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Synthèse de haut-niveau de contrôleurs ultra-faible consommation ...

Synthèse de haut-niveau de contrôleurs ultra-faible consommation ...

tel-00553143, version 1

tel-00553143, version 1 - 6 Jan 2011 138 Experimental setup and results 6.6 Effects of low duty-cycle and overall energy gain As mentioned earlier, a WSN-node has a very low active duty-cycle. If we look at Tables 6.2 through Table 6.5, they show the energy gains of the proposed technique during the active period only, whereas the overall energy gain of a micro-task-based WSN node can be obtained if complete time period of a task activation and the static energy dissipation during stand-by mode is considered. Figure 6.8 shows the simplified version of time distribution for sendFrame task activation having a wake-up time period of 100 ms which means that our transmitting node is sending a packet to the RF transceiver through SPI-bus every 100 ms. Such a task can be considered as highly reactive w.r.t. WSN application domain. It can be can clearly observed that the node has a pretty low duty cycle (even less than 1% of the whole time period). The switching delays of the MSP430 are taken from its datasheet whereas those of the micro-task block from the Section 6.1. The overall energy gain of our approach over the MSP430 can be presented by the following expression: Gaintot = where � Eactmsp 1 Eactmsp + (Pstbymsp × Tstbymsp ) + ( Pactmsp 2 × (Tonmsp + Toffmsp )) Eactmt + (Pslpmt × Tslpmt) + ( 1 2Pactmt × (Tonmt + Toffmt)) is the dynamic energy of the MSP430 (given in Table 6.1), � Pstbymsp is the static power consumption of the MSP430, � Tstbymsp is the time spent in standby mode by the MSP430, � Pactmsp is the dynamic power of the MSP430 (given in Table 6.1), � Tonmsp � Toffmsp � Eactmt � Pslpmt is the turn-on delay of the MSP430 (given in [129]), is the turn-off delay of the MSP430 (given in [129]), is the dynamic energy of the micro-task (given in Table 6.2) is the static power consumption of the micro-task, � Tslpmt is the time spent in sleep mode by the micro-task, � Pactmt is the dynamic power of the micro-task (given in Table 6.2), � Tonmt is the turn-on delay of the micro-task (c.f. Section 6.1), � Toffmt is the turn-off delay of the micro-task (c.f. Section 6.1). The MSP430F21x2 consumes approximately 1.54 µW in stand-by mode having a 512 Bytes of RAM. The static power of a micro-task in power-gated mode depends on the size of its RAM. We considered the same static power consumption (as that of the MSP430) for a micro-task and just scaled it down since a micro-task needs much smaller global memory (6 Bytes on average, see Table 6.7). Hence, average static power consumption of a micro-task would be around 18 nW. (6.1)

tel-00553143, version 1 - 6 Jan 2011 Effects of low duty-cycle and overall energy gain 139 Considering this static power, a time period of 100 ms and using the expression derived above, we calculated the overall energy gain for sendFrame micro-task over a complete period of task-activation. As a result, we gain approximately 138 x over the MSP430. To conclude, as our system uses much lower power in sleep mode (thanks to power gating) and relatively shorter output switching delays than a low-power MCU (e.g. the MSP430), one to two orders of savings in energy are obtained when a complete period of a WSN task activation is considered. With these results, we close our discussion on experimental setup and results obtained for static and dynamic power consumptions of hardware the micro-tasks and the SM generated through our design-flows. In next chapter, we conclude the work presented in this thesis and draw some future research directions.

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