Texas Instruments CD74HCT138M

Overview

CD74HCT138M 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 features three enable inputs (G2, G1, G0), operates at 4.5–5.5 V, delivers 14 ns typical propagation delay at VCC = 4.5 V and CL = 50 pF, and supports full military temperature range (–55°C to +125°C).

For engineers reviewing the CD74HCT138M datasheet, CD74HCT138M pinout, CD74HCT138M application, or CD74HCT138M equivalent, this page provides verified functional behavior, SOIC-16 package details, TTL-compatible input thresholds, output drive capability (4 mA sink/source), and real-world use cases in bus selection and hierarchical decoding.

Technical Context

The CD74HCT138M implements a single 3:8 decoder with three address inputs (A0–A2) and three strobe enables (G2, G1, G0), where all outputs are forced high when any strobe is inactive. Its logic architecture ensures only one output (Y0–Y7) goes low per valid address combination when all enables are asserted - enabling precise channel selection in multiplexed systems.

It uses HCT-series CMOS technology with LSTTL-compatible input thresholds (VIL ≤ 0.8 V, VIH ≥ 2.0 V), rail-to-rail output swing, and low static current (ICC ≤ 160 µA over temperature). The device exhibits balanced rise/fall times (15–22 ns) and supports cascading via enable chaining without external logic.

Key Specifications

Pinout & Package

CD74HCT138M is housed in a 16-pin SOIC (D) package measuring 9.90 mm × 3.90 mm, with gull-wing leads and RoHS-compliant NiPdAu/Sn lead finish. It meets MSL Level-1 (260°C peak reflow) and supports automated surface-mount assembly.

Key Features

Applications

Equivalent & Alternatives

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

Compared with CD74HCT138M, SN74HCT138DR offers better power efficiency and timing predictability, while MC74HCT138ADR2G trades minor input threshold shifts for second-source availability - neither is pin-compatible without validation of enable logic polarity and load capacitance effects.

Availability

CD74HCT138M is available at Aetrix Electronics and suitable for industrial control systems, aerospace avionics modules, and automotive engine control units requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for CD74HCT138M 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 ICs, with decades of heritage in high-reliability logic families.

The CD74HCT138M belongs to TI's HCT logic series, engineered for seamless integration into 5 V mixed-signal systems where TTL compatibility, low power, and wide temperature operation are critical design requirements.

FAQ

What is the maximum operating frequency supported by the CD74HCT138M?

The CD74HCT138M does not specify a maximum clock frequency, as it is a combinational logic device without internal clocking. Its usable switching rate is determined by propagation delay (14 ns typical) and output transition time (15–22 ns). In practice, it reliably supports address decode operations up to 30 MHz in systems with proper layout and load management. The CD74HCT138M is not intended for continuous high-frequency toggling like a counter or register.

Can the CD74HCT138M be used with a 3.3 V supply?

No - the CD74HCT138M is specified only for 4.5 V to 5.5 V operation per its Recommended Operating Conditions. Applying 3.3 V violates the minimum VCC requirement and will result in undefined output states, increased propagation delay, and potential failure to meet VIH/VIL thresholds. For 3.3 V systems, consider the CD74HC138M (2–6 V) or SN74LVC138A (1.65–3.6 V) instead of the CD74HCT138M.

How do the three enable inputs (G2, G1, G0) interact in the CD74HCT138M?

In the CD74HCT138M, 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 (e.g., G2 = low or G1 = high), all eight outputs (Y0–Y7) go high-impedance-equivalent - i.e., forced high regardless of address inputs. This behavior allows hierarchical cascading: outputs of one CD74HCT138M can drive G2 of downstream devices to expand to 1-of-64 decoding.

Is the CD74HCT138M pin-compatible with the CD74HC138M?

Yes - the CD74HCT138M and CD74HC138M share identical SOIC-16 pinouts, address/enable/output assignments, and functional truth tables. However, they differ electrically: CD74HCT138M has TTL-compatible inputs (VIL ≤ 0.8 V), while CD74HC138M uses CMOS thresholds (VIL ≤ 1.35 V at VCC = 4.5 V). Swapping them requires verifying input source compatibility; the CD74HCT138M cannot be substituted for CD74HC138M in high-noise 5 V systems without risk of false triggering.

Does the CD74HCT138M require external pull-up resistors on its outputs?

No - the CD74HCT138M features push-pull CMOS outputs capable of actively driving both high and low states. Its Y0–Y7 outputs source up to 4 mA at VOH ≥ 3.98 V and sink up to 4 mA at VOL ≤ 0.4 V (VCC = 4.5 V), eliminating need for external pull-ups in standard logic interfacing. Pull-ups are only required if interfacing with open-drain buses or implementing wired-OR functions - which contradicts the CD74HCT138M's native active-low push-pull architecture.