Texas Instruments CD74HCT238E

Overview

CD74HCT238E from Texas Instruments is a high-speed CMOS 3-to-8 line decoder/demultiplexer with active-high outputs, designed for memory address decoding and data routing in industrial control and instrumentation systems. It features three enable inputs (G2, G1, G0), operates from 4.5 V to 5.5 V, delivers 14 ns typical propagation delay at VCC = 4.5 V and CL = 15 pF, and supports full military temperature range (–55°C to +125°C).

For engineers reviewing the CD74HCT238E datasheet, CD74HCT238E pinout, CD74HCT238E application, or CD74HCT238E equivalent, this page provides verified functional behavior, validated package mapping, confirmed output polarity (active-high), and real-world selection guidance against comparable HCT-family decoders.

Technical Context

The CD74HCT238E implements a 3-input binary address decoder with three independent strobe inputs (G2, G1, G0) that collectively gate all eight outputs to logic low when asserted - enabling cascaded decoding and demultiplexing functions. Its TTL-compatible input thresholds (VIL ≤ 0.8 V, VIH ≥ 2.0 V) ensure direct interfacing with legacy LSTTL logic without level shifting.

Unlike the '138 variant, the '238 architecture forces unselected outputs high and the selected output high only when strobes are inactive - making it suitable for active-high enable paths in bus arbitration and peripheral selection. Propagation delays are balanced across address and strobe paths, and output transition times remain consistent (15–22 ns) over temperature and supply voltage.

Key Specifications

Pinout & Package

CD74HCT238E is supplied in a 16-pin plastic dual in-line package (PDIP-N), body size 25.40 mm × 6.35 mm, with through-hole mounting and industry-standard lead pitch (2.54 mm).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 3-to-8 decoder applications.

Compared with CD74HCT138E and SN74HCT238N, CD74HCT238E provides guaranteed active-high decode outputs essential for direct enable of high-active peripherals, while maintaining full pin and timing compatibility with both alternatives - making it the optimal choice when output polarity is fixed by system architecture.

Availability

CD74HCT238E is available at Aetrix Electronics and suitable for industrial control systems, test instrumentation, and aerospace avionics requiring stable component supply, long-term lifecycle support, and extended temperature-grade assurance.

Supply support for CD74HCT238E 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 company specializing in analog, embedded processing, and logic ICs, with leadership in industrial, automotive, and aerospace markets.

The CD74HCT238E belongs to TI's HCT logic family, engineered for seamless 5 V TTL-to-CMOS interfacing in mission-critical systems demanding wide temperature operation and high noise immunity.

FAQ

What is the output behavior of CD74HCT238E when all enable inputs are asserted?

When G2 is high and both G1 and G0 are low, CD74HCT238E performs normal decoding: exactly one output (Y0–Y7) goes high based on A2A1A0, and all others remain low. If any enable condition fails - e.g., G2 is low, or G1/G0 are high - all outputs are forced low. This active-low gating behavior is fundamental to CD74HCT238E's demultiplexing capability and differs from the '138's active-high disable scheme.

Can CD74HCT238E operate reliably at 4.5 V supply voltage?

Yes, CD74HCT238E is fully specified for operation from 4.5 V to 5.5 V. At VCC = 4.5 V, its typical propagation delay is 14 ns (CL = 15 pF), output drive remains ±4 mA, and input thresholds (VIL ≤ 0.8 V, VIH ≥ 2.0 V) are maintained - ensuring robust interoperability with 5 V TTL systems even at minimum rated supply.

How does CD74HCT238E differ from CD74HC238E in practical use?

CD74HCT238E features TTL-compatible inputs (VIL ≤ 0.8 V, VIH ≥ 2.0 V), allowing direct connection to legacy 5 V TTL outputs without level shifters. CD74HC238E requires CMOS-level inputs (VIL ≤ 1.35 V, VIH ≥ 3.15 V at VCC = 4.5 V), making it incompatible with standard TTL drivers. Both share identical output behavior, pinout, and timing, but CD74HCT238E is the correct choice for mixed-logic 5 V systems.

Is CD74HCT238E suitable for use in automotive applications?

CD74HCT238E is not AEC-Q100 qualified and lacks automotive-grade screening or extended reliability testing. While its –55°C to +125°C operating range overlaps with some automotive under-hood requirements, TI offers the pin-compatible CD74HCT238Q1 variant specifically qualified for automotive use. For production automotive designs, CD74HCT238Q1 - not CD74HCT238E - must be used.

What is the recommended bypass capacitor for CD74HCT238E?

Texas Instruments recommends a 0.1 µF ceramic capacitor placed as close as possible to the VCC pin (Pin 16) and GND pin (Pin 8) of CD74HCT238E. This minimizes high-frequency supply noise and prevents output glitches during fast transitions. For enhanced noise suppression, a parallel 1 µF capacitor may be added, but the 0.1 µF device is mandatory per TI layout guidelines.