74688 8-Bit Equality Detector/Comparator

Overview

The 74688 is an advanced 8-bit equality detector/comparator IC that utilizes silicon-gate CMOS technology to perform bit-for-bit comparison of two 8-bit words. This device provides a low-active output (PeQ) to indicate when the two input words are exactly equal. Designed with cascading capability through its active-low enable input, the 74688 is suitable for systems requiring multiple comparison stages or wider data word comparisons.

This component combines high-speed operation with the power efficiency of CMOS technology, making it ideal for both battery-powered and high-performance computing applications. The TTL-compatible inputs ensure easy integration with existing logic families while maintaining CMOS-level power characteristics.

Key Features

• Advanced CMOS Technology: Utilizes silicon-gate CMOS for optimal performance and power efficiency
• TTL Input Compatibility: Direct interface with TTL logic levels (74HCT series)
• High-Speed Operation: Typical propagation delay of 20 ns
• Low Power Consumption: Maximum quiescent current of 80 μA (74HCT series)
• Strong Output Drive: Capable of sourcing/sinking 4 mA
• Cascadable Design: Active-low enable input for multi-package expansion
• Wide Operating Voltage: Compatible with standard 5V logic systems
• Pin-compatible Alternative: Functional equivalent to HCT521

Applications

The 74688 finds use in numerous digital systems requiring precise data comparison:

• Memory Address Decoding: Compare address buses for memory selection
• Data Validation Systems: Verify data integrity in transmission systems
• Control Systems: State comparison in finite state machines
• Test Equipment: Expected vs. actual result comparison
• Security Systems: Code or password verification
• Arithmetic Logic Units: Equality comparison in processors
• Cascaded Comparison Networks: Multiple devices can be chained for wider word comparison

Electrical Characteristics

Absolute Maximum Ratings

• Supply Voltage (Vcc): -0.5V to +7V
• Input Voltage: -0.5V to Vcc + 0.5V
• Output Voltage: -0.5V to Vcc + 0.5V
• Operating Temperature Range: -55°C to +125°C (Military grade available)
• Storage Temperature Range: -65°C to +150°C
• Maximum Power Dissipation: 500 mW

Recommended Operating Conditions

• Supply Voltage (Vcc): 4.5V to 5.5V
• Input High Voltage: 2.0V minimum
• Input Low Voltage: 0.8V maximum
• Operating Temperature: 0°C to +70°C (Commercial), -40°C to +85°C (Industrial)

DC Characteristics

• Input Leakage Current: ±1 μA maximum at Vcc = 5.5V
• Output High Voltage: 3.84V minimum (Ioh = -4 mA)
• Output Low Voltage: 0.33V maximum (Iol = 4 mA)
• Quiescent Supply Current: 80 μA maximum (74HCT series)

AC Characteristics

• Propagation Delay (A/B to PeQ): 20 ns typical, 30 ns maximum
• Enable to Output Delay: 25 ns typical, 35 ns maximum
• Input Capacitance: 3 pF typical
• Power Dissipation Capacitance: 30 pF typical

Functional Description

The 74688 performs a bitwise comparison between two 8-bit inputs (A0-A7 and B0-B7). The equality output (PeQ) goes low when all corresponding bit pairs are equal (A0=B0, A1=B1, etc.) and the enable input (E) is active low. The enable input facilitates device cascading - when driven high, it forces the PeQ output high regardless of the input states, allowing the output to reflect the comparison result of a subsequent device in the chain.

Internal circuitry includes eight XNOR gates (for bit comparison) feeding into a 9-input AND gate (8 comparison results plus the inverted enable). This configuration ensures the output only goes low when all bits match AND the device is enabled.

Package Information

Available Packages

• 20-pin Plastic Dual In-line Package (PDIP): Standard through-hole mounting
• 20-pin Small Outline Package (SOIC): Surface-mount version
• 20-pin Shrink Small Outline Package (SSOP): Higher density surface-mount option

Pin Configuration

The 74688 is typically packaged in a 20-pin configuration with the following pinout:

• Pins 1-8: A0-A7 (First 8-bit input word)
• Pins 11-18: B0-B7 (Second 8-bit input word)
• Pin 9: Enable (E) - Active low
• Pin 10: Ground (GND)
• Pin 19: Equality Output (PeQ) - Active low
• Pin 20: Vcc (+5V supply)

Thermal Characteristics

• Thermal Resistance (θJA): 80°C/W (PDIP), 65°C/W (SOIC)
• Maximum Junction Temperature: 150°C

Design Considerations

When implementing the 74688 in system designs, consider the following:

• For high-speed applications, minimize trace lengths to reduce capacitive loading
• Use proper decoupling capacitors (0.1 μF ceramic) near the Vcc pin
• When cascading devices, account for additional propagation delay through the chain
• For unused inputs, tie them to either Vcc or GND (do not leave floating)
• In high-noise environments, consider adding small series resistors (100Ω) on inputs

Reliability Information

The 74688 meets or exceeds standard reliability requirements for commercial and industrial applications:

• ESD Protection: All inputs and outputs protected against electrostatic discharge
• Latch-up Immunity: Exceeds 100 mA per JEDEC Standard 17
• Burn-in: Available for high-reliability applications
• Quality Conformance: MIL-PRF-38535 available for military applications