;number&nbsp;of&nbsp;subsystems&nbsp;that&nbsp;need&nbsp;&nbsp;<br />
to&nbsp;be&nbsp;recertified&nbsp;when&nbsp;changes&nbsp;occur.&nbsp;This&nbsp;is&nbsp;a&nbsp;partitioning&nbsp;problem&nbsp;affecte&nbsp;<br />
d&nbsp;by&nbsp;potential&nbsp;design&nbsp;changes,&nbsp;technology&nbsp;insertion&nbsp;strategies,&nbsp;and&nbsp;regulato&nbsp;<br />
ry&nbsp;requirements.&nbsp;<br />
Design&nbsp;challenge:&nbsp;<br />
·&nbsp;Partitioning/synthesis&nbsp;to&nbsp;minimize&nbsp;recertification&nbsp;costs.&nbsp;<br />
5.3.&nbsp;Logistics&nbsp;and&nbsp;repair&nbsp;<br />
Whenever&nbsp;an&nbsp;embedded&nbsp;computer&nbsp;design&nbsp;is&nbsp;created&nbsp;or&nbsp;changed,&nbsp;it&nbsp;affects&nbsp;the&nbsp;d&nbsp;<br />
ownstream&nbsp;maintenance&nbsp;of&nbsp;the&nbsp;product.&nbsp;A&nbsp;failure&nbsp;of&nbsp;the&nbsp;computer&nbsp;can&nbsp;cause&nbsp;th&nbsp;<br />
e&nbsp;entire&nbsp;system&nbsp;to&nbsp;be&nbsp;unusable&nbsp;until&nbsp;the&nbsp;computer&nbsp;is&nbsp;repaired.&nbsp;In&nbsp;many&nbsp;cases&nbsp;<br />
&nbsp;embedded&nbsp;systems&nbsp;must&nbsp;be&nbsp;repairable&nbsp;in&nbsp;a&nbsp;few&nbsp;minutes&nbsp;to&nbsp;a&nbsp;few&nbsp;hours,&nbsp;which&nbsp;&nbsp;<br />
implies&nbsp;that&nbsp;spare&nbsp;components&nbsp;and&nbsp;maintenance&nbsp;personnel&nbsp;must&nbsp;be&nbsp;located&nbsp;clos&nbsp;<br />
e&nbsp;to&nbsp;the&nbsp;system.&nbsp;A&nbsp;fast&nbsp;repair&nbsp;time&nbsp;may&nbsp;also&nbsp;imply&nbsp;that&nbsp;extensive&nbsp;diagnosis&nbsp;&nbsp;<br />
and&nbsp;data&nbsp;collection&nbsp;capabilities&nbsp;must&nbsp;be&nbsp;built&nbsp;into&nbsp;the&nbsp;system,&nbsp;which&nbsp;may&nbsp;be&nbsp;<br />
&nbsp;at&nbsp;odds&nbsp;with&nbsp;keeping&nbsp;production&nbsp;costs&nbsp;low.&nbsp;<br />
Because&nbsp;of&nbsp;the&nbsp;long&nbsp;system&nbsp;lifetimes&nbsp;of&nbsp;many&nbsp;embedded&nbsp;systems,&nbsp;proliferation&nbsp;<br />
&nbsp;of&nbsp;design&nbsp;variations&nbsp;can&nbsp;cause&nbsp;significant&nbsp;logistics&nbsp;expenses.&nbsp;For&nbsp;example,&nbsp;<br />
&nbsp;if&nbsp;a&nbsp;component&nbsp;design&nbsp;is&nbsp;changed&nbsp;it&nbsp;can&nbsp;force&nbsp;changes&nbsp;in&nbsp;spare&nbsp;component&nbsp;in&nbsp;<br />
ventory,&nbsp;maintenance&nbsp;test&nbsp;equipment,&nbsp;maintenance&nbsp;procedures,&nbsp;and&nbsp;maintenance&nbsp;<br />
