Page 30 - Embedded Microprocessor Systems Real World Design

von Neumann Architecture Harvard Architecture
Single memory space

1E( iingle path for both code and data
limits performance but permits Sepamte &e and data
spaces aHow both to be
intermixing code and data. accessed simunanaously
bul require two address
and data paths.

Figure 1.3
Von Neumann versus Harvard Architecture.

implemented with a small number of pointers that can retrieve data from the code
space and with the inclusion of immediate instructions where a byte (or word) of
data is included in the instruction itself. Many singlechip microcontrollers use
the Harvard architecture, among them the 8031, Microchip PIC series, and Atmel
AVR9OS series. Figure 1.3 shows the relative characteristics of the von Neumann
and Harvard architectures.
The advantage of the Harvard architecture is that there are two separate memory
areas and often two separate data paths, so code and data can be fetched simulta-
neously, increasing the throughput of the processor. From an embedded system
point of view, the difference between the architectures is important if compiled
data tables are needed. For example, a stepper motor controller may have a number
of ramp tables embedded in the code space.
If you choose a processor with a modified Harvard architecture, be sure the table
lookup features of the instruction set will not bog down the code. If you are con-
sidering an 8031 for this application, you will find that it has several registers that
can be used as pointers into the data RAM but only one register (DPTR) that can
be used as a pointer into the code PROM. An application that must simultaneously
use two tables in PROM constantly switches DPTR between the two pointers. One
solution to this is to move one or both tables into RAM, but then you must make
sure enough additional RAM is available to hold the tables.

Power Requirements
In some designs, power is not an issue-you just put in whatever power supply you
need to run whatever the electronics requires. There are two areas in which power
is normally an issue. The first is designs that have a power restriction, such as the
need to operate from a wall-mounted power supply as found in many consumer

System Design 15