Texas Instruments CD74HC221M

Overview

CD74HC221M from Texas Instruments is a dual monostable multivibrator with independent reset, Schmitt-trigger B inputs, and buffered Q/Q̅ outputs. It operates from 2V to 6V, supports -55°C to +125°C temperature range, and delivers pulse widths from nanoseconds to microseconds via external RX/CX timing components-used in precision timing control, pulse shaping, and debounce circuits.

For engineers reviewing the CD74HC221M datasheet, CD74HC221M pinout, CD74HC221M application, or CD74HC221M equivalent, key selection considerations include its dual independent monostable architecture, edge-selective triggering (A = trailing, B = leading), overriding reset functionality, wide VCC range, and SOIC-16 package compatibility with industrial and aerospace-grade thermal requirements.

Technical Context

The CD74HC221M implements two fully independent monostable multivibrators, each with separate A (trailing-edge), B (leading-edge), and R (active-low reset) inputs. Triggering is asynchronous and immune to input rise/fall time due to Schmitt-triggered B inputs and internal edge-detection logic on A inputs.

Each monostable's output pulse width is determined by t_W = 0.7 × R_X × C_X at V_CC = 4.5V, with minimum R_X = 500Ω and C_X = 0pF. Outputs remain stable during power-up (auto-reset) and support TTL/CMOS fanout up to 15 LSTTL loads under temperature extremes.

Key Specifications

Pinout & Package

CD74HC221M is supplied in a 16-pin SOIC (D) package with 1.27 mm pitch, 7.5 mm × 10.3 mm body, and 1.75 mm max height. Pin 1 is located in Quadrant Q1 per tape-and-reel orientation.

Key Features

Applications

Equivalent & Alternatives

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

Compared with CD74HC221M96 and CD74HCT221M96, the CD74HC221M shares identical core functionality but is marked obsolete by TI; CD74HC221M96 provides identical performance with production-ready tape-and-reel logistics, while CD74HCT221M96 trades voltage flexibility for guaranteed TTL-level input compatibility.

Availability

CD74HC221M is available at Aetrix Electronics and suitable for industrial control systems, aerospace avionics, and test equipment requiring stable component supply across extended temperature ranges and long product lifecycles.

Supply support for CD74HC221M 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, embedded processing, and logic solutions for industrial, automotive, and communications markets.

The CD74HC221M belongs to TI's High-Speed CMOS Logic family, designed specifically for robust, low-power timing functions in harsh environments where reliability, wide temperature operation, and noise immunity are critical.

FAQ

What is the minimum external timing capacitor value supported by the CD74HC221M?

The CD74HC221M supports C_X = 0 pF as the minimum external timing capacitor value. Pulse width is then determined solely by R_X, though practical implementations typically use ≥28 pF for stability. The device's internal timing circuitry does not require a minimum capacitance to function, enabling ultra-short pulse generation down to ~140 ns with R_X = 2 kΩ and C_X = 28 pF at V_CC = 4.5 V.

Can the CD74HC221M operate reliably at 3.3V supply voltage?

Yes, the CD74HC221M operates reliably across 2V–6V, including 3.3V nominal systems. At V_CC = 3.3V, VIH = 2.31V (min), VIL = 0.99V (max), and propagation delays increase moderately versus 5V operation - t_PLH/t_PHL ≈ 50–65 ns (CL = 50 pF). All specifications in the HC family datasheet apply across the full voltage range, making CD74HC221M suitable for mixed-voltage 3.3V/5V board designs.

Does the CD74HC221M require external pull-up or pull-down resistors on unused inputs?

Yes - per the datasheet, unused A, B, or R inputs on either monostable must be terminated either high or low. Floating inputs risk undefined triggering, increased ICC, or erratic output behavior due to CMOS input sensitivity. For example, an unused A input should connect to V_CC or GND via ≤10 kΩ; unused B inputs may leverage their Schmitt-trigger hysteresis but still require DC bias. Leaving any input unconnected violates recommended operating conditions for CD74HC221M.

How does the reset function interact with an active output pulse on the CD74HC221M?

The reset input (R) on the CD74HC221M is asynchronous and overriding: asserting R low at any point during an active output pulse forces immediate termination of that pulse, driving Q low and Q̅ high regardless of remaining timing period. The monostable remains in reset until R returns high, after which it is ready to accept new triggers. This behavior is confirmed in the truth table and functional description - no setup/hold timing is required relative to the ongoing pulse.

Is the CD74HC221M pin-compatible with the CD74HCT221M series?

Yes, the CD74HC221M and CD74HCT221M share identical SOIC-16 pinouts, terminal assignments, and package dimensions. However, they differ electrically: CD74HC221M accepts 2V–6V supplies and has CMOS input thresholds, while CD74HCT221M is limited to 4.5V–5.5V and features TTL-compatible inputs (VIH = 2V min, VIL = 0.8V max). Interchangeability requires matching supply and input logic families - direct substitution is possible only if system voltage and driver types align.