Page 135 - The Art of Designing Embedded Systems

122 THE ART OF DESIGNING EMBEDDED SYSTEMS

memory on any conventional device programmer, but, since there’s no
window, you can never erase it. When it’s time to change the code, you’ll
toss the part out.
Intel sold OTP versions of their EPROMs many years ago, but they
never caught on. A system that uses discrete memory devices-RAM,
ROM, and the like-has intrinsically higher costs than one based on a mi-
crocontroller. In a system with $100 of parts, the extra dollar or two needed
to use erasable EPROMs (which are very forgiving of mistakes) is small.
The dynamics are a bit different with a minimal system. If the entire
computer is contained in a $2 part, adding a buck for a window is a huge
cost hit. OTP starts to make quite a bit of sense, assuming your code will
be stable.
This is not to diminish Flash memory, which has all of the benefits of
OTP, though sometimes with a bit more cost.
Using either technology, the code can be cast in concrete in small ap-
plications, since the entire program might require only tens to hundreds of
statements. Though I have to plead guilty to one or two disasters where it
seemed there were more bugs than lines of code, a program this small,
once debugged and thoroughly tested, holds little chance of an obscure
bug. The risk of going with OTP is pretty small.
You can’t pick up a magazine without reading about “time to mar-
ket.” Managers want to shrink development times to zero. One obvious so-
lution is to replace masked ROMs with their OTP equivalents, as
producing a processor with the code permanently engraved in a metaliza-
tion layer takes months . . . and suffers from the same risk factors as does
OTP. The masked part might be a bit cheaper in high volumes, but this
price advantage doesn’t help much if you can’t ship while waiting for parts
to come in.
Part of the art of managing a business is to preserve your options as
long as possible. Stuff happens. You can’t predict everything. Given op-
tions, even at the last minute, you have the flexibility to adapt to problems
and changing markets. For example, some companies ship multiple ver-
sions of a product, differing only in the code. A Flash or OTP part lets
them make a last-minute decision, on the production floor, about how
many of a particular widget to build. If you have a half million dollars tied
up in inventory of masked parts, your options are awfully limited.
Part of the 805 1’s success came from the wide variety of parts avail-
able. You could get EPROM or masked versions of the same part. Low-
volume applications always took advantage of the EPROM version. OTP
reduces the costs of the parts significantly, even when you’re only build-
ing a handful.