Page 279 - Embedded Microprocessor Systems Real World Design

Design Problems

Buying an off-the-shelf CPU means someone else has verified the design. However,
if subtle timing problems turn up in the hardware, you are dependent on the board
vendor to admit they exist and fix them. You have no schematics, programmable
logic device (PLD) equations, or the other information necessary to debug the
design yourself. And you do not want to; that is why you chose to buy instead
of build.

Some Solutions to These Problems

Some of these problems have been addressed and have solutions, but they all make
the resulting system a little less compatible with the PC:

BIOS. Kits are available that allow you to write a basic 1/0 system (BIOS) that
eliminates the keyboard, monitor, and other standard peripherals.
DOS in ROM. Although obsolete for desktop PCs, DOS and its variants still find
occasional use in embedded systems. Development kits are available from com-
panies such as Annasoft that allow DOS and your applications to be placed in
PROM or flash memory, eliminating the requirement for a disk drive. However,
not all operating systems can run from ROM or without a disk drive.
Passive Backplane. The problem of parts availability sometimes can be solved by
using a passive backplane. Essentially, this consists of the expansion slots from a
PC motherboard without the motherboard. A CPU board plugs into one of the
expansion slots; other standard boards can plug into the other slots. While these
backplane/CPU board combinations typically are more expensive than a clone
motherboard produced to the tune of 100,000 per month, they solve the problem
of not being able to buy the same board twice. But these boards are not perfect-
they still depend on availability of parts, such as PC chip sets, that may go out of
production.
RTOS. Real-time (that is, deterministic) operating systems that emulate DOS are
available. Of course, all of them do not work exact4 like DOS, which can cause
problems. Some, however, are close enough to DOS that they advertise as being
able to run Windows (or they did, before Windows 95/98/2000 replaced
Windows 3.1). One problem with using a non-DOS, non-Windows operating
system is that you will not always find drivers for every peripheral chipset for every
RTOS. For instance, you may find that one vendor’s motherboard uses an
Ethernet chipset for which your RTOS vendor has no driver. Using an RTOS
in a PC environment means you must make sure there is a match between the
PC hardware and the RTOS. In addition, if your hardware becomes obsolete, you

260 Embedded Macrqproc~~sor System