&nbsp;training.&nbsp;Furthermore,&nbsp;each&nbsp;design&nbsp;change&nbsp;should&nbsp;be&nbsp;tested&nbsp;for&nbsp;compatibilit&nbsp;<br />
y&nbsp;with&nbsp;various&nbsp;system&nbsp;configurations,&nbsp;and&nbsp;accommodated&nbsp;by&nbsp;the&nbsp;configuration&nbsp;&nbsp;<br />
management&nbsp;database.&nbsp;<br />
Design&nbsp;challenge:&nbsp;<br />
·&nbsp;Designs&nbsp;optimized&nbsp;to&nbsp;minimize&nbsp;spares&nbsp;inventory.&nbsp;<br />
·&nbsp;High-coverage&nbsp;diagnosis&nbsp;and&nbsp;self-test&nbsp;at&nbsp;system&nbsp;level,&nbsp;not&nbsp;just&nbsp;digital&nbsp;c&nbsp;<br />
omponent&nbsp;level.&nbsp;<br />
5.4.&nbsp;Upgrades&nbsp;<br />
Because&nbsp;of&nbsp;the&nbsp;long&nbsp;life&nbsp;of&nbsp;many&nbsp;embedded&nbsp;systems,&nbsp;upgrades&nbsp;to&nbsp;electronic&nbsp;co&nbsp;<br />
mponents&nbsp;and&nbsp;software&nbsp;may&nbsp;be&nbsp;used&nbsp;to&nbsp;update&nbsp;functionality&nbsp;and&nbsp;extend&nbsp;the&nbsp;lif&nbsp;<br />
e&nbsp;of&nbsp;the&nbsp;embedded&nbsp;system&nbsp;with&nbsp;respect&nbsp;to&nbsp;competing&nbsp;with&nbsp;replacement&nbsp;equipmen&nbsp;<br />
t.&nbsp;While&nbsp;it&nbsp;may&nbsp;often&nbsp;be&nbsp;the&nbsp;case&nbsp;that&nbsp;an&nbsp;electronics&nbsp;upgrade&nbsp;involves&nbsp;compl&nbsp;<br />
etely&nbsp;replacing&nbsp;circuit&nbsp;boards,&nbsp;it&nbsp;is&nbsp;important&nbsp;to&nbsp;realize&nbsp;that&nbsp;the&nbsp;rest&nbsp;of&nbsp;&nbsp;<br />
the&nbsp;system&nbsp;will&nbsp;remain&nbsp;unchanged.&nbsp;Therefore,&nbsp;any&nbsp;special&nbsp;behaviors,&nbsp;interfac&nbsp;<br />
es,&nbsp;and&nbsp;undocumented&nbsp;features&nbsp;must&nbsp;be&nbsp;taken&nbsp;into&nbsp;account&nbsp;when&nbsp;performing&nbsp;the&nbsp;<br />
&nbsp;upgrade.&nbsp;Also,&nbsp;upgrades&nbsp;may&nbsp;be&nbsp;subject&nbsp;to&nbsp;recertification&nbsp;requirements.&nbsp;<br />
Of&nbsp;special&nbsp;concern&nbsp;is&nbsp;software&nbsp;in&nbsp;an&nbsp;upgraded&nbsp;system.&nbsp;Legacy&nbsp;software&nbsp;may&nbsp;no&nbsp;<br />
t&nbsp;be&nbsp;executable&nbsp;on&nbsp;upgraded&nbsp;replacement&nbsp;hardware,&nbsp;and&nbsp;may&nbsp;not&nbsp;be&nbsp;readily&nbsp;cro&nbsp;<br />
ss-compiled&nbsp;to&nbsp;the&nbsp;new&nbsp;target&nbsp;CPU.&nbsp;Even&nbsp;worse,&nbsp;timing&nbsp;behavior&nbsp;is&nbsp;likely&nbsp;to&nbsp;&nbsp;<br />
be&nbsp;different&nbsp;on&nbsp;newer&nbsp;hardware,&nbsp;but&nbsp;may&nbsp;be&nbsp;both&nbsp;undocumented&nbsp;and&nbsp;critical&nbsp;to&nbsp;<br />
