Microchip Technology 24LC21-I/P

Overview

24LC21-I/P from Microchip Technology Inc. is a 128 × 8-bit I²C-compatible serial EEPROM with dual-mode operation (Transmit-only and Bidirectional), 2.5V–5.5V supply range, 100 kHz/400 kHz clock compatibility, and 1 million erase/write cycles. It serves as a DDC1/DDC2-compliant monitor identification memory in display interface systems.

For engineers reviewing the 24LC21-I/P datasheet, 24LC21-I/P pinout, 24LC21-I/P application, or 24LC21-I/P equivalent, key selection considerations include its unique VCLK-controlled Transmit-only mode, SCL-triggered mode transition, write-protection via VCLK logic level, page-write buffer (8 bytes), and industrial temperature rating (–40°C to +85°C).

Technical Context

The 24LC21-I/P implements two independent serial protocols: a unidirectional Transmit-only mode synchronized to VCLK (for legacy DDC monitor ID streaming), and a bidirectional I²C-compliant mode triggered by a high-to-low SCL edge. It uses Schmitt-trigger inputs on SCL/SDA for noise immunity and includes internal VCC threshold detection to disable writes below 1.5V.

Its memory architecture supports byte and page writes (up to 8 bytes per page), with automatic address wraparound in Transmit-only mode and internal address pointer incrementation in Bidirectional read modes. Write protection is enforced by holding VCLK low in Bidirectional mode, disabling all programming operations.

Key Specifications

Pinout & Package

24LC21-I/P is housed in an 8-pin PDIP (Plastic Dual In-line Package) with 300-mil body width, compliant with JEDEC MS-001. Pin 1 is bottom-left corner when notch faces up; pins are numbered counterclockwise.

Key Features

Applications

Equivalent & Alternatives

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

Compared with 24LC21-I/P, the 24LCS21A-I/P offers improved reliability and longevity for new designs, while the AT24C01C-PU sacrifices DDC streaming capability for broader voltage operation and simplified interface - making it suitable only when Transmit-only mode is unused.

Availability

24LC21-I/P is available at Aetrix Electronics and suitable for industrial display interfaces, monitor EDID storage, KVM switch configuration, and embedded video signal generator applications requiring stable component supply and long-term obsolescence management.

Supply support for 24LC21-I/P 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

Microchip Technology Inc. is a U.S.-based semiconductor manufacturer specializing in microcontrollers, analog devices, and nonvolatile memory solutions for embedded control applications.

The 24LC21-I/P belongs to Microchip's legacy serial EEPROM product line, designed specifically for display interface systems requiring DDC1/DDC2 compliance and dual-mode memory access in industrial environments.

FAQ

What is the primary function of the VCLK pin on the 24LC21-I/P?

The VCLK pin on the 24LC21-I/P serves two distinct functions: in Transmit-only mode, it acts as the clock input that synchronizes serial output of memory contents on the SDA pin; in Bidirectional mode, it functions as a hardware write-enable control - a logic-high level is required to permit write operations, and pulling it low disables all writes. This dual role is central to the 24LC21-I/P's DDC compliance and robust configuration protection.

How does the 24LC21-I/P enter Bidirectional mode from power-up?

The 24LC21-I/P powers up in Transmit-only mode and remains there until a valid high-to-low transition is detected on the SCL pin. This transition triggers an internal state change, switching the device into Bidirectional mode where SCL becomes the active clock and VCLK is ignored for timing (though still required high for writes). The 24LC21-I/P cannot return to Transmit-only mode without a full power cycle.

Can the 24LC21-I/P be used as a standard I²C EEPROM without leveraging its Transmit-only mode?

Yes, the 24LC21-I/P fully supports standard I²C Bidirectional mode operation with slave address 1010xxxR/W, byte and page writes, and current/random/sequential reads - identical to common I²C EEPROMs. Its Transmit-only mode is optional and inactive unless VCLK is actively clocked and SCL is held high; leaving SCL unconnected or pulled high allows exclusive use of I²C functionality in the 24LC21-I/P.

What is the maximum page-write size supported by the 24LC21-I/P?

The 24LC21-I/P supports a page-write buffer of exactly 8 bytes. Page writes must remain within a single physical page boundary - defined by addresses 0x00–0x07, 0x08–0x0F, etc. Attempting to write across a page boundary causes wraparound within the current page, overwriting earlier bytes. This constraint is inherent to the 24LC21-I/P's internal page latch architecture and must be managed in firmware.

Why does the 24LC21-I/P require VCLK to be held high during write operations in Bidirectional mode?

The 24LC21-I/P uses VCLK as a hardware write-enable signal in Bidirectional mode: only when VCLK is logic-high will the internal programming circuitry accept write commands. This provides fail-safe protection against unintended writes during power transients, noise events, or software errors. If VCLK is low or floating during a write attempt, the 24LC21-I/P ignores the command and returns no acknowledge - a deliberate safety feature documented in the 24LC21-I/P datasheet Section 6.0.