Page 28 - Embedded Microprocessor Systems Real World Design

ufacturer, who creates a mask for the version of the IC that has an internal ROM.
This provides the lowest production cost. However, the following caveats exist:

There is a mask charge to produce the ROM. This charge is usually several
thousand dollars and is usually tied to a minimum purchase requirement. If
the product volumes are less than expected or (get your risumi ready) a bug
is discovered in the program after the ROM is created, the mask charge is not
recoverable. A new NRE (nonrecurring expense) is required for a new mask,
and all the old parts must be scrapped because the ROM program cannot be
changed.
Some manufacturers are so swamped with mask order requests that they have
stopped accepting them. This can be disastrous if your entire product pricing
strategy is based on the availability of mask ROM parts. A list of these manufac-
turers, even assuming I knew who they all were this week, would be useless by
the time this book reaches print. Check into this before deciding to use a ROM
part.

Even though the production costs are low, the high upfront costs prevent many
designers from using mask ROM parts. If your volume is too low or you know the
design will change before the end of product life, then mask ROMs usually are a
poor choice.
One additional consideration is that not all devices are available in all flavors.
For example, the Motorola 68HC05 series parts are designed for extremely high-
volume applications. Not all parts in the series (and there seem to be more every
month) are available in the EPROM version. Some parts are available only in the
ROM version. Development is done on a similar part for which an EPROM version
is available. The catch is, if you cannot justify the ROM costs, you cannot select
these ROManly devices, and the nearest equivalent EPROM part may be too costly
for your use.
Another example is the 8031 family parts, which are available in EPROM, OTP,
and ROM versions. As of this writing, the cost of the EPROM version is about 10
times the cost of the ROM version, and the OTP is about 60 percent of the EPROM
version, depending, of course, on your volume and where you buy the parts. The
basic ROM 8031 may be the cheapest choice, but if you will not have the volume
to use it, the OTP version of a different processor may be cheaper than the OTP
8031. The device with the lowest cost in a ROM version may not be the cheapest
in the OTP. In addition, for some devices, the OTP is not available. Your choices
are EPROM or ROM, which can make these parts a real cost problem in low-volume
applications. Be sure to research which varieties of a part are available based on
your volume and other product requirements.
Finally, remember that once a design is committed to mask ROM, it has the same
inflexibility as a non-microprocessor-based hardware design. Once you go to ROM,

System Design 13