11 Layout

11.1 Layout Guidelines

When using multiple-input and multiple-channel logic devices inputs must not ever be left floating. In many

cases, functions or parts of functions of digital logic devices are unused; for example, when only two inputs of

a triple-input AND gate are used. Such unused input pins must not be left unconnected because the undefined

voltages at the outside connections result in undefined operational states. All unused inputs of digital logic

devices must be connected to a logic high or logic low voltage, as defined by the input voltage specifications, to

prevent them from floating. The logic level that must be applied to any particular unused input depends on the

function of the device. Generally, the inputs are tied to GND or VCC, whichever makes more sense for the logic

function or is more convenient.

11.2 Layout Example

1

2

3

4

5

6

7

14

13

12

11

10

9

8

1A

1B

1C

1D

1Y

GND

V

CC

2D

2C

2B

2A

2Y

GND

V

CC

NC

NC

0.1

F

Unused inputs

tied to VCC

Bypass capacitor

placed close to

the device

Avoid 90°

corners for

signal lines

Recommend GND flood fill for

improved signal isolation, noise

reduction, and thermal dissipation

Figure 11-1. Example layout for the SN74HC21

SN74HC21, SN54HC21

SCLS087F – DECEMBER 1982 – REVISED JUNE 2021

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