Page 44 - Embedded Microprocessor Systems Real World Design

Hardware Design I 2

Once the system is designed and the hardware requirements are established, the
next step is to design the actual hardware. Of course, you will document the design
to make life easy for the software engineers, right?
Embedded microprocessors fall into two broad categories: Single-chip embed-
ded solutions with onchip memory like the 8031 and embedded systems using a
microprocessor with external memory and I/O. Examples of these are a 68000-,
80186, or 386EX-based embedded system.
Figure 2.1 shows the simplest single-chip microprocessor designs and multichip
designs. Note that they are basically the same except that the single-chip design has
everything inside the chip (inside the dashed line) and the multichip design has
everything except the processor itself outside.

Single-Chip Designs

Single-chip microprocessors (or microcontrollers) usually provide erasable pro-
grammable read-only memory (EPROM; or ROM or flash memory), random access
memory (RAM), and 1/0 ports. Most also have internal timers, serial interfaces,
or other peripherals. The 1/0 ports are flexible, permitting each bit to be assigned
as input or output.
The actual design of single integrated circuit (IC) systems is straightforward.
Before starting the design, you already know (or should know) that there are suf-
ficient 1/0 port pins, enough internal memory, and sufficient processor speed to
do the job.
A single-IC design often requires an external timebase. This can be a clock from
some master source (such as a higher-level control processor), a crystal, a ceramic
resonator, or even a resistor/capacitor timing circuit on some processors. What you
use depends on your cost requirements and how accurate the timebase needs to
be. If you are using a crystal or resonator, connect it according to the manufac-

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