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MEMORY
INTERFACING
Table
6-1. Common Logic Families*
74xxx The original
TIL
family. Now obsolete.
74Lxxx Low-power version of standard
TTL.
Very slow and now obsolete.
74LSxxx Low-power Schottky
TIL.
Lower power and higher speed than standard
TIL.
Widely
used
in
microprocessor systems.
74Sxxx Schottky
TIL.
High speed and high power consumption. Now obsoleted by newer
families.
74ALSxxx
Advanced low-powe'r Schottky
TIL.
An
improved version of
LS
TIL,
providing faster
speed, lower power, and better-defined specifications.
74ASxxx Advanced Schottky
TIL.
A replacement for Schottky
TIL,
with higher speeds and
lower power.
74Fxxx Fairchild Advanced Schottky
TIL.
A competitor of
AS.
There are also several families of CMOS logic that are pin-compatible with the
TIL
families and use
the same type number designations.
CMOS logic has the advantage of very low power consump-
tion. Unlike
TIL,
however, the power consumption of CMOS logic increases linearly with switching
frequency.
There are several major families of
CMOS logic,
as
follows:
74Cxxx
CMOS equivalents of standard
TIL
devices, but considerably slower. Obsoleted by
newer families.
74HCxxx High-sPeed
CMOS logic. Speeds comparable to
LS
TIL.
74HCTxxx
High-speed
CMOS logic with TIL-compatible input thresholds. Widely used as a low-
power
LS
TIL
replacement.
74ACxxx Advanced
CMOS,
logic, with higher speeds than
HC.
This nomenclature
is
used for
several families, including Fairchild's
FACT
family and the ACL family from
TI,
Signet-
ics, and Phillips.
74ACTxxx
Same
as
74AC, with TIL-compatible thresholds
*Microprocessor Based Design. Michael
Slater; © 1987PLD Devices
6.2.2
PLD
Devices
Many design examples in this manual use PLDs (Programmable Logic Devices) and
EPLDs (Erasable Programmable Logic Devices), which can be programmed
by
the user
to implement. random logic. A
PLD device can be used
as
a state machine or a signal
decoder, for example. The advantages
of
PLDs include the followjng:
1.
PLD pinout
is
determined
by
the designer, which can simplify board layout
by
mov-
ing signals as required.
2.
PLDs are inexpensive
as
compared to dedicated bus controllers.
6-3