Embedded Systems September 2000 Vol13_10

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FIGURE 7 State machine for the soda machine system 1/null, CR/ CHR, CS/CD 1/ null, CR/ CHR CS/ null 1/ null, CR/ CHR ~ 1/LIGHT I COKE_OUT / SYS_INIT =ON \ ,___ ___ Q/ null / SYS_INIT=ON _.\ COKE_OUT 1 Q/ null =FALSE ~71N_COUNT I l-------1~1 =FALSE f SYS INIT=ON 1 COKE_OUT J Q/ null 1.~_1NIT=:~ ~fiN_COUN/ ~~) l-----..... 1 =FALS COKE OUT E Q/ null CS/ null done is define a rule for a function for this software system. This process is rooted in the mathematics of function theory, which maps a domain (valid inputs or stimuli to a system) to a range (the correct responses) within a co-domain (all possible responses) . See Figure 8. Referring back to the use case described earlier, you will notice that we can map the behavior from this use case to parts, but not all, of the enu- meration. Certain parts of the enu- meration describe the story being told by the use case. The sequence enu- meration goes a step furth er, defining the complete behavior that was origi- nally described in the story told in the use case. This is where the synergy between use cases and sequence enu- meration becomes apparent. Use cases and scenarios are an effective tool to decide what we have to do for the various stakeholders of the system. The sequence enumera- tion drives home the completeness 126 SEPTEMBER 2ooo Embedded Systems Programming and consistency that will give your developers a clear picture of what to do in all circumstances of use, as well as resolve any inconsistencies between the stakeholders. Rob Oshana is an engineering manager in esp the Software Develojnnent Systems group at Texas Instruments. He has over 18 years of experience in the management and develojJ- ment of real-time embedded systems. He also teaches requirements and design courses at Southern Methodist University. His e-mail address is oshana@airmail. net. FIGURE 8 A fundion mĀ·aps' il domai'ri to ? r(!ng~ within a co-domain /- I I a b c d e g h Software system :' I I I FUNCTION f(n) / I I I I I / DOMAIN

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