Page 31 - Embedded Microprocessor Systems Real World Design

applications. In most of these cases, the cost requirements of the design will keep
you away from high-current solutions anyway.
The second area in which power is a consideration is in battery-operated equip
ment. In some cases, you must choose a microprocessor with a specific maximum
current to match the battery. In other cases, you must pick a microprocessor with
a reasonable current requirement and then pick a battery to match. In either case,
you need to know the total operational current.
A related issue is sleep current. Many microprocessors have a low-power mode
of operation in which the CPU goes to “sleep,” turning off internal peripherals to
conserve power. Some microcontrollers have very low current in this mode; on
others it saves so little power that I’ve wondered why the manufacturer bothered
with it. Either way, you need to get an estimate on the amount of time the system
will spend in this mode to have a good handle on battery life.

Environmental Requirements
For the purpose of choosing a microprocessor, environmental requirements typi-
cally translate into temperature. If your design must operate over an extended tem-
perature range, such as designs for military or automotive purposes, your choices
of available parts are more limited than if you have normal industrial temperature
requirements. Note that extended temperature devices are nearly always more
expensive, so don’t base your cost estimates on the industrial parts if you really need
high-temperature parts.

Life Cycle Costs
Are you making a VCR or a piece of industrial equipment? If you are making a
VCR, you probably don’t need to consider the need to reprogram the unit in the
field or worry about long-term availability of replacement parts. VCRs are throw-
away consumer items. On the other hand, if you are building some kind of indus-
trial equipment that costs thousands of dollars and will be operating for many years,
you have a different set of considerations. You must pick a processor and/or
memory architecture that can be upgraded. You probably also want to design in
some excess program memory so you will have room for upgrades, and you might
make long-term availability of the microprocessor more important than cost.
Life cycle costs are also a factor on the front end of a design. The more widgets
you will produce, the more upfront development cost you can stand. If you are
selling VCRs, you might pick a very cheap microcontroller and spend a lot of money
developing the software, making the software do everything to save on hardware
costs. If you are building an expensive piece of industrial equipment, you may sell
only a few thousand over the life of the product. In that case, you want to minimize
the development cost. In addition, your product cost is not likely to be as sensitive
to the electronics cost. In that case, you would probably pick a processor that has

16 Embedded Microprocessor Systems