NXP Semiconductors 74HC646D,653

Overview

74HC646D,653 from Nexperia is an octal bus transceiver/register with 3-state outputs, designed for bidirectional data flow control between two 8-bit buses. It features independent A/B registers, multiplexed real-time and stored data paths, and supports clocked data latching (CPAB/CPBA), direction control (DIR), output enable (OE), and source selection (SAB/SBA). It operates at VCC = 2.0–6.0 V, delivers 11 ns typical propagation delay (An/Bn to Bn/An, 5 V), and is used in microcontroller system bus interfacing and memory-mapped I/O expansion.

For engineers reviewing the 74HC646D,653 datasheet, 74HC646D,653 pinout, 74HC646D,653 application, or 74HC646D,653 equivalent, key selection considerations include its dual-register architecture, non-inverting data path, 3-state bus driver capability, clock-edge-triggered storage, and compatibility with 74HCT logic levels in mixed-voltage systems.

Technical Context

The 74HC646D,653 integrates eight D-type flip-flops per bus (A and B), enabling synchronous capture of data on LOW-to-HIGH clock transitions at CPAB (A→B) or CPBA (B→A). Its DIR input determines data direction during active OE (LOW), while SAB/SBA select between transparent (real-time) or registered (stored) data sources.

It supports three operational modes: isolation (OE = HIGH), real-time transceiver (OE = LOW + DIR = L/H), and register storage (clock edges with OE HIGH or LOW). Input functions remain enabled even when outputs are in high-impedance state, allowing concurrent data sampling and storage without bus contention.

Key Specifications

Pinout & Package

74HC646D,653 is housed in a 24-pin SO24 (Small Outline) package, 7.5 mm body width, with standard JEDEC MS-013AC outline and 1.27 mm pitch.

Key Features

Applications

Equivalent & Alternatives

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

Compared with 74HC646D,653, the 74HCT646D,653 offers improved DC noise immunity in mixed-logic systems but trades off speed; the SN74LVTH16646DGGR provides scalable width and voltage translation but lacks pin compatibility and adds layout complexity.

Availability

74HC646D,653 is available at Aetrix Electronics and suitable for microcontroller bus expansion, memory-mapped I/O interface, dual-port register buffering, and legacy system bus isolation requiring stable component supply and long-term industrial availability.

Supply support for 74HC646D,653 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 high-volume production of discrete, logic, and PowerMOS semiconductors, spun off from NXP's Standard Products division in 2017 and focused on automotive, industrial, computing, and consumer markets.

The 74HC646D,653 belongs to Nexperia's 74HC logic family, engineered for high-speed, low-power CMOS operation in general-purpose digital systems requiring robust noise immunity and broad supply voltage range.

FAQ

What is the function of the SAB and SBA pins on the 74HC646D,653?

The SAB (Select A-to-B) and SBA (Select B-to-A) pins on the 74HC646D,653 determine whether real-time bus data or registered (latched) data is routed to the output. When SAB = HIGH, real-time A-bus data appears on the B-bus outputs; when SAB = LOW, stored data from the B-register is output. Similarly, SBA controls source selection for A-bus outputs. This enables flexible data arbitration without changing clock or direction signals.

Does the 74HC646D,653 support operation at 3.3 V?

Yes, the 74HC646D,653 supports VCC = 2.0 V to 6.0 V, making it fully functional at 3.3 V. At 3.3 V, typical propagation delay increases to ~15 ns (An/Bn to Bn/An), and maximum clock frequency reduces to ~45 MHz. All DC and AC specifications remain valid per the 74HC family datasheet, with no derating required for 3.3 V use in commercial temperature ranges.

How does the 74HC646D,653 differ from the 74HC648?

The 74HC646D,653 and 74HC648 share identical pinout and register functionality, but the 74HC646D,653 has non-inverting data paths, whereas the 74HC648 inverts data during transceiver operation. This means that for the same input pattern, the 74HC646D,653 outputs identical logic levels on the destination bus, simplifying timing closure and eliminating need for complementary logic in signal paths.

Can the 74HC646D,653 drive standard TTL loads?

Yes, the 74HC646D,653 can drive standard TTL loads due to its bus-driver output stage, rated for IOH = −7.8 mA and IOL = 7.8 mA at VCC = 4.5 V. Its output voltage levels meet TTL interface requirements (VOH ≥ 2.4 V, VOL ≤ 0.4 V) under these conditions, enabling direct connection to 74LS or 74F inputs without pull-up resistors or level translators.

What happens to data inputs when outputs are disabled on the 74HC646D,653?

When OE = HIGH (outputs disabled), the 74HC646D,653 retains full input functionality: data present on A0–A7 or B0–B7 continues to be sampled and latched on every LOW-to-HIGH transition of CPAB or CPBA. This allows concurrent data capture and storage even while the device is isolated from the bus, supporting background register updates without interrupting system operation.