&nbsp;system&nbsp;operation.&nbsp;<br />
Design&nbsp;challenge:&nbsp;<br />
·&nbsp;Ensuring&nbsp;complete&nbsp;interface,&nbsp;timing,&nbsp;and&nbsp;functionality&nbsp;compatibility&nbsp;when&nbsp;<br />
&nbsp;upgrading&nbsp;designs.&nbsp;<br />
5.5.&nbsp;Long-term&nbsp;component&nbsp;availability&nbsp;<br />
When&nbsp;embedded&nbsp;systems&nbsp;are&nbsp;more&nbsp;than&nbsp;a&nbsp;few&nbsp;years&nbsp;old,&nbsp;some&nbsp;electronic&nbsp;compone&nbsp;<br />
nts&nbsp;may&nbsp;no&nbsp;longer&nbsp;be&nbsp;available&nbsp;for&nbsp;production&nbsp;of&nbsp;new&nbsp;equipment&nbsp;or&nbsp;replacemen&nbsp;<br />
ts.&nbsp;This&nbsp;problem&nbsp;can&nbsp;be&nbsp;especially&nbsp;troublesome&nbsp;with&nbsp;obsolete&nbsp;processors&nbsp;and&nbsp;&nbsp;<br />
small-sized&nbsp;dynamic&nbsp;memory&nbsp;chips.&nbsp;<br />
When&nbsp;a&nbsp;product&nbsp;does&nbsp;reach&nbsp;a&nbsp;point&nbsp;at&nbsp;which&nbsp;spare&nbsp;components&nbsp;are&nbsp;no&nbsp;longer&nbsp;ec&nbsp;<br />
onomically&nbsp;available,&nbsp;the&nbsp;entire&nbsp;embedded&nbsp;computer&nbsp;must&nbsp;sometimes&nbsp;be&nbsp;redesig&nbsp;<br />
ned&nbsp;or&nbsp;upgraded.&nbsp;This&nbsp;redesign&nbsp;might&nbsp;need&nbsp;to&nbsp;take&nbsp;place&nbsp;even&nbsp;if&nbsp;the&nbsp;system&nbsp;i&nbsp;<br />
s&nbsp;no&nbsp;longer&nbsp;in&nbsp;production,&nbsp;depending&nbsp;on&nbsp;the&nbsp;availability&nbsp;of&nbsp;a&nbsp;replacement&nbsp;sy&nbsp;<br />
stem.&nbsp;This&nbsp;problem&nbsp;is&nbsp;a&nbsp;significant&nbsp;concern&nbsp;on&nbsp;the&nbsp;Distributed&nbsp;example&nbsp;syste&nbsp;<br />
m.&nbsp;<br />
Design&nbsp;challenge:&nbsp;<br />
·&nbsp;Cost-effectively&nbsp;update&nbsp;old&nbsp;designs&nbsp;to&nbsp;incorporate&nbsp;new&nbsp;components.&nbsp;<br />
6.&nbsp;Business&nbsp;model&nbsp;<br />
The&nbsp;business&nbsp;models&nbsp;under&nbsp;which&nbsp;embedded&nbsp;systems&nbsp;are&nbsp;developed&nbsp;can&nbsp;vary&nbsp;as&nbsp;w&nbsp;<br />
idely&nbsp;as&nbsp;the&nbsp;applications&nbsp;themselves.&nbsp;Costs,&nbsp;cycle&nbsp;time,&nbsp;and&nbsp;the&nbsp;role&nbsp;of&nbsp;pro&nbsp;<br />
duct&nbsp;families&nbsp;are&nbsp;all&nbsp;crucial&nbsp;business&nbsp;issues&nbsp;that&nbsp;affect&nbsp;design&nbsp;decisions.&nbsp;<br />
6.1.&nbsp;Design&nbsp;vs.&nbsp;production&nbsp;costs&nbsp;<br />
Design&nbsp;costs,&nbsp;also&nbsp;called&nbsp;Non-Recurring&nbsp;Engineering&nbsp;costs&nbsp;(NRE),&nbsp;are&nbsp;of&nbsp;majo&nbsp;<br />
