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Copyright Ó 2001 All Rights Reserved.
March 2001
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There are a number of potential system advantages to using one processor for both
real-time control and signal processing tasks:
Reduce board space
Lower system power consumption
Lower system cost
Provide additional functionality to the system
Simplify system development
To many Microcontroller engineers a DSP is an unfamiliar entity. Microcontrollers have
regular instruction sets, either accumulator based or register based, with a friendly
program model. "Friendly" doesn't mean that the MCU will buy you a coke; "friendly" in
this case translates into either lots of general-purpose registers or a small number of
specialized ones. DSPs have lots of specialized registers and multiple buses, a concept
that makes DSPs more difficult to program compared to a microcontroller's more
register-friendly architecture.
A software engineer would much rather stick with what is a known entity; that is, an
engineer familiar with microcontrollers will use a microcontroller to do light-duty DSP
tasks. If a microcontroller is available that has all the features and memory needed to
perform control tasks while also providing enough performance to satisfy the signal
processing requirements, the engineer will favor this solution. If the two critical issues of
price/performance and development tool quality are met, the microcontroller would be
the first choice for the application, especially if the engineer is already familiar with the
development tools and/or the microcontroller. If the engineer is experienced (i.e. he's
been screwed by a vendor before), then the full processor product definition discussed
earlier, as well as the company's reputation in the embedded market, will already have
been considered.
Signal processing performance in a microcontroller can be enhanced in a number of
different ways:
A fast multiply instruction, or even more preferable a hardware multiply,
allows certain filters to be implemented more efficiently.
Regular cycle execution, that is, all instructions execute in the same number
of internal clock cycles, greatly enhances deterministic behavior.
Programmable interrupt controllers are useful in control applications; if
signal processing is also being performed, effective use of a programmable
interrupt control unit insures that signal processing has both interrupt priority
and permission levels. 
A DMA helps to keep data flowing efficiently through the data paths and can
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