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In the previous
chapter, we discussed

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how to encode information
as sequences of bits.

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In this chapter, we
turn our attention

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to finding a useful
physical representation

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for bits, our first
step in building devices

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that can process information.

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So, what makes a
good bit, i.e., what

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properties do we want our
physical representation of bits

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to have?

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Well, we’ll want a lot of them.

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We expect to carry
billions of bits

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around with us,
e.g., music files.

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And we expect to have access
to trillions of additional bits

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on the web for
news, entertainment,

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social interactions, commerce
— the list goes on and on.

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So we want bits to be
small and inexpensive.

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Mother Nature has a suggestion:
the chemical encoding embodied

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in DNA, where sequences of the
nucleotides Adenine, Thymine,

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Guanine and Cytosine
form codons that

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encode genetic
information that serve

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as the blueprint for
living organisms.

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The molecular scale meets our
size requirements and there’s

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active research underway on how
to use the chemistry of life

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to perform interesting
computations on a massive

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scale.

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We’d certainly like our bits
to be stable over long periods

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of time — “once
a 0, always a 0!”

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The Rosetta Stone, shown here
as part of its original tablet

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containing a decree from
the Egyptian King Ptolemy V,

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was created in 196 BC and
encoded the information needed

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for archeologists
to start reliably

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deciphering Egyptian
hieroglyphics almost 2000

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years later.

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But, the very property
that makes stone engravings

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a stable representation
of information

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makes it difficult to
manipulate the information.

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Which brings us to the final
item on our shopping list:

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we’d like our representation of
bits to make it easy to quickly

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access, transform,
combine, transmit and store

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the information they encode.

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Assuming we don’t want to carry
around buckets of gooey DNA

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or stone chisels, how
should we represent bits?

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With some engineering we
can represent information

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using the electrical
phenomenon associated

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with charged particles.

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The presence of
charged particles

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creates differences in
electrical potential energy

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we can measure as voltages, and
the flow of charged particles

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can be measured as currents.

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We can also encode information
using the phase and frequency

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of electromagnetic fields
associated with charged

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particles — these latter
two choices form the basis

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for wireless communication.

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Which electrical phenomenon
is the best choice

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depends on the
intended application.

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In this course, we’ll use
voltages to represent bits.

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For example, we might choose
0V to represent a 0-bit and 1V

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to represent a 1-bit.

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To represent
sequences of bits we

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can use multiple
voltage measurements,

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either from many
different wires,

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or as a sequence of voltages
over time on a single wire.

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A representation using
voltages has many advantages:

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electrical outlets provide an
inexpensive and mostly reliable

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source of electricity and,
for mobile applications,

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we can use batteries
to supply what we need.

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For more than a century, we’ve
been accumulating considerable

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engineering knowledge about
voltages and currents.

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We now know how to build very
small circuits to store, detect

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and manipulate voltages.

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And we can make those circuits
run on a very small amount

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of electrical power.

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In fact, we can
design circuits that

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require close to zero power
dissipation in a steady state

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if none of the encoded
information is changing.

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However, a voltage-based
representation

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does have some challenges:

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voltages are easily affected by
changing electromagnetic fields

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in the surrounding environment.

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If I want to transmit
voltage-encoded information

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to you, we need to be
connected by a wire.

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And changing the
voltage on a wire

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takes some time since the
timing of the necessary flow

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of charged particles is
determined by the resistance

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and capacitance of the wire.

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In modern integrated circuits,
these RC time constants

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are small, but sadly not zero.

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We have good engineering
solutions for these challenges,

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so let’s get started!