STMicroelectronics M74HCT138B1R

Overview

M74HCT138B1R from STMicroelectronics is a high-speed CMOS 3-to-8 line decoder with inverting outputs, used for address decoding in memory systems and I/O selection logic. It features tPD = 16 ns (typ.) at VCC = 4.5 V, symmetrical output drive (|IOH| = IOL = 4 mA min), and TTL-compatible inputs (VIH ≥ 2.0 V, VIL ≤ 0.8 V). Its three enable inputs (G1, G2A, G2B) support cascaded decoding in multi-chip memory maps.

For engineers reviewing the M74HCT138B1R datasheet, M74HCT138B1R pinout, M74HCT138B1R application, or M74HCT138B1R equivalent, key selection criteria include propagation delay matching across enable and address paths, guaranteed 4 mA output drive under 5 V operation, input voltage compatibility with legacy TTL buses, and DIP-16 mechanical fit for through-hole prototyping and industrial control backplanes.

Technical Context

The M74HCT138 implements active-low 3-input binary decoding with three independent enable controls: G1 (active-high), G2A and G2B (both active-low). All eight outputs (Y0–Y7) are inverted and driven to logic high when disabled - no output floats. The device uses silicon gate C2MOS technology, enabling TTL-level input thresholds while maintaining CMOS quiescent current (ICC ≤ 4 µA).

Propagation delays are balanced (tPLH ≅ tPHL) and specified separately for address-to-output (17 ns typ.), G1-to-output (16 ns typ.), and G2-to-output (19 ns typ.) paths under CL = 50 pF. Input capacitance is 10 pF max; power dissipation capacitance CPD is 52 pF, supporting accurate dynamic current estimation via ICC(opr) = CPD × VCC × fIN + ICC.

Key Specifications

Pinout & Package

Package: Plastic DIP-16 (0.300" wide), 16-pin dual in-line package with 2.54 mm pitch, suitable for through-hole PCB assembly and socket-based test fixtures.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HCT138N and 74VHC138N, the M74HCT138B1R uniquely combines full –55°C to +125°C operation, true TTL input thresholds, and DIP-16 packaging - making it the only choice for ruggedized, drop-in legacy replacements in military, avionics, and industrial control systems.

Availability

M74HCT138B1R is available at Aetrix Electronics and suitable for memory address decoding, peripheral selection logic, cascaded decoder trees, and legacy system emulation requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for M74HCT138B1R 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

STMicroelectronics is a global semiconductor leader headquartered in Geneva, designing and manufacturing analog, digital, and mixed-signal ICs for automotive, industrial, and consumer markets.

The M74HCT138 belongs to ST's high-speed CMOS logic family, engineered specifically for pin-compatible upgrades of legacy 74LS logic in systems demanding lower power, wider temperature operation, and improved noise immunity.

FAQ

Is M74HCT138B1R compatible with 5 V TTL logic families?

Yes. Its input thresholds meet TTL specifications: VIH ≥ 2.0 V (min) and VIL ≤ 0.8 V (max) over VCC = 4.5–5.5 V. Output drive strength (4 mA min) matches 74LS fan-out requirements, enabling direct interface with 74LS, 74F, and other 5 V TTL devices without level shifters or pull-ups.

What is the maximum clock frequency supported for address decoding?

The M74HCT138 is not a clocked device, but its propagation delay (16 ns typ. G1→Y) sets the upper bound for address transition rates. For reliable decoding, address stability must be maintained for ≥30 ns before and after enable assertion - supporting effective address update rates up to ~15 MHz in static memory systems with appropriate timing margins.

Does M74HCT138B1R require external pull-up resistors on outputs?

No. Outputs are actively driven high (IOH ≥ 4 mA) and low (IOL ≥ 4 mA); no pull-ups are needed. However, if interfacing with open-collector loads or wired-AND configurations, external pull-ups may be required - but this is not inherent to the M74HCT138B1R's operation.

Can M74HCT138B1R operate from a 3.3 V supply?

No. Its recommended VCC range is strictly 4.5 V to 5.5 V. At 3.3 V, input thresholds (VIH = 2.0 V min) become marginal, and output drive degrades significantly below specification. For 3.3 V systems, consider the 74LVC138 or 74AHC138, which are designed for 3.3 V operation.