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always cause pipeline stalls. Arithmetic instructions with register operands cost 1 cycle,
and memory instructions or memory addressing cost 2 processing cycles.
9.5.2 16-bit MSP430 Microcontroller
The 16-bit MSP430F1611 produced by Texas Instruments differs in many ways from
the Atmel chip. The MSP430 has a more traditional architecture that uses mainly
memory-to-memory operations rather than the more classic RISC load-store approach
of the ATmega128. Its instruction set is limited to 27 instructions, but the variety of 7
different addressing modes offers a lot of flexibility in data manipulation. This orthog-
onal architecture allows every instruction to be used with every addressing mode. An
interesting feature of this CPU is the existence of an external hardware multiplier that
performs fast multiplication of 8- and 16-bit integers. The MSP430 provides 12 gen-
eral-purpose registers. The remaining 4 registers, r0-r3, are used for Program Counter,
Stack Pointer, Status Register, and Constant Generator, respectively. Registers r4-r15
are general purpose and available for use at all times. This CPU also offers 10 KB of
RAM, but only 48 KB of ROM, which might be a problem in the case of large pro-
grams. A very nice feature of the MSP430 is its ultra low power consumption, which
is especially important on tiny devices in distributed environments where the available
energy is very limited.
9.5.3 Marvell PXA271
The 32-bit Marvell PXA271 processor is built around the ARM core and differs com-
pletely from the two previously described platforms. It is a much more advanced and
powerful CPU, with superior capabilities. In addition, the memory resources on this
processor are a few orders of magnitude higher than on the other two devices (32 MB
of ROM and 256 KB of RAM). This range of CPUs is commonly used in smartphones
(e.g., the iPhone) and PDA class devices and opens new possibilities when used in the
WSN environment. The ARM processor has a standard RISC load-store architecture,
with several innovative features, including effectively free shifting of operands at no
extra cost (using a built-in barrel shifter) and conditional execution of instructions. It
has 16 registers, although 3 of these are reserved for special purposes—r13 is a stack
pointer, r14 holds a function return address, and r15 is the program counter. If needed,
the content of r14 can be pushed onto the stack, and an extra register is then available
until the end of the called function. The PXA271 offers full 32-bit processing, and its
clock rate can be changed dynamically by adjusting the input voltage. This interesting
feature permits a tradeoff between energy consumption and processing power during
the CPU operation.
Table 9.3 brings together many important parameters of popular WSN platforms.
This table shows that most of the popular sensor platforms have limited computational
and storage capacity. Hence, implementing security on such resource-constrained
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