8 Detailed Description

8.1 Overview

This device contains four independent 2-input NOR gates. Each gate performs the Boolean function Y = A + B

in positive logic.

8.2 Functional Block Diagram

xA

xB

xY

Figure 8-1. Logic Diagram (Positive Logic) for the CD74HCT02

8.3 Balanced CMOS Push-Pull Outputs

This device includes balanced CMOS push-pull outputs. The term "balanced" indicates that the device can sink

and source similar currents. The drive capability of this device may create fast edges into light loads so routing

and load conditions should be considered to prevent ringing. Additionally, the outputs of this device are capable

of driving larger currents than the device can sustain without being damaged. It is important for the output power

of the device to be limited to avoid damage due to overcurrent. The electrical and thermal limits defined in the

Absolute Maximum Ratings must be followed at all times.

Unused push-pull CMOS outputs should be left disconnected.

8.4 TTL-Compatible CMOS Inputs

This device includes TTL-compatible CMOS inputs. These inputs are specifically designed to interface with TTL

logic devices by having a reduced input voltage threshold.

TTL-compatible CMOS inputs are high impedance and are typically modeled as a resistor in parallel with the

input capacitance given in the Electrical Characteristics. The worst case resistance is calculated with the

maximum input voltage, given in the Absolute Maximum Ratings, and the maximum input leakage current, given

in the Electrical Characteristics, using Ohm's law (R = V ÷ I).

TTL-compatible CMOS inputs require that input signals transition between valid logic states quickly, as defined

by the input transition time or rate in the Recommended Operating Conditions table. Failing to meet this

specification will result in excessive power consumption and could cause oscillations. More details can be found

in Implications of Slow or Floating CMOS Inputs.

Do not leave TTL-compatible CMOS inputs floating at any time during operation. Unused inputs must be

terminated at VCC or GND. If a system will not be actively driving an input at all times, a pull-up or pull-down

resistor can be added to provide a valid input voltage during these times. The resistor value will depend on

multiple factors, however a 10-kΩ resistor is recommended and will typically meet all requirements.

8.5 Clamp Diode Structure

The inputs and outputs to this device have both positive and negative clamping diodes as depicted in Electrical

Placement of Clamping Diodes for Each Input and Output.

CAUTION

Voltages beyond the values specified in the Absolute Maximum Ratings table can cause damage to

the device. The input and output voltage ratings may be exceeded if the input and output clamp-

current ratings are observed.

CD54HCT02, CD74HCT02

SCHS400 – DECEMBER 2020

www.ti.com

8

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