r&nbsp;importance&nbsp;when&nbsp;few&nbsp;of&nbsp;a&nbsp;particular&nbsp;embedded&nbsp;system&nbsp;are&nbsp;being&nbsp;built.&nbsp;Conve&nbsp;<br />
rsely,&nbsp;production&nbsp;costs&nbsp;are&nbsp;important&nbsp;in&nbsp;high-volume&nbsp;production.&nbsp;Embedded&nbsp;sy&nbsp;<br />
stems&nbsp;vary&nbsp;from&nbsp;single&nbsp;units&nbsp;to&nbsp;millions&nbsp;of&nbsp;units,&nbsp;and&nbsp;so&nbsp;span&nbsp;the&nbsp;range&nbsp;of&nbsp;&nbsp;<br />
tradeoffs&nbsp;between&nbsp;design&nbsp;versus&nbsp;production&nbsp;costs.&nbsp;<br />
At&nbsp;the&nbsp;low-volume&nbsp;end&nbsp;of&nbsp;the&nbsp;spectrum,&nbsp;CAD&nbsp;tools&nbsp;can&nbsp;help&nbsp;designers&nbsp;complete&nbsp;<br />
&nbsp;their&nbsp;work&nbsp;with&nbsp;a&nbsp;minimum&nbsp;of&nbsp;effort.&nbsp;However,&nbsp;at&nbsp;the&nbsp;high-volume&nbsp;end&nbsp;of&nbsp;the&nbsp;<br />
&nbsp;spectrum&nbsp;the&nbsp;designs&nbsp;may&nbsp;be&nbsp;simple&nbsp;enough&nbsp;and&nbsp;engineering&nbsp;cost&nbsp;such&nbsp;a&nbsp;small&nbsp;<br />
&nbsp;fraction&nbsp;of&nbsp;total&nbsp;system&nbsp;cost&nbsp;that&nbsp;extensive&nbsp;hand-optimization&nbsp;is&nbsp;performed&nbsp;<br />
&nbsp;in&nbsp;order&nbsp;to&nbsp;reduce&nbsp;production&nbsp;costs.&nbsp;<br />
CAD&nbsp;tools&nbsp;may&nbsp;be&nbsp;able&nbsp;to&nbsp;outperform&nbsp;an&nbsp;average&nbsp;engineer&nbsp;at&nbsp;all&nbsp;times,&nbsp;and&nbsp;a&nbsp;&nbsp;<br />
superior&nbsp;engineer&nbsp;on&nbsp;very&nbsp;large&nbsp;designs&nbsp;(because&nbsp;of&nbsp;the&nbsp;limits&nbsp;of&nbsp;human&nbsp;capa&nbsp;<br />
city&nbsp;to&nbsp;deal&nbsp;with&nbsp;complexity&nbsp;and&nbsp;repetition).&nbsp;However,&nbsp;in&nbsp;small&nbsp;designs&nbsp;some&nbsp;<br />
&nbsp;embedded&nbsp;computer&nbsp;designers&nbsp;believe&nbsp;that&nbsp;a&nbsp;superior&nbsp;human&nbsp;engineer&nbsp;can&nbsp;outp&nbsp;<br />
erform&nbsp;CAD&nbsp;tools.&nbsp;In&nbsp;the&nbsp;Small&nbsp;system&nbsp;example&nbsp;a&nbsp;programmer&nbsp;squeezed&nbsp;software&nbsp;<br />
&nbsp;into&nbsp;a&nbsp;few&nbsp;hundred&nbsp;bytes&nbsp;of&nbsp;memory&nbsp;by&nbsp;hand&nbsp;when&nbsp;the&nbsp;compiler&nbsp;produced&nbsp;overl&nbsp;<br />
y&nbsp;large&nbsp;output&nbsp;that&nbsp;needed&nbsp;more&nbsp;memory&nbsp;than&nbsp;was&nbsp;available.&nbsp;It&nbsp;can&nbsp;readily&nbsp;be&nbsp;<br />
