Texas Instruments SN74HC646DWR

Overview

SN74HC646DWR from Texas Instruments is an octal bus transceiver and register IC with 3-state outputs, designed for bidirectional data flow control between two 8-bit buses (A and B) in synchronous digital systems. It features independent clocked D-type registers per bus, dual select controls (SAB/SBA), direction control (DIR), and output-enable (OE), operating across 2 V to 6 V with typical propagation delay of 11 ns at 5 V and ±6-mA drive capability.

For engineers reviewing the SN74HC646DWR datasheet, SN74HC646DWR pinout, SN74HC646DWR application, or SN74HC646DWR equivalent, key selection considerations include its dual-register storage capability, real-time vs. stored data multiplexing, 24-pin SOIC (DW) package compatibility, and support for LSTTL load driving up to 15 units.

Technical Context

The SN74HC646DWR integrates two independent 8-bit D-type flip-flop registers clocked on low-to-high transitions of CLKAB (A→B) or CLKBA (B→A), enabling synchronized capture of data from either bus. Its transceiver logic supports four core bus-management functions: real-time transfer, stored-data transfer, simultaneous A/B storage, and isolation mode - all controlled via OE, DIR, SAB, and SBA inputs.

Each register retains state during OE high (3-state disabled), allowing concurrent storage and isolation. The device uses standard HC CMOS logic with input thresholds scaled to VCC (VIH = 70% VCC, VIL = 30% VCC), ensuring robust noise immunity and compatibility with mixed-voltage system interfaces.

Key Specifications

Pinout & Package

SN74HC646DWR is housed in a 24-pin SOIC (DW) package with standard 0.300-inch body width and 1.27-mm pitch. Pin 1 is located in Quadrant Q1 per tape-and-reel orientation standards.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar octal transceiver-and-register applications.

Compared with SN74HCT646DWR and SN74LVTH16646DLR, the SN74HC646DWR offers optimal balance of wide voltage range (2–6 V), low static power (80 µA), and proven interoperability with both CMOS and LSTTL loads - making it the preferred choice for industrial retrofits and mixed-supply embedded gateways where pin compatibility and voltage flexibility are primary constraints.

Availability

SN74HC646DWR is available at Aetrix Electronics and suitable for industrial PLC backplane interfaces, microcontroller peripheral expansion, legacy system bus bridging, and automated test equipment signal routing requiring stable component supply and long-term lifecycle support.

Supply support for SN74HC646DWR 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

Texas Instruments is a global semiconductor leader specializing in analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.

The SN74HC646DWR belongs to TI's classic 74HC logic family, engineered for robust, low-power, pin-compatible replacements of legacy TTL bus interface components in space-constrained and thermally demanding applications.

FAQ

What is the maximum clock frequency supported by SN74HC646DWR?

The SN74HC646DWR supports a maximum clock frequency of 31 MHz at VCC = 4.5 V and 36 MHz at VCC = 6 V, as specified in the timing requirements table. This applies to both CLKAB and CLKBA inputs under recommended operating conditions with CL = 50 pF. Real-world performance may vary based on PCB layout, load capacitance, and supply stability.

Can SN74HC646DWR operate reliably at 3.3 V supply?

Yes, SN74HC646DWR is fully specified for operation at 3.3 V (within its 2 V to 6 V range). At VCC = 3.3 V, VIH = 2.31 V min and VIL = 0.99 V max ensure compatibility with standard 3.3 V CMOS logic families. Output drive remains functional at ±4.3 mA typical, sufficient for most 3.3 V bus loads.

How does the SN74HC646DWR handle bus contention during power-up?

To prevent bus contention during power-up or power-down, the OE pin of SN74HC646DWR must be tied to VCC through a pull-up resistor. TI recommends sizing the resistor based on the driver's current-sinking capability - typically 10 kΩ suffices for most applications - ensuring outputs remain in high-impedance state until system initialization completes.

Does SN74HC646DWR support hot insertion or live bus swapping?

SN74HC646DWR is not rated for hot insertion. Its absolute maximum ratings do not include live insertion stress testing, and uncontrolled voltage sequencing during insertion can exceed input clamp current limits (±20 mA). For hot-swap applications, external protection circuitry (e.g., series resistors, TVS diodes) and controlled power-rail ramping are required.

Are the A and B registers in SN74HC646DWR truly independent?

Yes, the A and B registers in SN74HC646DWR are physically and functionally independent: CLKAB clocks data from the A bus into the A register, while CLKBA clocks data from the B bus into the B register. Storage, readback, and multiplexing operations for each register occur without interference - confirmed by the function table and logic diagram in the official datasheet SCLS150C.