NXP Semiconductors HEF4521BT,652

Overview

HEF4521BT,652 from Nexperia is a 24-stage CMOS binary frequency divider with integrated crystal/RC oscillator circuitry, operating from 3 V to 15 V supply. It delivers precise division ratios up to 2²⁴ = 16,777,216, features asynchronous master reset (MR), and provides seven buffered output stages (Q18–Q24) for flexible timing distribution in low-power clock trees.

For engineers reviewing the HEF4521BT,652 datasheet, HEF4521BT,652 pinout, HEF4521BT,652 application, or HEF4521BT,652 equivalent, key selection criteria include its wide VDD range (3–15 V), low-power crystal oscillator capability, static operation, -40 °C to +85 °C industrial temperature rating, and SO16 package compatibility with legacy 4000-series logic designs.

Technical Context

The HEF4521BT,652 implements a cascaded chain of 24 toggle flip-flops, advancing on the HIGH-to-LOW transition of the A2 clock input. Its input circuit supports three modes: crystal oscillator (with VSS1/VDD1 bias resistors), external oscillator buffer, or RC oscillator (using A1/A2 with external R/C network).

Each stage divides by two, enabling programmable division factors from 2¹⁸ (262,144) to 2²⁴ (16,777,216) via dedicated outputs Q18–Q24. The device operates fully statically, with propagation delays scaling inversely with VDD (e.g., A2→Q18 typ. 950 ns at 5 V, 220 ns at 15 V) and supports dynamic power calculation using supply voltage, input/output frequencies, and load capacitance.

Key Specifications

Pinout & Package

HEF4521BT,652 is housed in a plastic small outline package (SO16), SOT109-1, with 16 leads and 3.9 mm body width. Pin 1 is marked by a notch or dot; pin numbering follows standard SO16 convention (counterclockwise from top-left corner).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 24-stage frequency divider applications.

Compared with CD4521BE and 74HC4060, the HEF4521BT,652 uniquely combines full 24-bit division, dedicated high-order outputs, and integrated low-power crystal oscillator control - making it the only option for precision long-interval timing where power, pinout, and division depth are simultaneously constrained.

Availability

HEF4521BT,652 is available at Aetrix Electronics and suitable for industrial timer systems, legacy equipment clock synthesis, low-power remote sensor nodes, and multi-rate communication clocking requiring stable component supply across extended product lifecycles.

Supply support for HEF4521BT,652 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

Nexperia is a global semiconductor expert specializing in high-performance, reliable logic, analog, and discrete components for automotive, industrial, and consumer applications.

The HEF4521B belongs to Nexperia's legacy-compatible CMOS logic family, designed specifically for robust, low-power timing and frequency division in industrial control, instrumentation, and retro-design replacement applications.

FAQ

What oscillator configurations does the HEF4521BT,652 support?

The HEF4521BT,652 supports three distinct oscillator modes: (1) crystal oscillator using A2/Y2 with external crystal and VSS1/VDD1 bias resistors; (2) external oscillator buffer using A1/Y1; and (3) RC oscillator using A1/A2 with external resistor and capacitor. Each mode is selected by wiring configuration - no internal register programming is required.

Can unused inputs be left floating?

No. All unused inputs (including A1, A2, MR, Y1, Y2) must be tied to VDD, VSS, or another valid logic level. Floating CMOS inputs cause increased power consumption, potential oscillation, and reduced noise immunity due to undefined gate thresholds - this is explicitly prohibited in the datasheet's General Description section.

How is the master reset (MR) function implemented?

MR is an asynchronous, active-HIGH input that forces all 24 flip-flops to logic LOW immediately, independent of the A2 clock edge. Its minimum pulse width is 15 ns at 15 V supply, and recovery time after deassertion is 0 ns - meaning counting resumes on the next valid A2 HIGH-to-LOW transition without delay or metastability risk.

What is the purpose of separate VSS1 and VDD1 pins?

VSS1 and VDD1 provide isolated bias paths for the internal crystal oscillator circuitry, reducing noise coupling from the main logic supply. When used with external resistors, they enable low-power crystal operation by limiting current through the crystal - a feature absent in standard 4000-series dividers like CD4521BE.