Fairchild 74HCT service manual

An Introduction to and
Comparison of 74HCT TTL
Compatible CMOS Logic

An Introduction to and Comparison of 74HCT TTL Compatible CMOS Logic AN-368

Fairchild Semiconductor
Application Note 368
March 1984

The 54HC/74HC series of high speed CMOS logic is unique
in that it has a sub-family of components, designated
54HCT/74HCT. Generally,when one encounters a 54/74 se­ries number,the following letters designate some speed and
power performance, usually determined by the technology
used. Of course, the letters HC designate high speed CMOS
with the same pinouts and functions as the 54LS/74LS se­ries. The sub-family of HC, called HCT, is nearly identical to
HC with the exception that its input levels are compatible
with TTL logic levels.

This simple difference can, however,lead to some confusion
as to why HCT is needed; how HCT should be used; how it
is implemented; when it should be used; and how its perfor­mance compares to HC or LS. This paper will attempt to an­swer these questions.

It should also be noted that not all HCTs are the same. That
is, HCTs from other vendors may have some characteristics
that are different. Thus, when discussing general character­istics this paper will directly address Fairchild Semiconduc­tor’s 54HCT/74HCT which is compatible with JEDEC stan-
dard 7. Other vendors’ ICs which also meet this standard will
probably have similar characteristics.

WHY DOES HCT EXIST?

Ideally, when a designer sits down to design a low power
high speed system, he would like to use 54HC/74HC, and
CMOS LSI components. Unfortunately, due to system re­quirements he may have to use NMOS microprocessors and
their NMOS or bipolar peripherals or bipolar logic (54S/74S,

54F/74F, 54ALS/74ALS, or 54AS/74AS) because either the
specific function does not exist in CMOS or the CMOS de­vice may not have adequate performance. Since the system
designer still desires to use HC where possible, he will mix
HC with these products. If these devices are specified to be
TTL compatible, incompatibilities may result at the interface
between the TTL, NMOS, etc. and HC.

More specifically, in the case of where a TTL or NMOS out­put may drive an HC input, a specification incompatibility re­sults.

Table1

patible outputs with the input specifications of 54HC/74HC.
Notice that the output high level of a TTL specified device will
not be guaranteed to have a logic high output voltage level
that will be guaranteed to be recognized as a valid logic high
input level by HC. A TTL output will be equal to or greater
than 2.4V, but an HCMOS input needs at least 3.15V. It
should be noted that in an actual application the TTL output
will pull-up probably to about V
and HC will accept voltages as low as 3V as a valid one level
so that in almost all cases there is no problem driving HC
with TTL.

Even with the specified incompatibility, it is possible to im­prove the TTL-CMOS interface without using HCT.
illustrates this solution. By merely tying a pull-up resistor
from the TTL output to V
voltage to go to V
very easily to TTL. This works very well for systems with a
few lines requiring pull-ups, but for many interfacing lines,
HCT will be a better solution.

lists the output drive specifications of TTL com-

minus 2 diode voltages,

CC

Figure 1

, this will force the output high

CC

. Thus, HC can be directly interfaced

CC

AN006751-1

FIGURE 1. Interfacing LS-TTL Outputs to Standard

CMOS Inputs Using a Pull-Up Resistor

© 1998 Fairchild Semiconductor Corporation AN006751 www.fairchildsemi.com

The input high logic level of HC is the only source of incom­patibility. 54HC/74HC can drive TTL easily and its input low
level is TTL compatible. Again referring to

Table 1

, the logic
output of the TTL type device will be recognized to be a valid
logic low (0) level, so there is no incompatibility here.

Table2

shows that the specified output drive of HC is capable of
driving many LS-TTL inputs, so there is no incompatibility
here either (although one should be aware of possible fanout
restrictions similar to that encountered when designing with
TTL).

The question then arises: since only the input high level must
be altered, why not design CMOS logic to be TTL compat­ible? 54HC/74HC was designed to optimize performance in
all areas, and making a completely TTL compatible logic
family would sacrifice significant performance. Most impor­tantly, there is a large loss of AC noise immunity, and there
are speed and/or die size penalties when trying to design for
TTL input levels.

Thus, since it is obvious that there is a need to interface with
TTL and TTL compatible logic, yet optimum performance
would be sacrificed, a limited sub-family of HCT devices was
created. It is completely TTL input compatible, which en­ables guaranteed direct connection of TTL outputs to its in­puts. In addition, HCT still provides many of the other advan­tages of 54HC/74HC.

WHEN TO USE 54HCT/74HCT LOGIC

The 54HCT/74HCT devices are primarily intended to be
used to provide an easy method of interfacing between TTL
compatible microprocessor and associated peripherals and
bipolar TTL logic to 54HC/74HC. There are essentially two
application areas where a designer will want to perform this
interface.

1. The first case is illustrated in

Figure 2

. In this case the
system is a TTL compatible microprocessor. This figure
shows an NS16XXX (any NMOS µP may be substituted)
that is in a typical system and therefore must be inter­faced to 54HC/74HC. In this instance, the popular gate,
buffer, decoder, and flip-flop functions provided in the
54HCT/74HCT sub-family can be used to interface the
many lines that come from TTL compatible outputs. It is
also easy to upgrade this configuration to an

all

CMOS
system once the CMOS version of the microprocessor is
available by replacing the HCT with HC.

2. Asecond application is, when in speed-critical situations
a faster logic element than HC, probably ALS or AS,
must be used in a predominantly 54HC/74HC system, or
a specific logic function unique to TTL is placed into an
HC design. This situation is illustrated in

Figure 3

. In this
case, pull-up resistors on an HC input may be sufficient,
but if not, then an HCT can be used to provide the guar­anteed interface.

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