Texas Instruments CD74HCT138MT

Overview

CD74HCT138MT 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 (CL = 50 pF), supports -55°C to +125°C temperature range, and features three enable inputs (G2, G1, G0) for cascading.

For engineers reviewing the CD74HCT138MT datasheet, CD74HCT138MT pinout, CD74HCT138MT application, or CD74HCT138MT equivalent, this page provides verified functional behavior, real-world timing specs, validated package mapping, confirmed LSTTL-compatible input thresholds, and direct alternative part comparisons for system-level selection.

Technical Context

The CD74HCT138MT implements a single 3:8 decoder with three address inputs (A2–A0) and three strobe inputs (G2, G1, G0). When any strobe is asserted, all eight outputs (Y0–Y7) are forced high; only the selected output goes low when strobes are inactive and address matches.

It uses TTL-compatible input logic (VIH ≥ 2.0 V, VIL ≤ 0.8 V), draws ≤160 µA supply current over full temperature range, and maintains balanced transition times (15–22 ns) with 50 pF load - enabling reliable interfacing with legacy LSTTL and modern CMOS buses without level-shifting.

Key Specifications

Pinout & Package

CD74HCT138MT is packaged in a 16-pin SOIC (D) with 9.90 mm × 3.90 mm body size and standard 1.27 mm pitch. Pinout conforms to TI's industry-standard 16-pin decoder layout.

Key Features

Applications

Equivalent & Alternatives

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

Compared with CD74HCT138MT, SN74HCT138DR offers improved manufacturing consistency and lifecycle support, while MC74HCT138ADR2G provides second-source flexibility at the cost of marginal timing degradation - both require no PCB changes but differ in qualification scope and long-term supply assurance.

Availability

CD74HCT138MT is available at Aetrix Electronics and suitable for industrial control systems, test instrumentation, and aerospace subsystems requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for CD74HCT138MT 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, embedded processing, and logic solutions for industrial, automotive, and communications markets.

CD74HCT138MT belongs to TI's HCT logic family, engineered for seamless 5 V TTL-to-CMOS interfacing in legacy and mixed-technology systems where reliability across extreme temperatures is critical.

FAQ

What is the maximum operating frequency supported by CD74HCT138MT?

The CD74HCT138MT does not specify a maximum clock frequency, but its 14 ns typical propagation delay (address-to-output, CL = 50 pF) supports reliable operation up to approximately 35 MHz in synchronous demultiplexing applications. System-level timing must account for worst-case delays (44 ns max over -40°C to +85°C), PCB trace delays, and load capacitance effects before final frequency validation.

Can CD74HCT138MT be used with a 3.3 V microcontroller?

No - CD74HCT138MT requires 4.5–5.5 V supply and has TTL-compatible inputs (VIH ≥ 2.0 V, VIL ≤ 0.8 V), making it incompatible with direct 3.3 V logic interfacing. Using it with a 3.3 V MCU requires level translation on all inputs (A2–A0, G0–G2) and outputs (Y0–Y7); for native 3.3 V operation, consider CD74HC138M instead.

Is CD74HCT138MT pin-compatible with CD74HC138MT?

Yes - CD74HCT138MT and CD74HC138MT share identical 16-pin SOIC (D) packaging and pinout. However, they differ electrically: CD74HCT138MT accepts TTL-level inputs and operates only at 4.5–5.5 V, while CD74HC138MT supports 2–6 V and CMOS-level inputs. Swapping them requires verifying supply voltage and input source compatibility.

What is the purpose of the three enable inputs (G2, G1, G0) on CD74HCT138MT?

The three enable inputs provide hierarchical control: G2 is active-high, G1 and G0 are active-low. All three must be simultaneously asserted (G2 = high, G1 = low, G0 = low) for decoding to occur. This allows cascading multiple CD74HCT138MT devices - e.g., using one unit's outputs to control G2 of downstream units - enabling 1-of-64 or larger decoding without additional logic gates.

Does CD74HCT138MT require external pull-up resistors on its outputs?

No - CD74HCT138MT features push-pull (totem-pole) outputs capable of actively driving both high and low logic states. Its outputs can directly interface with LSTTL or CMOS inputs without pull-ups. External pull-ups are unnecessary unless open-drain emulation or wired-OR logic is intentionally implemented - which is not supported by this device's architecture.