Hardware Reference
In-Depth Information
In the previous sections we saw how to implement truth tables and other sim-
ple circuits using individual gates. In practice, few circuits are actually constructed
gate-by-gate anymore, although this once was common. Nowadays, the usual
building blocks are modules containing a number of gates. In the following sec-
tions we will examine these building blocks more closely and see how they are
used and how they can be constructed from individual gates.
Gates are not manufactured or sold individually but rather in units called
Inte-
grated Circuits
, often called
ICs
or
chips
. An IC is a rectangular piece of silicon
of varied size depending on how many gates are required to implement the chip's
components. Small dies will measure about 2 mm
×
2 mm, while larger dies can be
as large as 18 mm
18 mm. ICs are mounted into plastic or ceramic packages that
can be much larger than the dies they house, if many pins are required to connect
the chip to the outside world. Each pin connects to the input or output of some
gate on the chip or to power or to ground.
Figure 3-10 shows a number of common IC packages used for chips today.
Smaller chips, such as those used to house microcontrollers or RAM chips, will
use
Dual Inline Packages
or
DIPs
. A DIP is a package with two rows of pins that
fit into a matching socket on the motherboard. The most common DIP packages
have 14, 16, 18, 20, 22, 24, 28, 40, 64, or 68 pins. For large chips, square packages
with pins on all four sides or on the bottom are often used. Two common packages
for larger chips are
Pin Grid Arrays
or
PGAs
and
Land Grid Arrays
or
LGAs
.
PGA have pins on the bottom of the package, which fit into a matching socket on
the motherboard. PGA sockets often utilize a zero-insertion-force mechanism in
which the PGA can be placed into the socket without force, then a lever can be
thrown which will apply lateral pressure to all of the PGA's pins, holding it firmly
in the PGA socket. LGAs, on the other hand, have small flat pads on the bottom of
the chip, and an LGA socket will have a cover that fits over the LGA and applies a
downward force on the chip, ensuring that all of the LGA pads make contact with
the LGA socket pads.
Because many IC packages are symmetric in shape, figuring out which orienta-
tion is correct is a perennial problem with IC installation. DIPs typically have a
notch in one end which matches a corresponding mark on the DIP socket. PGAs
typically have one pin missing, so if you attempt to insert the PGA into the socket
incorrectly, the PGA will not insert. Because LGAs do not have pins, correct in-
stallation is enforced by placing a notch on one or two sides of the LGA, which
matches a notch in the LGA socket. The LGA will not enter the socket unless the
two notches match.
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