SN54/74HCT
CMOS Logic Family
Applications and Restrictions
SCLA011
May 1996
1
IMPORTANT NOTICE
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Copyright
©
1996, Texas Instruments Incorporated
2
Contents
Title
Page
Introduction
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
TTL/HC Interface
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
Operating Voltages of HCT Circuits
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
Noise of HCT Circuits
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
Power Consumption of HCT Circuits
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
Delay Times
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
Bergeron Analysis
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
Summary
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
List of Illustrations
Figure
1
2
3
4
5
6
7
8
9
10
11
12
Title
Page
TTL-CMOS Interface With Open-Collector Output and Pullup Resistor
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
Noise Margin
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
Specified Noise Margins for HC, HCT, and TTL Devices
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
Crosstalk (First Case)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
Crosstalk (Second Case)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
Input-Stage Structure of HC and HCT Circuits
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
Supply Current as a Function of the Input Voltage
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
Current Consumption as a Function of Frequency
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
Bergeron Diagram, SN74ALS245 Driver
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
Line Reflections, SN74ALS245 Driver
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
Bergeron Diagram, SN74HC245 Driver
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
Line Reflections, SN74HC245 Driver
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
iii
Introduction
This report describes applications, features, and system design of the SN54/74HCT high-speed CMOS family. To simplify
interfacing of TTL outputs to high-speed CMOS inputs, Texas Instruments (TI) introduced HCT circuits, a subgroup of its HC
family. HCT features and functions are identical to HC devices with the exception of modified input circuitry, which makes
the input threshold voltage compatible with TTL circuits. HCT outputs are similar to the HC family.
TTL/HC Interface
TTL output voltages and HC input voltages are incompatible, especially between the TTL high-level output voltage (V
OH
)
and the HC high-level input voltage (V
IH
). This problem can be solved in three different ways. The first way is to use HCT
devices with their TTL-compatible input voltages to interface between TTL and HC circuits. Another solution is to provide
pullup resistors at the TTL outputs to ensure an adequate high-level TTL output voltage. The third method requires the use
of level shifters.
Of the three alternatives mentioned, using HCT circuits to solve the incompatibility problem, is the most convenient. Designed
to meet the requirements of this application, HCT devices allow the engineer to benefit from the advantages of HC devices
(low power consumption) without using discrete components, such as pullup resistors.
+5 V
Rp
Figure 1. TTL-CMOS Interface With Open-Collector Output and Pullup Resistor
Using pullup resistors to accommodate TTL output signals to interface with HC input circuits (see Figure 1), the design
engineer has to choose the resistance that is appropriate for the application. The minimum value of the resistor is determined
by the maximum current I
OL
that a TTL circuit can supply at the low-level output (V
OL
).
R
p
min
+
V I max
*
V I min
)
n
CC
OL
OL
IL
(1)
where n is the number of HC inputs to be driven, and I
IL
is their input current. I
IL
, having a value of only a few nanoamperes,
is negligible in all calculations.
In the case of a SN74ALS03, the following equation defines R
p
min:
R
p
min
+
5.5 V
*
0.4 V
+
640
W
8 mA
+
V n
*
VI min
CC
IH
IH
(2)
To calculate the upper limit of this resistor, a sufficient V
IH
high level must be ensured.
R
p
max
(3)
In this situation, the input current of HC devices is negligible and very high values also are obtained.
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