Texas Instruments CD74HC238PWT

Overview

CD74HC238PWT from Texas Instruments is a high-speed CMOS 3-to-8 line decoder/demultiplexer with active-high outputs, 16-pin TSSOP package, 2–6 V supply range, -55°C to +125°C operating temperature, and 13 ns typical propagation delay at V_CC = 4.5 V and C_L = 15 pF. It serves as an address selector in memory decoding or data routing circuits where output-enable logic must assert high on selection.

For engineers reviewing the CD74HC238PWT datasheet, CD74HC238PWT pinout, CD74HC238PWT application, or CD74HC238PWT equivalent, key selection criteria include its noninverting output behavior, three independent enable inputs (G2, G1, G0) for cascading, LSTTL-compatible fanout (10 loads), and wide industrial temperature support without derating.

Technical Context

The CD74HC238PWT implements a silicon-gate CMOS 3:8 decoder with address inputs A2–A0 and three strobe inputs (G2 = standard enable, G1/G0 = active-low enables). When any strobe is asserted, all eight Y0–Y7 outputs are forced low - unlike the '138 variant which forces them high.

Its functional mode is defined by Table 7-2: only when G2 = H, G1 = L, G0 = L and a valid 3-bit address is applied does exactly one output go high; all others remain low. This noninverting decode behavior enables direct interface with active-high load logic without external inversion.

Key Specifications

Pinout & Package

TSSOP-16 package (5.00 mm × 4.40 mm body size, 0.65 mm pitch), thin shrink small-outline package optimized for space-constrained industrial and portable designs.

Key Features

Applications

Equivalent & Alternatives

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

Compared with CD74HC138PW and SN74HC238PWR, the CD74HC238PWT provides identical noninverting decode functionality and timing, but differs in packaging format (obsolete tube vs. active tape-and-reel) and output polarity - making it irreplaceable in systems requiring guaranteed active-high outputs without layout changes.

Availability

CD74HC238PWT is available at Aetrix Electronics and suitable for memory address decoding, peripheral I/O selection, programmable logic interfaces, and industrial control signal distribution requiring stable component supply across extended temperature ranges.

Supply support for CD74HC238PWT 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 company headquartered in Dallas, Texas, delivering analog and embedded processing solutions for industrial, automotive, personal electronics, and communications markets.

The CD74HC238PWT belongs to TI's HC-series high-speed CMOS logic family, designed specifically for low-power, wide-voltage-range digital interfacing in harsh-environment applications where reliability and signal integrity are critical.

FAQ

What is the output behavior of the CD74HC238PWT compared to the CD74HC138?

The CD74HC238PWT features active-high outputs: only the selected Y0–Y7 pin goes high when enabled, while all others remain low. In contrast, the CD74HC138 drives the selected output low and holds others high. This noninverting behavior eliminates external inverters in systems requiring high-true enables, such as driving LED anodes or enabling active-high peripherals. The CD74HC238PWT's truth table (Table 7-2) confirms this functional distinction.

Is the CD74HC238PWT still in active production?

No - the CD74HC238PWT is marked as obsolete in TI's official packaging addendum. Its active replacement is the CD74HC238PWR, which uses the same TSSOP-16 package, identical electrical specifications, and tape-and-reel packaging (2000 pcs per reel). Engineers designing new products should specify CD74HC238PWR; CD74HC238PWT may only be sourced from legacy distributor stock.

Can the CD74HC238PWT operate at 3.3 V supply voltage?

Yes - the CD74HC238PWT is fully specified for 2 V to 6 V operation, including 3.3 V. At V_CC = 3.3 V, its typical propagation delay is 18 ns (max 33 ns), output drive is ±4 mA, and input thresholds scale proportionally (V_IH ≈ 2.3 V, V_IL ≈ 0.99 V). It interfaces seamlessly with 3.3 V microcontrollers and FPGAs without level translation.

What is the maximum clock/data rate supported by the CD74HC238PWT?

The CD74HC238PWT is not a clocked device but a combinational decoder; its speed is governed by propagation delay and transition time. With t_pd = 13 ns (typ) and t_t = 13 ns (typ) at V_CC = 4.5 V, it supports reliable operation in address/data paths up to ~30 MHz (1/(2 × t_pd)). For higher-frequency multiplexing, ensure setup/hold margins relative to controlling clock edges and limit capacitive loading to ≤15 pF.

How should unused inputs be handled on the CD74HC238PWT?

All unused inputs (A0–A2, G0–G2) on the CD74HC238PWT must be tied to a defined logic level - either V_CC or GND - to prevent floating states that cause increased current draw, erratic outputs, or ESD susceptibility. TI recommends connecting unused address inputs to GND and unused enable inputs to V_CC (for G2) or GND (for G1/G0) to maintain default disabled state unless otherwise required by system logic.