Page 27 - Embedded Microprocessor Systems Real World Design

the hardware. A RISC microcontroller can execute instructions very fast, but
in a given application it may not be as fast as a CISC (complex instruction set
computer) with an instruction set that can perform complex operations. For
example, multiplying two 16-bit numbers may take one instruction and only a
few clock cycles on a CISC processor or a single cycle on a DSP with multiplier
hardware. On a RISC processor that has no multiply instruction or multiply
hardware, this operation must be implemented in some kind of loop that uses
several instructions and a large number of clock cycles. On the other hand, an
application that does a lot of bit flipping and sensor reading, with little or no
complex math, may get better performance from a RISC processor.
Not evaluating the architecture. The ADSP-2100 family parts from Analog
Devices are DSPs that lend themselves well to embedded applications. These
parts are optimized for signal processing, which means that they have some
powerful data manipulation capabilities such as hardware multiply and barrel
shifters. However, they also have some limitations. Some operations require an
extra instruction to move a value from RAM to a register before it can be
used, whereas other, slower processors allow the value in RAM to be
manipulated or tested directly.
These are typical and by no means unique. Every processor has its quirks, and
these are not dark secrets. You just must understand the data sheets on the part
before you use it. Take the data book or CD-ROM home. Read it. Study the timing
diagrams, especially the worstcase numbers. Understand how everything in your
system will connect to and be controlled by this processor. If you do not understand
something, you are not ready to start the design.

ROMability
This consideration applies only to those devices that execute their programs from
internal ROM. These devices usually are chosen for an application where cost,
rather than being no object, is a key factor. If the finished design is going to be a
very high-volume (thousands per year) product, it may be worthwhile to select a
processor that has a ROM version.
Most engineering projects use EPROM or flash memory for their development
phases. These erasable and reprogrammable memories allow a part to be reused
instead of thrown away. When the part goes to production, the EPROM parts can
be replaced with one-time programmable (OTP) devices. These usually are just
EPROM-based parts in a plastic package with no erasure window. Since the expen-
sive ceramic package and quartz window are not required, the OTP parts are
cheaper than the EPROM parts to manufacture, thus reducing product costs.
If the production volume is high enough, the EPROM part can be replaced with
a mask ROM version. The designer supplies the finished program to the IC man-

12 Embedded Microprocessor System