STMicroelectronics M74HC688B1R

Overview

M74HC688B1R from STMicroelectronics is a high-speed CMOS 8-bit equality comparator with pin- and function-compatibility to the 74-series 688. It performs bit-by-bit comparison of two 8-bit words (P0–P7 and Q0–Q7), outputs active-low equality signal (P=Q), features 17ns typical propagation delay at 6V, 2V–6V supply range, and 4µA max quiescent current at 25°C - used in digital logic systems requiring fast word matching such as address decoding and bus arbitration.

For engineers reviewing the M74HC688B1R datasheet, M74HC688B1R pinout, M74HC688B1R application, or M74HC688B1R equivalent, key selection criteria include active-low enable (G), symmetrical output drive (±4mA), wide VCC range (2–6V), noise immunity (28% VCC), and cascading capability for multi-byte comparisons.

Technical Context

The M74HC688 implements combinational logic to assert P=Q only when all eight corresponding Pn/Qn bits match and G is low. Its silicon gate C2MOS process enables low static power and high noise immunity without internal latching or clocking.

Propagation delays are balanced (tPLH ≅ tPHL), and input/output voltage thresholds scale with VCC (VIH = 0.7×VCC min, VIL = 0.3×VCC max), ensuring robust operation across the full 2–6V supply range and temperature span from –55°C to +125°C.

Key Specifications

Pinout & Package

Package: Plastic DIP-20 (0.300" width), JEDEC MS-001, body dimensions 25.4 × 8.5 mm.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HC688N and 74ACT688PC, the M74HC688B1R offers optimal balance of wide supply range, low power, and proven interoperability in legacy and mixed-voltage industrial logic - making it preferred for cost-sensitive, thermally constrained, or multi-rail applications.

Availability

M74HC688B1R is available at Aetrix Electronics and suitable for memory address decoding, bus arbitration logic, programmable logic controller (PLC) input matching, and digital test equipment pattern recognition requiring stable component supply and long-term industrial lifecycle support.

Supply support for M74HC688B1R 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 M74HC688 belongs to ST's high-speed CMOS logic family, engineered for pin-compatible upgrades of legacy 74-series devices while delivering lower power, wider voltage tolerance, and enhanced noise immunity in industrial control and instrumentation.

FAQ

What is the function of the G (Enable) pin on the M74HC688B1R?

The G pin is an active-low enable input. When G is high, the P=Q output is forced high regardless of P and Q inputs. Only when G is low does the device perform bit-by-bit comparison of P0–P7 and Q0–Q7, asserting P=Q low upon full match. This enables cascading multiple comparators for wider word comparisons.

Can the M74HC688B1R operate reliably at 3.3V?

Yes - the M74HC688B1R is fully specified from 2V to 6V, including 3.3V operation. At VCC = 3.3V, VIH is guaranteed ≥2.31V and VIL ≤0.99V, providing 0.6V noise margin. Propagation delay increases to ~40ns (typ), and output drive remains ≥4mA, making it suitable for 3.3V logic interfacing without level shifters.

Is the M74HC688B1R pinout compatible with older 74LS688 devices?

Yes - the M74HC688B1R uses identical pin numbering and function assignment as the 74LS688, including G (pin 1), P0–P7 (pins 2,4,6,8,11,13,15,17), Q0–Q7 (pins 3,5,7,9,12,14,16,18), P=Q (pin 19), GND (pin 10), and VCC (pin 20). However, voltage thresholds differ: HC logic requires VIH ≥0.7×VCC, not fixed TTL levels.

Does the M74HC688B1R require external pull-up resistors on the P=Q output?

No - the P=Q output is a standard CMOS totem-pole driver capable of sinking and sourcing current (±4mA min). Pull-ups are unnecessary unless open-drain behavior is explicitly needed. For direct connection to HC/AC inputs, the output drives cleanly to rail; for TTL loads, verify VOH ≥2.4V at IOH = –0.4mA (met at VCC ≥4.5V).