NXP Semiconductors PCA8550PW/DG,118

Overview

PCA8550PW/DG,118 from NXP Semiconductors is a 4-bit 2-to-1 I²C multiplexer with integrated 5-bit non-volatile EEPROM and a latched 1-bit non-multiplexed output. It functions as a programmable DIP switch replacement in Pentium Pro/Pentium II motherboard configurations, supporting 2.5 V multiplexed outputs, 3.3 V latched non-multiplexed output, and 5 V tolerant inputs. Its core role is to select between external MUX_IN signals or internal register data for routing to four MUX_OUT pins under I²C control.

For engineers reviewing the PCA8550PW/DG,118 datasheet, PCA8550PW/DG,118 pinout, PCA8550PW/DG,118 application, or PCA8550PW/DG,118 equivalent, this device serves as a configurable logic-level selection element with persistent configuration storage, override safety control, and write-protected I²C register access - critical for jumperless system initialization and BIOS configuration in legacy x86 platforms.

Technical Context

The PCA8550PW/DG,118 implements a dual-path selection architecture: when MUX_SELECT = 0, all four MUX_OUT pins drive values from the 5-bit non-volatile register (bits D3–D0); when MUX_SELECT = 1, they route MUX_IN_A through MUX_IN_D. The NON_MUXED_OUT pin is transparent-latched on the rising edge of MUX_SELECT, capturing register bit D4 independently.

I²C communication uses a fixed 7-bit address (0x4C) with R/W bit, enabling read/write of the full 5-bit register (D4:D0), where D4 maps to NON_MUXED_OUT and D3:D0 map to MUX_OUT D:A. Write protection is enforced by the active-low WP pin, and the active-low OVERRIDE_N forces all MUX_OUT pins to logic 0 regardless of other controls.

Key Specifications

Pinout & Package

TSSOP16 package: plastic thin shrink small outline, 16 leads, body width 4.4 mm, lead pitch 0.65 mm, JEDEC MO-153 compliant.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar I²C-configurable DIP switch applications.

Compared with PCA9554PW,118 and PCA8574PW,118, the PCA8550PW/DG,118 uniquely integrates hardware-selectable multiplexing, non-volatile storage, and safety override - making it irreplaceable for legacy x86 strap configuration where deterministic, power-cycle-persistent, and fail-safe behavior is mandatory.

Availability

PCA8550PW/DG,118 is available at Aetrix Electronics and suitable for legacy computing platform manufacturing, industrial control system refurbishment, and embedded diagnostic subsystem design requiring stable component supply and long-term lifecycle support.

Supply support for PCA8550PW/DG,118 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

NXP Semiconductors is a global semiconductor company headquartered in Eindhoven, Netherlands, specializing in secure connectivity solutions for automotive, industrial, and consumer applications.

The PCA8550 product line was designed specifically for jumperless configuration of Intel Pentium Pro and Pentium II-based systems, delivering EEPROM-backed, I²C-controllable logic-level strapping with hardware safety features.

FAQ

What is the primary function of the PCA8550PW/DG,118?

The PCA8550PW/DG,118 is a 4-bit 2-to-1 I²C multiplexer with integrated 5-bit non-volatile EEPROM and a latched 1-bit non-multiplexed output. Its primary function is to replace physical DIP switches on Pentium Pro/Pentium II motherboards by allowing firmware to select between external input signals or stored register values for routing to MUX_OUT pins. The PCA8550PW/DG,118 provides deterministic, power-cycle-persistent configuration without requiring manual hardware changes.

How does the OVERRIDE_N pin affect PCA8550PW/DG,118 operation?

The OVERRIDE_N pin is an active-low safety control that forces all four MUX_OUT pins to logic 0 regardless of MUX_SELECT state or register contents. When OVERRIDE_N is asserted (logic 0), the PCA8550PW/DG,118 enters a fail-safe output state, overriding both multiplexer paths. This feature ensures predictable hardware behavior during system faults or diagnostic modes. The PCA8550PW/DG,118 maintains this forced-zero state until OVERRIDE_N is deasserted (logic 1).

Can the PCA8550PW/DG,118 be used with 5 V logic systems?

Yes - the PCA8550PW/DG,118 features 5 V tolerant inputs on MUX_IN A–D, OVERRIDE_N, MUX_SELECT, and WP pins, allowing direct connection to 5 V logic without level-shifting. However, its supply voltage (VCC) must remain within 3.0 V to 3.6 V, and its MUX_OUT outputs operate at 2.5 V CMOS levels while NON_MUXED_OUT operates at 3.3 V TTL levels. Therefore, the PCA8550PW/DG,118 interfaces safely with 5 V control signals but drives only 2.5 V/3.3 V loads.

What is the role of the NON_MUXED_OUT pin in PCA8550PW/DG,118?

The NON_MUXED_OUT pin delivers bit D4 of the 5-bit EEPROM register as a latched TTL-level (3.3 V) output. Unlike the multiplexed outputs, it is not selected by MUX_SELECT but is instead transparently updated when MUX_SELECT = 0 and latched on the rising edge of MUX_SELECT. This allows the PCA8550PW/DG,118 to provide one dedicated, stable configuration signal independent of the 4-bit multiplexer path - commonly used for processor-specific strapping like LINT0/INTR or A20M in Pentium systems.

How is write protection implemented in the PCA8550PW/DG,118?

Write protection in the PCA8550PW/DG,118 is controlled by the WP (Write Protect) pin: when WP = logic 0, I²C writes to the 5-bit EEPROM register are permitted; when WP = logic 1, all I²C write attempts to the register are blocked. The WP pin has an internal pull-up resistor, so it defaults to write-protected unless actively pulled low. This hardware-enforced protection prevents accidental overwrites of critical configuration data during normal system operation or firmware updates - a key reliability feature of the PCA8550PW/DG,118.