STMicroelectronics M74HC138B1R

Overview

M74HC138B1R from STMicroelectronics is a high-speed CMOS 3-to-8 line decoder with inverting outputs, designed for address decoding in memory systems and logic-level selection. It operates across 2V–6V supply, delivers 13ns typical propagation delay at 6V, supports symmetrical ±4mA output drive, and features three enable inputs (G1, G2A, G2B) for cascading. Used in microcontroller peripheral decoding where low-power, noise-immune binary selection is required.

For engineers reviewing the M74HC138B1R datasheet, M74HC138B1R pinout, M74HC138B1R application, or M74HC138B1R equivalent, key selection criteria include its 3-enable architecture, guaranteed 28% VCC noise immunity, -55°C to +125°C operating range, and DIP-16 package compatibility with legacy 74-series layouts.

Technical Context

The M74HC138B1R implements a fully static, silicon gate C2MOS-based 3-input binary decoder with active-low outputs. Its enable logic requires G1 = HIGH and both G2A = LOW and G2B = LOW to activate decoding; any violation forces all eight Y0–Y7 outputs HIGH.

Propagation delays are balanced (tPLH ≅ tPHL), and output impedance is symmetrical (|IOH| = IOL ≥ 4mA), enabling consistent rise/fall timing in bus-driven or wired-OR configurations. Input thresholds meet 28% VCC noise margin minimums across the full 2V–6V VCC range.

Key Specifications

Pinout & Package

Package: Plastic DIP-16 (0.300" wide), JEDEC MS-001, body dimensions 20.0 × 8.5 × 3.3 mm.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HC138N and 74AHC138PW, the M74HC138B1R uniquely supports 6V operation and -55°C startup, making it the only choice for military-grade or wide-temperature industrial memory decoding where reliability trumps raw speed.

Availability

M74HC138B1R is available at Aetrix Electronics and suitable for memory address decoding, peripheral select logic, industrial I/O expansion, and legacy bus interface applications requiring stable component supply and long-term obsolescence management.

Supply support for M74HC138B1R 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, Switzerland, designing and manufacturing analog, digital, and mixed-signal ICs for automotive, industrial, and consumer markets.

The M74HC138B1R belongs to ST's high-speed CMOS logic family, engineered specifically for robust, low-power address decoding and signal routing in mission-critical embedded systems.

FAQ

What is the maximum clock frequency supported by the M74HC138B1R?

The M74HC138B1R is a combinational logic device with no internal clock; its usable frequency depends on propagation delay and system timing margins. With 13ns typical tPD at 6V, it supports reliable operation in address decode paths up to ~40MHz (based on 2×tPD worst-case setup window). System-level timing validation is required for >25MHz buses.

Can the M74HC138B1R drive LEDs directly?

Yes - each output can sink up to 4mA continuously (min), sufficient to drive low-current indicator LEDs with appropriate series resistors (e.g., 1kΩ at 5V yields ~4.3mA). For higher-brightness or multiplexed LEDs, external transistor buffering is recommended due to cumulative current limits across multiple outputs.

Is the M74HC138B1R RoHS compliant?

Yes - the M74HC138B1R (DIP-16 package) manufactured by STMicroelectronics meets RoHS Directive 2011/65/EU requirements, with lead content below 0.1% by weight and full compliance verified per JEDEC J-STD-609A standards. Certificate of Compliance is available upon request.

How does the enable logic work across G1, G2A, and G2B?

G1 is active-HIGH; G2A and G2B are both active-LOW. The decoder activates only when G1 = HIGH AND G2A = LOW AND G2B = LOW. Any deviation (e.g., G1 = LOW or G2A = HIGH) forces all Y0–Y7 outputs HIGH, regardless of A/B/C inputs - enabling clean, predictable disable behavior in cascaded memory maps.