Texas Instruments SN74HC684N

Overview

SN74HC684N from Texas Instruments is an 8-bit magnitude comparator IC that performs binary or BCD word comparison between two 8-bit inputs (P₀–P₇ and Q₀–Q₇), delivering P = Q and P > Q logic-level outputs. It operates from 2 V to 6 V, delivers ±4-mA output drive at 5 V, exhibits typical propagation delay of 22 ns at 6 V, and draws ≤80 µA ICC, making it suitable for low-power digital control and address decoding in industrial microcontroller systems.

For engineers reviewing the SN74HC684N datasheet, SN74HC684N pinout, SN74HC684N application, or SN74HC684N equivalent, this device is selected for precise 8-bit equality/greater-than logic in bus arbitration, memory mapping, and programmable logic interconnect where CMOS-compatible input thresholds, high noise immunity, and LSTTL-load drive capability are required.

Technical Context

The SN74HC684N implements a combinatorial magnitude comparison function using cascaded bit-level logic to generate two active-high outputs: P = Q asserts only when all eight corresponding bits match, while P > Q asserts when the unsigned binary value of input P exceeds that of Q. No internal latching or clocking is present - operation is fully asynchronous and level-sensitive.

Input thresholds follow standard HC logic families: VIH ≥ 3.15 V and VIL ≤ 1.35 V at VCC = 4.5 V, ensuring robust noise margins. The device supports direct interface with both HC/HCT and legacy LSTTL loads via its ±4-mA drive strength and 10-LSTTL-load capability, eliminating need for buffer stages in mixed-logic designs.

Key Specifications

Pinout & Package

SN74HC684N is housed in a 20-pin plastic dual in-line package (PDIP-N), with 0.3-inch body width and through-hole mounting compatibility. Pin spacing follows JEDEC MS-001 standard (0.1-inch pitch).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 8-bit magnitude comparison applications.

Compared with SN74HC684N, SN74LS684N trades power efficiency and voltage flexibility for TTL compatibility, while SN74HC688N reduces functionality to equality-only comparison - neither offers drop-in replacement without design review of output requirements and supply constraints.

Availability

SN74HC684N is available at Aetrix Electronics and suitable for memory address decoding, bus arbitration logic, and programmable logic interconnect requiring stable component supply, long-term industrial lifecycle support, and through-hole assembly compatibility.

Supply support for SN74HC684N 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 U.S.-based semiconductor company specializing in analog, embedded processing, and logic solutions with over 50 years of industrial-grade product development.

The SN74HC684N belongs to TI's 74HC high-speed CMOS logic family, designed for low-power, wide-voltage-range digital control and interface functions in industrial, automotive, and communications equipment.

FAQ

What logic family does SN74HC684N belong to, and how does it differ from 74LS variants?

SN74HC684N belongs to the 74HC (High-Speed CMOS) logic family. It differs from 74LS (Low-Power Schottky TTL) variants like SN74LS684N by supporting 2 V–6 V operation, drawing ≤80 µA quiescent current versus ~25 mA, and offering CMOS input thresholds that scale with VCC. SN74HC684N also provides faster typical propagation delay (22 ns vs. 35 ns) at 5 V but requires careful handling of unused inputs - they must be tied to VCC or GND to prevent floating-node oscillation.

Can SN74HC684N generate a P < Q output, and if so, how?

Yes, SN74HC684N can generate a P < Q output using external logic. The device provides active-high P = Q and P > Q outputs; connecting both to a 2-input NAND gate yields an active-high P < Q signal, as confirmed in the function table and logic diagram of the official datasheet. This method requires only one additional gate and introduces minimal added delay - typically under 10 ns with HC-series NAND devices - preserving overall timing integrity in critical paths.

What is the maximum capacitive load SN74HC684N can drive while maintaining specified timing?

SN74HC684N switching characteristics (including tpd = 22 ns typical) are specified with CL = 50 pF, which includes probe and fixture capacitance per Figure 1 of the datasheet. Driving loads significantly exceeding 50 pF will increase propagation delay and may degrade edge rates; for loads >75 pF, designers should verify timing margins in-system or add a buffer stage. The output drive strength (±4 mA at 5 V) supports moderate fan-out but is not intended for transmission-line driving or heavy capacitive bus loading.

Is SN74HC684N suitable for use with 3.3 V microcontrollers, and what interface considerations apply?

Yes, SN74HC684N is fully suitable for use with 3.3 V microcontrollers. Its 2 V–6 V supply range allows direct VCC connection to 3.3 V rails, and its CMOS inputs accept VIH ≥ 2.31 V (70% of 3.3 V) and VIL ≤ 0.99 V (30% of 3.3 V), comfortably accommodating standard 3.3 V logic levels. No level-shifting circuitry is required. Ensure microcontroller outputs meet SN74HC684N's input current limit (±1 µA max) - standard GPIO pins satisfy this condition.

Does SN74HC684N require decoupling capacitors, and what value is recommended?

Yes, SN74HC684N requires local decoupling. A 0.1 µF ceramic capacitor placed as close as possible between VCC (pin 18) and GND (pin 17) is recommended per standard HC-family practice. This suppresses transient current spikes during output switching, prevents supply rail disturbance, and maintains stable logic thresholds. For boards with multiple HC devices, a bulk 4.7 µF–10 µF tantalum or aluminum electrolytic capacitor near the power entry point further improves low-frequency regulation.