NXP Semiconductors 74HCT646DB,118

Overview

74HCT646DB,118 from Nexperia is an octal bus transceiver/register with 3-state outputs, designed for bidirectional data flow control between two 8-bit buses in TTL-compatible systems. It features independent A/B registers, real-time and stored data multiplexing, direction-controlled transceivers, and operates at VCC = 4.5–5.5 V with 85 MHz max clock frequency. It is used in microprocessor bus interfacing and memory-mapped I/O expansion.

For engineers reviewing the 74HCT646DB,118 datasheet, 74HCT646DB,118 pinout, 74HCT646DB,118 application, or 74HCT646DB,118 equivalent, key selection criteria include propagation delay (13 ns typ. An→Bn), 3-state enable/disable timing (21–40 ns), clock pulse width (16 ns min.), register edge-triggered clocks (CPAB/CPBA), and HCT-level TTL input compatibility.

Technical Context

The 74HCT646DB,118 implements dual 8-bit D-type flip-flop registers synchronized to LOW-to-HIGH clock edges on CPAB and CPBA, enabling independent latching of A- or B-bus data. Its transceiver logic uses DIR to set data direction and OE to enable/disable outputs while preserving input functionality during isolation.

Real-time (transparent) and registered data paths are selected via SAB/SBA inputs, allowing dynamic switching between live bus signals and stored values. All control inputs - DIR, OE, SAB, SBA, CPAB, CPBA - are TTL-compatible (VIH = 2.0 V min., VIL = 0.8 V max.) and support operation across −40 °C to +125 °C.

Key Specifications

Pinout & Package

74HCT646DB,118 is housed in a 24-pin SSOP (Shrink Small Outline Package) with 0.65 mm pitch, JEDEC MO-153 compliant, body width 5.3 mm, and thermal resistance θJA ≈ 150 K/W.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar octal bus transceiver/register applications.

Compared with 74HCT646DB,118, the SO24 variant offers easier handling and thermal derating margin, while the PDIP version provides maximum mechanical reliability and zero reflow constraints - both retain identical register behavior, timing, and pinout but differ solely in package geometry and thermal performance.

Availability

74HCT646DB,118 is available at Aetrix Electronics and suitable for microprocessor bus expansion, memory-mapped I/O interface, and industrial PLC backplane interface requiring stable component supply, long-term lifecycle assurance, and guaranteed traceability.

Supply support for 74HCT646DB,118 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 leader in discrete, logic, and PowerMOS semiconductors, spun off from NXP in 2017 and focused on automotive, industrial, computing, and consumer markets.

The 74HCT646DB,118 belongs to Nexperia's 74HCT logic family - engineered for TTL-compatible operation in noise-sensitive, thermally demanding applications where reliability and pin-for-pin upgrade paths from legacy 74LS are required.

FAQ

What is the maximum operating temperature range for the 74HCT646DB,118?

The 74HCT646DB,118 is rated for operation from −40 °C to +125 °C, meeting industrial and extended-temperature automotive requirements. This range is validated per Nexperia's qualification standards and applies to all DC and AC specifications in the official datasheet. The 74HCT646DB,118 maintains full functionality-including register retention, 3-state timing, and propagation delay-across this entire range without derating.

Does the 74HCT646DB,118 support true bidirectional data flow without external control logic?

Yes, the 74HCT646DB,118 supports fully autonomous bidirectional data flow using its DIR and OE inputs. When OE is LOW, DIR selects direction: LOW enables A→B transfer, HIGH enables B→A transfer. The 74HCT646DB,118 internally gates outputs to prevent contention, requiring no external logic for safe direction switching.

Can the 74HCT646DB,118 operate with a 3.3 V supply?

No, the 74HCT646DB,118 is specified only for VCC = 4.5 V to 5.5 V. Its TTL-compatible inputs require VIH ≥ 2.0 V, but the internal circuitry is not characterized below 4.5 V. For 3.3 V systems, use the 74LVC646 or 74ALVC646 families instead - the 74HCT646DB,118 will not meet timing or drive specifications at 3.3 V.

How does the 74HCT646DB,118 handle simultaneous clock edges on CPAB and CPBA?

The 74HCT646DB,118 treats CPAB and CPBA as independent clock inputs: a LOW-to-HIGH edge on CPAB latches A-bus data into the B-register, while a separate edge on CPBA latches B-bus data into the A-register. Both can occur simultaneously without conflict, enabling synchronized dual-bus snapshot capture - a feature confirmed in the functional diagram and timing waveforms of the Nexperia datasheet for 74HCT646DB,118.

Is the 74HCT646DB,118 pin-compatible with the 74HC646DB,118?

Yes, the 74HCT646DB,118 is pin-compatible and functionally identical to the 74HC646DB,118, differing only in input threshold levels (TTL vs. CMOS) and AC timing. Both share identical pinout, package, register architecture, and control logic. The 74HCT646DB,118 substitutes directly where TTL-level inputs or tighter timing margins are needed - no PCB changes are required.