Texas Instruments SN74HCT645DWR

Overview

SN74HCT645DWR from Texas Instruments is an octal bus transceiver IC designed for asynchronous two-way data flow between parallel buses in 5-V digital systems. It features direction control (DIR), output-enable (OE), ±6-mA drive strength at 5 V, 14-ns typical propagation delay, and TTL-compatible inputs. It is used in microcontroller peripheral interfacing, memory expansion, and system-level bus isolation.

For engineers reviewing the SN74HCT645DWR datasheet, SN74HCT645DWR pinout, SN74HCT645DWR application, or SN74HCT645DWR equivalent, key selection criteria include bidirectional bus control logic, 3-state output isolation timing (ten/tdis), operating voltage range (4.5–5.5 V), and SOIC-20 thermal performance (θJA = 58°C/W).

Technical Context

This device implements dual 8-bit bidirectional bus transceivers with independent DIR and OE controls per transceiver bank. Logic operation follows positive-logic truth table: DIR = L enables B→A transfer; DIR = H enables A→B transfer; OE = H forces all outputs into high-impedance state regardless of DIR.

It uses HCT (High-Speed CMOS with TTL input thresholds) process technology, ensuring compatibility with legacy TTL logic while delivering CMOS-level power efficiency. Input clamp current is ±20 mA; continuous output current per pin is ±35 mA; absolute max VCC is 7 V.

Key Specifications

Pinout & Package

SN74HCT645DWR is housed in a 20-pin SOIC (DW) package, 7.5 mm wide, 12.8 mm long, 2.65 mm max height, with 1.27-mm lead pitch and NIPDAU lead finish. Moisture sensitivity level is Level-1 (unlimited reflow).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HCT245DW, SN74HCT645DWR simplifies direction control with one pin instead of two enables; compared with 74LCX541MTCX, it avoids level-shifting complexity but operates strictly in 5-V domains - making it optimal for cost-sensitive, legacy-compatible 5-V bus isolation.

Availability

SN74HCT645DWR is available at Aetrix Electronics and suitable for microcontroller bus expansion, memory subsystem interfacing, and industrial backplane isolation requiring stable component supply, full traceability, and long-term lifecycle support.

Supply support for SN74HCT645DWR 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 founded in 1930, specializing in analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.

The SN74HCT645DWR belongs to TI's 74HCT logic family, engineered for seamless TTL-to-CMOS bridging in 5-V systems where reliability, noise immunity, and backward compatibility are critical design priorities.

FAQ

What is the maximum clock rate supported by SN74HCT645DWR in a synchronous bus application?

The SN74HCT645DWR does not contain internal clock circuitry and is not a clocked device. Its 14-ns typical propagation delay (tpd) at 5.5 V and 50-pF load supports reliable data toggling up to approximately 35 MHz in edge-triggered synchronous bus designs, assuming setup/hold margins are met by the controlling logic. The actual usable rate depends on PCB trace length, loading, and controller timing margins - not an intrinsic clock specification of the SN74HCT645DWR itself.

Can SN74HCT645DWR operate with a 3.3-V supply?

No, SN74HCT645DWR is specified only for 4.5 V to 5.5 V operation per its recommended conditions. Applying 3.3 V violates the minimum VCC requirement and will result in undefined logic states, increased propagation delay, and potential failure to drive LSTTL loads. For 3.3-V systems, consider TI's 74LVC645 or similar families - the SN74HCT645DWR must be used exclusively in 5-V domains.

How does the DIR pin interact with OE in SN74HCT645DWR?

In SN74HCT645DWR, OE has priority over DIR: when OE is high (disabled), all A and B pins enter high-impedance state regardless of DIR level. Only when OE is low (enabled) does DIR determine direction - DIR = low enables B→A transfer; DIR = high enables A→B transfer. This hierarchy ensures deterministic bus isolation during system reset or arbitration.

Is SN74HCT645DWR pin-compatible with SN74HCT245DW?

No, SN74HCT645DWR and SN74HCT245DW are not pin-compatible. While both are 20-pin SOIC octal transceivers, SN74HCT645DWR uses Pin 1 for DIR and Pins 19/20 for OE/VCC; SN74HCT245DW uses Pins 1/19 for dual OE controls and has no DIR pin. Their pin functions differ fundamentally - direct PCB replacement is not possible without layout revision.

What thermal derating applies to SN74HCT645DWR in high-ambient environments?

SN74HCT645DWR in SOIC-DW package has θJA = 58°C/W. At 85°C ambient, maximum allowable power dissipation is (125°C − 85°C)/58°C/W ≈ 690 mW. With typical ICC = 80 µA and dynamic power dominated by Cpd × V² × f, sustained 10-MHz toggling across all 8 lines yields ~15 mW - well within limit. Derating becomes critical only above 85°C ambient or under heavy capacitive loading (>100 pF per line).