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

1CLR

1D

1PRE

1Q

1Q

GND

VCC

2CLR

2D

2PRE

2Q

2Q

GND

VCC

2CLK

1CLK

0.1

F

Unused input

tied to GND

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

Unused output

left floating

Unused inputs

tied to VCC

Figure 11-1. Example layout for the SN74HC74

SN74HC74, SN54HC74

SCLS094F – DECEMBER 1982 – REVISED JUNE 2021

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