Page 56 - The Art of Designing Embedded Systems

Stop Writing Sig Programs 43

the NRE. Smaller technology companies often act like cowboys and
figure that NRE is just the cost of doing business; if we are prof-
itable, then the product’s price somehow (!) reflects all engineering
expenses.
Increasing NRE costs drives up the product’s price (most likely
making it less competitive and thus reducing profits), or directly re-
duces profits.
Making an NRE versus COGS decision requires a delicate bal-
ancing act that deeply mirrors the nature of your company’s product
pricing. A $1 electronic greeting card cannot stand any extra com-
ponents; minimize COGS above all. In an automobile the quantities
are so large that engineers agonize over saving a foot of wire. The
converse is a one-off or short-production-run device. The slightest
development hiccup costs tens of thousands-easily-which will
have to be amortized over a very small number of units.
Sometimes it’s easy to figure the tradeoff between NRE and
COGS. You should also consider the extra complication of opportu-
nity costs-”If I do this, then what is the cost of not doing that?” As
a young engineer I realized that we could save about $5000 a year by
changing from EPROMS to masked ROMs. I prepared a careful
analysis and presented it to my boss, who instantly turned it down
because making the change would shut down my other engineering
activities for some time. In this case we had a tremendous backlog of
projects, any of which could yield more revenue than the measly $5K
saved. In effect, my boss’s message was, “You are more valuable
than what we pay you.” (That’s what drives entrepreneurs into busi-
ness-the hope they can get the extra money into their own pockets!)

Follow these guidelines to be successful in simplifying software
through multiple CPUs:
Break out nasty real-time hardware functions into independent
CPUs. Do interrupts come at 1000/second from a device? Partition
it to a controller and offload all of that ISR overhead from the main
processor.
Think microcontrollers, not microprocessors. Controllers are in-
herently limited in address space, which helps keep firmware size
under control. Controllers are cheap (some cost less than 40 cents
in quantity). Controllers have everything you need on one chip-
RAM, ROM, 110, etc.