Page 41 - ARM Based Microcontroller Projects Using MBED

3.3 THE ARM PROCESSOR ARCHITECTURE 27
execute more instructions for a given task. Normally, each instruction in a RISC-based design
occupies one memory location. These instructions are fetched from the memory and are ex-
ecuted in one cycle each as fast as possible. RISC-based CPU designs also allow for several
levels of pipelining where the next instructions are fetched from the memory while the cur-
rent instruction is being executed, thus resulting in higher throughput. Also, because the
RISC-based designs are simpler, they consume less power which is extremely important in
battery operated mobile applications, such as in mobile phones, tablets, GPS systems, or
games consoles. This is perhaps the major reason why the ARM architecture is widely used
in mobile devices requiring very fast processing times as well as low power consumptions.

3.3.2 Why ARM?
The choice of a microcontroller for a particular application depends on many factors such
as the following:
• cost
• speed
• power consumption
• size
• number of digital and analog input-output ports
• digital input-output port current capacity
• analog port resolution and accuracy
• program and data memory sizes
• interrupt support
• timer support
• USART support
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• special bus support (e.g., USB, CAN, SPI, I C, and so on)
• ease of system development (e.g., programming)
• working voltage
For example, if we need to develop a battery powered portable device such as a mobile
phone, a tablet, or a games console then high clock speed as well as long battery life are
the main requirements. But if on the other hand, for example, we wish to develop a liquid
level control system or a heating control system then high speed or low power consumption
are not among the desired requirements. In general, as the clock speed goes up so does the
power consumption and as a result a trade-off should be made in choosing a microcontroller
for a specific application.
ARM processors are based on an instruction set called Thumb. With clever design this in-
struction set takes 32-bit instructions and compresses them down to 16-bits, thus reduces the
hardware size, which also reduces the overall cost and the power consumption. The processor
makes use of multistage pipelined architecture that is easier to learn, build, and program.
ARM’s core architecture is only a processor and it does not include graphics, input-output
ports, USB, serial communication, wireless connectivity, or any other form of peripheral mod-
ules. Chip manufacturers build their systems around the ARM core design and this is why
different manufacturers offer different types of ARM-based microcontrollers.