&nbsp;debated&nbsp;whether&nbsp;CAD&nbsp;tools&nbsp;or&nbsp;humans&nbsp;are&nbsp;&quot;better&quot;&nbsp;designers,&nbsp;but&nbsp;CAD&nbsp;tools&nbsp;f&nbsp;<br />
ace&nbsp;skepticism&nbsp;in&nbsp;areas&nbsp;that&nbsp;require&nbsp;extraordinary&nbsp;optimization&nbsp;for&nbsp;size,&nbsp;pe&nbsp;<br />
rformance,&nbsp;or&nbsp;cost.&nbsp;<br />
Design&nbsp;challenge:&nbsp;<br />
·&nbsp;Intelligently&nbsp;trade&nbsp;off&nbsp;design&nbsp;time&nbsp;versus&nbsp;production&nbsp;cost.&nbsp;<br />
6.2.&nbsp;Cycle&nbsp;time&nbsp;<br />
The&nbsp;cycle&nbsp;time&nbsp;between&nbsp;identification&nbsp;of&nbsp;a&nbsp;product&nbsp;opportunity&nbsp;and&nbsp;product&nbsp;d&nbsp;<br />
eployment&nbsp;(also&nbsp;called&nbsp;Time&nbsp;to&nbsp;Market)&nbsp;can&nbsp;be&nbsp;quite&nbsp;long&nbsp;for&nbsp;embedded&nbsp;system&nbsp;<br />
s.&nbsp;In&nbsp;many&nbsp;cases&nbsp;the&nbsp;electronics&nbsp;are&nbsp;not&nbsp;the&nbsp;driving&nbsp;force;&nbsp;instead,&nbsp;product&nbsp;<br />
&nbsp;schedules&nbsp;are&nbsp;driven&nbsp;by&nbsp;concerns&nbsp;such&nbsp;as&nbsp;tooling&nbsp;for&nbsp;mechanical&nbsp;components&nbsp;&nbsp;<br />
and&nbsp;manufacturing&nbsp;process&nbsp;design.&nbsp;Superficially,&nbsp;this&nbsp;would&nbsp;seem&nbsp;to&nbsp;imply&nbsp;th&nbsp;<br />
at&nbsp;design&nbsp;time&nbsp;for&nbsp;the&nbsp;electronics&nbsp;is&nbsp;not&nbsp;an&nbsp;overriding&nbsp;concern,&nbsp;but&nbsp;this&nbsp;is&nbsp;<br />
&nbsp;only&nbsp;partially&nbsp;true.&nbsp;<br />
Because&nbsp;the&nbsp;computer&nbsp;system&nbsp;may&nbsp;have&nbsp;the&nbsp;most&nbsp;malleable&nbsp;design,&nbsp;it&nbsp;may&nbsp;absor&nbsp;<br />
b&nbsp;the&nbsp;brunt&nbsp;of&nbsp;changes.&nbsp;For&nbsp;example,&nbsp;redesign&nbsp;of&nbsp;hardware&nbsp;was&nbsp;required&nbsp;on&nbsp;th&nbsp;<br />
e&nbsp;Mission&nbsp;Critical&nbsp;example&nbsp;system&nbsp;when&nbsp;it&nbsp;was&nbsp;found&nbsp;that&nbsp;additional&nbsp;sensors&nbsp;&nbsp;<br />
and&nbsp;actuators&nbsp;were&nbsp;needed&nbsp;to&nbsp;meet&nbsp;system&nbsp;performance&nbsp;goals.&nbsp;On&nbsp;the&nbsp;Small&nbsp;exa&nbsp;<br />
mple&nbsp;system,&nbsp;delays&nbsp;in&nbsp;making&nbsp;masked&nbsp;ROM&nbsp;changes&nbsp;in&nbsp;order&nbsp;to&nbsp;revise&nbsp;software&nbsp;<br />
&nbsp;dominate&nbsp;concerns&nbsp;about&nbsp;modifications&nbsp;(and&nbsp;programmable&nbsp;memory&nbsp;is&nbsp;too&nbsp;expen&nbsp;<br />