075)
3600
= 0.2618mA. (4)
isensor: Current drawn by continuously active sensors. Its value depends on
which sensors are kept continuously active. The current consumption for
the sensors used on the XSM appear in Table 2.
erepr: Energy spent in one wireless reprogramming. One reprogramming of
55 kByte program (the size of ExScal program) requires every XSM to
stay awake for approximately 45 minutes. Assuming the XSMs are not in
the LPL mode during reprogramming, just staying awake for 45 minutes
consumes 18 mA-hr of energy11. The reprogramming of 55 kByte requires
11 Keeping XSMs in the LPL mode during reprogramming will cause a higher energy
consumption because the XSMs will need to send long preambles, consuming
275 Chapter 11 Maximizing the Lifetime of an Always-On WSN Application
Santosh Kumar, Anish Arora, and Ten H. Lai
the most constrained XSM to send out 1,942 packets, where each packet
has 29 bytes of data. Assuming a retransmission factor of 1.43, approximately
2,771 packets are sent by the most energy constrained XSM. One
packet transmission takes 20 ms and consumes an extra current of 8 mA.
Therefore, the additional energy spent in transmission of 2771 packets
is 0.12 mA-hr (8*2771*0.02/3600.). Writing 55 kByte to flash takes less
than four seconds. Each second, it consumes 18.4 mA of current. So, total
energy consumed in flash writing is less than 0.02 mA-hr. Adding up the
energy consumed in staying awake for 45 minutes (18 mA-hr), in transmissions
(0.
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