Renesas X24C44S

Overview

X24C44S from Xicor is a serial 256-bit nonvolatile static RAM (NOVRAM) with 16 × 16 organization, integrating bit-mapped EEPROM and SRAM on a single die. It operates from a single 5V ±10% supply, supports hardware STORE/RECALL via dedicated pins, and delivers sub-2µs RECALL and ≤5ms STORE cycles - ideal for microcontroller-based systems requiring persistent memory with zero wait-state RAM access.

For engineers reviewing the X24C44S datasheet, X24C44S pinout, X24C44S application, or X24C44S equivalent, this device serves as a legacy serial NOVRAM solution where deterministic nonvolatile data retention, hardware-triggered memory transfer, and TTL/CMOS-compatible serial interface are required in space-constrained industrial control or instrumentation designs.

Technical Context

The X24C44S implements a dual-array architecture: a 256-bit static RAM overlaid bit-for-bit with a nonvolatile EEPROM array, enabling atomic STORE (RAM→EEPROM) and RECALL (EEPROM→RAM) operations. Its instruction register accepts 8-bit opcodes via DI/SK, with MSB-first serial protocol and static clocking support.

Operation requires two latches: a software-set "write enable" latch (via WREN) and a "previous recall" latch (set by RCL or RECALL pin), both reset on power-up. Power-up RECALL loads EEPROM to RAM but does not set the previous recall latch - a deliberate design to prevent accidental STOREs before valid RAM state is confirmed.

Key Specifications

Pinout & Package

Package: 8-lead SOIC (Package Code S8), surface-mount compatible with standard 1.27mm pitch footprint and 0.197" width.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar serial nonvolatile RAM applications.

Compared with X24C44S, X24C04S provides pin-compatible downsizing for lower-complexity designs, while FM24C04 trades deterministic hardware triggers for higher endurance and speed - requiring architectural changes rather than drop-in replacement.

Availability

X24C44S is available at Aetrix Electronics and suitable for industrial control, medical instrumentation, automotive body electronics, and test equipment requiring stable component supply amid obsolescence challenges.

Supply support for X24C44S 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

Xicor, Inc. was a fabless semiconductor company specializing in nonvolatile memory and programmable analog ICs, acquired by Intersil in 2001 and later integrated into Renesas Electronics.

The X24C44S belongs to Xicor's serial NOVRAM product line, designed specifically for embedded systems needing byte- or word-level nonvolatile storage with hardware-controlled persistence and zero-wait-state RAM access.

FAQ

What is the function of the RECALL pin on the X24C44S?

The RECALL pin (Pin 6) on the X24C44S is an active-LOW hardware trigger that initiates an immediate transfer of data from the internal EEPROM array to the static RAM array in ≤2µs. Unlike software-initiated RCL instructions, this pin allows asynchronous, firmware-independent restoration of saved configuration - critical for fail-safe recovery in power-loss scenarios. The X24C44S requires the "previous recall" latch to be set for STORE operations, but RECALL pin assertion satisfies this condition independently of CPU execution.

Does the X24C44S require external pull-up resistors on its STORE and RECALL pins?

Yes, the X24C44S requires external pull-up resistors on both STORE (Pin 7) and RECALL (Pin 6) pins when used in hardware-triggered mode. The datasheet specifies that tying these pins to V_CC via pull-ups prevents unintended STORE/RECALL during power transitions. Typical values range from 4.7kΩ to 10kΩ; selection depends on system noise immunity requirements and drive strength of controlling logic. Without pull-ups, floating inputs risk spurious activation due to noise or leakage, potentially corrupting nonvolatile data.

How does the X24C44S handle power-up sequence timing?

The X24C44S performs an automatic RECALL operation upon V_CC stabilization but does not set the "previous recall" latch during this process. Engineers must wait ≥200µs (t_PUR) after V_CC reaches nominal 5V before issuing commands - this ensures internal circuitry is fully operational. To enable subsequent STORE operations, a separate RECALL (via RCL instruction or RECALL pin) must be executed first. This two-phase power-up behavior prevents accidental writes before RAM contents are validated, a key reliability feature of the X24C44S architecture.

Can the X24C44S be interfaced directly with an 8051 microcontroller?

Yes, the X24C44S is explicitly designed for direct interface with 8051-family microcontrollers. Its serial protocol matches COPS™ (Compatible One Pin Serial) timing, uses TTL/CMOS-compatible voltage levels (V_IL ≤ 0.8V, V_IH ≥ 2.0V), and requires no external level shifters. The 8051's I/O ports can drive CE, SK, DI, and sample DO using bit-banged GPIO, while STORE and RECALL pins connect to interrupt-capable pins for hardware-triggered persistence. The X24C44S datasheet confirms compatibility with 8051 timing margins and static clock operation.

What is the purpose of the "write enable" latch in the X24C44S?

The "write enable" latch in the X24C44S is an internal security mechanism that must be explicitly set via the WREN instruction before any RAM write or STORE operation is permitted. It is automatically reset on power-up and after each STORE completion, preventing accidental overwrites during initialization or noise events. This latch works in conjunction with the "previous recall" latch - both must be asserted for STORE to execute - forming a dual-safety interlock that ensures EEPROM data integrity, especially during uncontrolled power-down sequences where the X24C44S remains susceptible to corruption.