Texas Instruments CD74HC123E

Overview

CD74HC123E from Texas Instruments is a dual retriggerable monostable multivibrator with independent resets, designed for precise pulse generation and timing control in digital systems. It operates from 2V to 6V, features Schmitt-trigger inputs on both A and B trigger pins, delivers buffered Q and Q̅ outputs, and supports output pulse widths adjustable from nanoseconds to milliseconds via external RX/CX networks. It is widely used in debounce circuits, event timing, and waveform shaping in industrial control and test equipment.

For engineers reviewing the CD74HC123E datasheet, CD74HC123E pinout, CD74HC123E application, or CD74HC123E equivalent, key selection considerations include its retriggerable architecture, edge-selective triggering (A = trailing, B = leading), overriding reset functionality, wide temperature range (–55°C to +125°C), and compatibility with LSTTL loads under extended thermal conditions.

Technical Context

The CD74HC123E 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 input thresholds, ensuring noise immunity and consistent edge detection regardless of input slew rate. The output pulse width t_W = 0.45·R_XC_X at V_CC = 5V, with R_X ≥ 5 kΩ and C_X ≥ 0 pF, enabling flexible timing from ~10 ns to >100 ms.

Each monostable is fully retriggerable: a new valid trigger during an active output pulse extends t_W, while a LOW on R terminates the pulse immediately. Unused inputs must be tied HIGH or LOW to prevent floating states. Propagation delays (e.g., 60 ns typical A→Q at V_CC = 4.5V, CL = 50 pF) and matched pulse widths (±2% between internal monos) support deterministic timing in synchronous logic interfaces.

Key Specifications

Pinout & Package

CD74HC123E is housed in a 16-lead plastic dual in-line package (PDIP), 0.300" wide, with standard through-hole footprint and 0.100" pin pitch. Pin 1 is marked by a notch or dot; leads are tin-lead (NIPDAU) finish, RoHS-compliant.

Key Features

Applications

Equivalent & Alternatives

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

Compared with CD74HC423E and CD74HCT123E, the CD74HC123E uniquely combines retriggerability, wide 2V–6V operation, and Schmitt-trigger inputs - making it the only option among the three for adaptive timing in mixed-voltage, noise-prone, or event-chaining applications.

Availability

CD74HC123E is available at Aetrix Electronics and suitable for industrial control panels, test instrumentation, and aerospace subsystems requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for CD74HC123E 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 delivering analog and embedded processing solutions, with deep expertise in logic, timing, and interface ICs since the 1960s.

The CD74HC123E belongs to TI's HC-series high-speed CMOS logic family, engineered for low-power, noise-immune timing functions in harsh environments - specifically targeting applications where reliability, wide V_CC tolerance, and deterministic pulse control are critical.

FAQ

What is the minimum external resistor value for timing the CD74HC123E?

The minimum recommended external resistor (R_X) for CD74HC123E is 5 kΩ. Using lower values may cause excessive current draw, timing inaccuracies, or output instability due to internal node loading. This value is specified in the Absolute Maximum Ratings and validated across –55°C to +125°C operation.

Can the CD74HC123E operate reliably at 3.3V supply?

Yes, the CD74HC123E operates reliably from 2V to 6V, including 3.3V nominal systems. At V_CC = 3.3V, input thresholds scale proportionally (VIH ≈ 2.3V, VIL ≈ 1.0V), and propagation delays increase modestly (~100 ns typical), maintaining full functionality in mixed-voltage designs without level shifters.

How does the CD74HC123E differ from the CD74HC423E?

The CD74HC123E is retriggerable: a new trigger during an active output pulse extends the pulse width. The CD74HC423E is non-retriggerable - subsequent triggers are ignored until the current pulse completes. Both share identical pinout, supply range, and temperature rating, but their timing behavior is fundamentally distinct.

What is the purpose of the Schmitt-trigger inputs on the CD74HC123E?

The Schmitt-trigger inputs on A and B pins provide hysteresis (typically 0.8V at V_CC = 5V), ensuring clean, single transitions even with slow-rising or noisy input signals. This eliminates false triggering and guarantees reliable edge detection in electrically noisy industrial or automotive environments.

Is the CD74HC123E pin-compatible with other packages in the same family?

Yes, the CD74HC123E (PDIP) shares identical pinout and electrical behavior with CD74HC123M (SOIC), CD74HC123NSR (SOP), and CD74HC123PWR (TSSOP). All variants use the same 16-pin functional mapping, allowing drop-in replacement across packages when layout and thermal constraints permit.