Texas Instruments CD74HCT123ME4

Overview

CD74HCT123ME4 from Texas Instruments is a dual retriggerable monostable multivibrator with independent resets, Schmitt-trigger inputs on both A and B trigger lines, buffered Q and Q̅ outputs, and external RC timing control (t_W = 0.45·R_XC_X at V_CC = 5 V). It operates from 4.5 V to 5.5 V, supports LSTTL-compatible input thresholds (V_IL ≤ 0.8 V, V_IH ≥ 2.0 V), and delivers precise pulse widths across –55 °C to +125 °C for industrial timing control applications.

For engineers reviewing the CD74HCT123ME4 datasheet, CD74HCT123ME4 pinout, CD74HCT123ME4 application, or CD74HCT123ME4 equivalent, key selection criteria include its dual-monostable architecture with edge-selective triggering (A = trailing-edge, B = leading-edge), overriding reset functionality, wide temperature range, HCT-level noise immunity, and compatibility with legacy TTL logic systems requiring stable, retriggerable one-shot behavior without external microcontroller intervention.

Technical Context

The CD74HCT123ME4 implements two independent monostable circuits, each with separate A (trailing-edge), B (leading-edge), and R (active-low reset) inputs. Triggering is enabled by Schmitt-trigger inputs that reject noise and ensure clean edge detection regardless of input rise/fall time. Pulse width is externally set via R_X and C_X, with minimum R_X = 5 kΩ and C_X = 0 pF, enabling output pulse widths from nanoseconds to milliseconds.

Each monostable is fully retriggerable: a new trigger during an active output pulse extends t_W, while a LOW on R terminates the pulse immediately. The device uses CMOS circuitry with TTL-compatible input thresholds and provides buffered complementary outputs (Q and Q̅) with fanout of 10 LSTTL loads over full temperature range.

Key Specifications

Pinout & Package

CD74HCT123ME4 is housed in a 16-pin SOIC (D) package (5.3 mm × 10.2 mm, 1.75 mm height), RoHS-compliant with NiPdAu lead finish and moisture sensitivity level 1 (unlimited floor life at ≤30 °C/60% RH).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar dual retriggerable monostable applications.

Compared with CD74HC123M96 and SN74LS123N, the CD74HCT123ME4 uniquely balances LSTTL input compatibility, wide temperature operation, Schmitt-trigger noise rejection, and retriggerable flexibility-making it the optimal choice for robust, drop-in upgrades in industrial and automotive timing subsystems.

Availability

CD74HCT123ME4 is available at Aetrix Electronics and suitable for industrial motor control, automotive powertrain timing, test equipment pulse shaping, and legacy system interface logic requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for CD74HCT123ME4 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 solutions, with over 90 years of innovation in high-reliability components.

The CD74HCT123ME4 belongs to TI's 74HCT logic family, designed specifically to bridge TTL and CMOS domains with pin-compatible replacements offering improved power efficiency, noise immunity, and extended temperature performance for industrial and automotive applications.

FAQ

What is the recommended external RC network for a 100 µs output pulse width using CD74HCT123ME4?

For a 100 µs output pulse width at V_CC = 5 V, use the formula t_W = 0.45·R_XC_X. Solving gives R_XC_X ≈ 222 kΩ·pF. A practical combination is R_X = 22 kΩ and C_X = 10 nF. Ensure C_X is low-leakage (e.g., ceramic or film) and placed close to CD74HCT123ME4 Pins 8/9 (Monostable 2) or 11/12 (Monostable 1) to minimize noise coupling.

Can CD74HCT123ME4 be used with a 3.3 V microcontroller output as a trigger source?

No-CD74HCT123ME4 requires V_IH ≥ 2.0 V (min) and V_IL ≤ 0.8 V (max) for guaranteed TTL compatibility, but a 3.3 V microcontroller's logic HIGH (typically ~3.3 V) exceeds the absolute maximum input voltage of 7 V and may violate DC input voltage limits if V_CC = 5 V. Use a resistive divider or level shifter to scale 3.3 V signals to 5 V logic levels before connecting to CD74HCT123ME4 inputs.

Does CD74HCT123ME4 support simultaneous triggering of both monostables?

Yes-CD74HCT123ME4 allows independent triggering of Monostable 1 (via Pins 1A/1B/1R) and Monostable 2 (via Pins 2A/2B/2R). There is no internal coupling; both can be triggered concurrently or sequentially. However, shared V_CC and GND require adequate local decoupling to prevent supply bounce during simultaneous output transitions.

What is the minimum external resistance value allowed for timing networks on CD74HCT123ME4?

The minimum recommended external resistance R_X is 5 kΩ, as specified in the datasheet. Using lower values risks excessive current draw through the internal timing node, increased power dissipation, reduced timing accuracy, and potential degradation of output pulse shape. For precision timing, R_X ≥ 10 kΩ and C_X ≥ 100 pF are preferred to minimize parasitic effects.

How does the reset function behave during an active output pulse on CD74HCT123ME4?

When a LOW is applied to the reset pin (1R or 2R) during an active output pulse, CD74HCT123ME4 immediately forces Q to LOW and Q̅ to HIGH, terminating the pulse within ≤65 ns (max propagation delay). This overriding reset is asynchronous and takes priority over ongoing retrigger events-ensuring deterministic, fail-safe pulse cancellation for safety-critical timing paths.