Texas Instruments CD74HCT138E

Overview

CD74HCT138E from Texas Instruments is a high-speed CMOS 3-to-8 line decoder/demultiplexer with active-low outputs, designed for memory address decoding and data routing in industrial control and embedded systems. It operates at 4.5–5.5 V, delivers 14 ns typical propagation delay (V_CC = 4.5 V, C_L = 50 pF), supports -55°C to +125°C temperature range, and features three enable inputs (G2, G1, G0) for cascading.

For engineers reviewing the CD74HCT138E datasheet, CD74HCT138E pinout, CD74HCT138E application, or CD74HCT138E equivalent, this page provides verified functional behavior, validated pin-level circuit roles, confirmed thermal and switching specifications across full temperature range, and real-world selection guidance against comparable HCT-family decoders.

Technical Context

The CD74HCT138E implements a single 3:8 binary decoder with three address inputs (A2–A0) and three independent strobe enables (G2, G1, G0). When any strobe is inactive (G2 = low, or G1/G0 = high), all eight outputs (Y0–Y7) are forced high; only the selected output goes low when all enables are asserted and address matches.

Its TTL-compatible input thresholds (V_IL ≤ 0.8 V, V_IH ≥ 2.0 V) ensure direct interfacing with legacy LSTTL logic, while CMOS-level output drive (±4 mA at V_CC = 4.5 V) maintains signal integrity in mixed-voltage systems without level shifters.

Key Specifications

Pinout & Package

CD74HCT138E is supplied in a 16-pin plastic dual in-line package (PDIP) with 0.3-inch body width (25.40 mm × 6.35 mm), through-hole mountable, RoHS-compliant with NiPdAu lead finish.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HCT138N and MC74HCT138DG, CD74HCT138E offers identical pin compatibility and identical enable-input logic structure, but provides tighter propagation delay distribution and broader qualification across military-grade temperature extremes - making it preferred for aerospace and high-reliability embedded deployments.

Availability

CD74HCT138E is available at Aetrix Electronics and suitable for memory address decoding, industrial I/O expansion, and legacy bus interface applications requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for CD74HCT138E 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 delivering analog and embedded processing solutions, with over 90 years of innovation in logic, power management, and signal chain technologies.

CD74HCT138E belongs to TI's CD74HCT high-speed CMOS logic family, engineered for seamless integration into 5 V TTL-compatible systems requiring robust performance across extreme temperatures and low static power consumption.

FAQ

What is the maximum operating frequency supported by the CD74HCT138E?

The CD74HCT138E does not specify a maximum clock frequency because it is a combinational logic device without an internal clock. Its usable speed is determined by propagation delay: 14 ns typical (V_CC = 4.5 V, C_L = 50 pF), supporting reliable operation in address decode paths up to ~50 MHz. For synchronous bus applications, ensure setup/hold times of downstream devices are met using this t_pd value. The CD74HCT138E datasheet confirms no minimum pulse width limitation beyond standard CMOS timing constraints.

Can CD74HCT138E be used with 3.3 V microcontrollers?

No - CD74HCT138E requires 4.5–5.5 V supply and has TTL-compatible inputs (V_IH ≥ 2.0 V, V_IL ≤ 0.8 V), so 3.3 V logic levels fall outside its guaranteed input recognition range. Direct connection risks metastability or incorrect decoding. To interface with 3.3 V controllers, use level-shifting buffers or select the CD74HC138E (2–6 V operation) instead. The CD74HCT138E is strictly intended for 5 V systems.

How do the three enable inputs (G2, G1, G0) interact in CD74HCT138E?

In CD74HCT138E, all three enables must be simultaneously asserted for normal decoding: G2 must be high, and both G1 and G0 must be low. If any enable is violated (G2 = low, or G1/G0 = high), all eight outputs (Y0–Y7) are forced high - overriding address inputs. This allows hierarchical decoding: e.g., G2 driven by a higher-order address bit, while G1/G0 serve as chip-selects for subsystems. The CD74HCT138E function table explicitly defines this AND-logic behavior across all combinations.

Is CD74HCT138E pin-compatible with CD74HC138E?

Yes - CD74HCT138E and CD74HC138E share identical PDIP-16 packaging, pinout, and terminal functions. However, they differ electrically: CD74HCT138E uses TTL-compatible inputs (V_IH ≥ 2.0 V) and 4.5–5.5 V operation, while CD74HC138E uses CMOS inputs (V_IH ≥ 70% V_CC) and 2–6 V operation. Swapping them requires verifying supply voltage and driver compatibility. Both devices retain the same pin mapping and mechanical footprint.

What is the purpose of the "demultiplexing" capability described for CD74HCT138E?

Demultiplexing with CD74HCT138E is achieved by holding address inputs (A2–A0) constant to select one output (e.g., Y3), then using an enable input (e.g., G0) as the data input: when G0 toggles, only Y3 follows its state (inverted), while all other outputs remain high. This converts 1-bit serial data into 1-of-8 parallel control - useful for LED matrix scanning, relay bank control, or test signal routing. The CD74HCT138E datasheet confirms this mode in Section 7.1 and Figure 7-1.