NXP Semiconductors N74F521D,623

Overview

N74F521D,623 from NXP Semiconductors (formerly Philips Semiconductors) is an 8-bit identity comparator IC that performs bit-for-bit equality comparison between two parallel 8-bit words and asserts a low-active QA=B output when matched. It features 7.0 ns typical propagation delay, 24 mA typical supply current at VCC = 5 V, and operates across 0°C to +70°C. It is used in address decoding, bus arbitration, and memory-mapped I/O validation circuits.

For engineers reviewing the N74F521D,623 datasheet, N74F521D,623 pinout, N74F521D,623 application, or N74F521D,623 equivalent, key selection considerations include its active-low expandable enable (IA=B), TTL-compatible input/output levels, 20-pin SO package, and ripple/parallel expansion capability for wider word comparisons.

Technical Context

The N74F521D,623 implements a combinatorial identity comparison function using fast F-series TTL logic. Its internal architecture evaluates all eight A/B bit pairs simultaneously and gates the result with the IA=B expansion input, enabling cascaded multi-byte comparisons without external logic.

It supports both ripple expansion (daisy-chained QA=B outputs) and parallel expansion (shared IA=B line), allowing flexible scaling to 16-, 24-, or 32-bit comparisons. The device requires no clock or control sequencing - operation is fully asynchronous and level-sensitive.

Key Specifications

Pinout & Package

Package: 20-lead plastic small outline (SO20), body width 7.5 mm, SOT163-1 footprint - surface-mount compatible with standard reflow profiles.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar identity comparator applications.

Compared with SN74F688N and 74ACT521M, the N74F521D,623 offers optimal balance of speed, TTL compatibility, and SO-package suitability for space-constrained commercial designs - especially where active-Low assertion and ripple expansion are required.

Availability

N74F521D,623 is available at Aetrix Electronics and suitable for memory decoding, bus arbitration, ROM validation, and I/O configuration matching requiring stable component supply and long-term industrial availability.

Supply support for N74F521D,623 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

NXP Semiconductors is a global semiconductor company formed from the spin-off of Philips Semiconductors in 2006, specializing in secure connectivity solutions and automotive, industrial, and IoT applications.

The 74F logic family - including the N74F521D,623 - was designed for high-speed commercial-grade digital systems requiring reliable TTL-compatible performance with improved power-speed trade-offs over earlier 74LS series.

FAQ

What is the primary function of the N74F521D,623?

The N74F521D,623 is an 8-bit identity comparator that outputs a low signal (QA=B) when all corresponding bits of two 8-bit input words (A0–A7 and B0–B7) match exactly. It uses TTL-compatible logic levels and supports expansion for wider comparisons. Its core role is deterministic, zero-latency equality detection in digital control and addressing circuits.

Does the N74F521D,623 require a clock signal to operate?

No, the N74F521D,623 is a purely combinatorial device with no internal clock or sequential elements. It responds asynchronously to changes on its A/B inputs or IA=B enable line, producing QA=B output within 7.0 ns typical propagation delay. This makes it suitable for real-time, cycle-critical applications like address decoding and bus arbitration.

How is the N74F521D,623 expanded for comparing more than 8 bits?

The N74F521D,623 supports two expansion modes: ripple expansion connects QA=B of one device to IA=B of the next, propagating match status serially; parallel expansion ties all IA=B inputs together and ORs QA=B outputs externally. Both methods preserve the 7 ns per-stage delay and maintain full 8-bit parallel evaluation per device.

What is the recommended pull-up configuration for the QA=B output of the N74F521D,623?

The QA=B output of the N74F521D,623 is a totem-pole output capable of sinking 20 mA but not sourcing current. A pull-up resistor to VCC (typically 1–4.7 kΩ) is required to establish a valid HIGH logic level. For driving multiple TTL inputs, a 1 kΩ pull-up ensures VIH compliance while limiting current to ~5 mA per stage in cascaded configurations.

Is the N74F521D,623 compatible with modern 3.3 V logic systems?

The N74F521D,623 is specified for 4.5–5.5 V operation and TTL input thresholds (VIH = 2.0 V min, VIL = 0.8 V max). While it may function with 3.3 V inputs in some cases, it is not guaranteed - VIH exceeds 3.3 V × 0.7 = 2.31 V, risking marginal HIGH recognition. Use level translators or select 74ACT/74LVC alternatives for native 3.3 V compatibility.