STMicroelectronics M74HC688TTR

Overview

M74HC688TTR from STMicroelectronics is a high-speed CMOS 8-bit equality comparator with pin- and function-compatibility to the 74-series 688. It compares two 8-bit words (P0–P7 and Q0–Q7) bit-for-bit and asserts an active-low P=Q output when identical. Key specs include tPD = 17 ns (typ. at VCC = 6 V), ICC ≤ 4 µA (max. at TA = 25°C), and operation across 2 V to 6 V supply.

For engineers reviewing the M74HC688TTR datasheet, M74HC688TTR pinout, M74HC688TTR application, or M74HC688TTR equivalent, this device serves in digital logic systems requiring fast, low-power word comparison-especially in address decoding, data validation, and multi-chip bus arbitration where cascaded 8-bit comparisons are needed via its active-low enable (G) input.

Technical Context

The M74HC688TTR implements a combinatorial equality check using silicon gate C2MOS technology, delivering symmetrical propagation delays (tPLH ≅ tPHL) and balanced output drive (|IOH| = IOL ≥ 4 mA). Its single active-low enable (G) allows daisy-chaining multiple units for >8-bit comparisons without external logic.

All inputs feature integrated protection against ESD and transient overvoltage, and the device meets wide operating conditions: VCC = 2–6 V, Top = –55°C to +125°C, with VIH/VIL thresholds defined as 70%/30% of VCC across voltage grades-ensuring robust noise immunity (VNIH = VNIL ≥ 28% VCC min).

Key Specifications

Pinout & Package

TSSOP-20 package: 4.4 mm × 6.5 mm body, 0.65 mm pitch, thin-profile surface-mount construction with exposed pad not present; RoHS-compliant, moisture sensitivity level (MSL) 1.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 8-bit equality comparator applications.

Compared with SN74HC688N and 74ACT688SCX, the M74HC688TTR uniquely balances ultra-low ICC (≤4 µA), 2–6 V flexibility, and TSSOP-20 space efficiency-making it optimal for modern low-power, mixed-voltage embedded controllers where thermal and board area constraints preclude DIP or ACT-family solutions.

Availability

M74HC688TTR is available at Aetrix Electronics and suitable for memory address decoding, microcontroller bus arbitration, firmware integrity checks, and industrial PLC I/O mapping requiring stable component supply across extended temperature and voltage ranges.

Supply support for M74HC688TTR 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

STMicroelectronics is a global semiconductor leader headquartered in Geneva, designing and manufacturing analog, digital, and mixed-signal ICs for automotive, industrial, and consumer markets.

The M74HC688TTR belongs to ST's high-speed HC logic product line, engineered for low-power, wide-supply digital interfacing in space- and energy-constrained embedded systems.

FAQ

What is the maximum recommended operating frequency for cascaded M74HC688TTR devices?

The M74HC688TTR has no internal clock; its maximum effective comparison rate depends on propagation delay and fan-out. With tPD = 17 ns (typ.) and G-to-P=Q delay of 10 ns (typ. at 6 V), cascaded configurations support up to ~25 MHz sustained comparison cycles when driving ≤1 TTL load per stage and using proper PCB layout practices.

Can the P=Q output drive an LED directly?

No. The P=Q output is active-low and designed for logic-level interfacing-not power switching. Its IOL is rated ≥4 mA (min.), insufficient for typical LEDs requiring 10–20 mA. An external NPN transistor or buffer IC is required to sink LED current while preserving logic integrity and avoiding VOL degradation.

Is the M74HC688TTR compatible with 3.3 V LVTTL logic families?

Yes, with qualification. At VCC = 3.3 V, VIH = 2.31 V (70% of 3.3 V) and VIL = 0.99 V (30% of 3.3 V), matching LVTTL thresholds (VIH = 2.0 V min., VIL = 0.8 V max.). Output VOH = 3.15 V (min. at IO = –20 µA) meets LVTTL VOH requirement, confirming interoperability in mixed-voltage designs.

Does the M74HC688TTR require external pull-up resistors on P=Q?

Yes. The P=Q output is an active-low, totem-pole (not open-drain) structure, but lacks internal pull-up. A 4.7 kΩ–10 kΩ resistor to VCC is required to assert logic HIGH when words differ. Without it, the output floats high-impedance during mismatch, risking metastability in downstream logic.