Microchip Technology 24LC21A-I/SN

Overview

24LC21A-I/SN from Microchip Technology Inc. is a 128 × 8-bit dual-mode I²C-compatible serial EEPROM designed for display monitor identification (DDC1/DDC2) and configuration storage. It operates in Transmit-Only mode (VCLK-clocked, unidirectional EDID streaming) or Bidirectional mode (SCL-clocked, byte-addressable read/write), supports 2.5–5.5V supply, delivers 1 mA active current, and features 1M endurance with >200-year data retention.

For engineers reviewing the 24LC21A-I/SN datasheet, 24LC21A-I/SN pinout, 24LC21A-I/SN application, or 24LC21A-I/SN equivalent, key selection considerations include DDC compliance, dual-clock interface (VCLK + SCL), write-protection via VCLK logic level, SOIC-8 industrial-temp packaging, and guaranteed 100 kHz (2.5V) / 400 kHz (5V) I²C timing.

Technical Context

The 24LC21A-I/SN implements two distinct operational modes sharing one SDA line: Transmit-Only mode uses VCLK as clock input to stream EDID data continuously from address 00h to 7Fh (MSB-first, 9th null bit), requiring no master control; Bidirectional mode uses SCL for standard I²C protocol with slave address 1010000X and supports byte/page writes (up to 8 bytes) and random/sequential reads.

Mode transition occurs on first valid high-to-low SCL edge after power-up; once in Bidirectional mode, only power cycle resets it to Transmit-Only. VCLK serves dual roles: clock source in Transmit-Only mode and write-enable control (high = writable, low = ROM lock) in Bidirectional mode - critical for system-level DDC security and initialization robustness.

Key Specifications

Pinout & Package

24LC21A-I/SN is housed in an 8-pin SOIC package (3.90 mm wide), RoHS-compliant, rated for industrial temperature range (−40°C to +85°C). Pin 1 is marked with a dot or notch; pin numbering follows standard SOIC convention (counterclockwise from top-left corner).

Key Features

Applications

Equivalent & Alternatives

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

Compared with 24LC21A-I/SN, the 24LC21A-I/P offers identical dual-mode behavior in a through-hole package for prototyping, while the AT24C01C lacks VCLK-driven EDID streaming - making it unsuitable for DDC1-compliant plug-and-play monitors without additional firmware overhead.

Availability

24LC21A-I/SN is available at Aetrix Electronics and suitable for monitor EDID storage, display controller configuration, embedded video interfaces, and KVM switch firmware storage requiring stable component supply across industrial temperature ranges and long-lifecycle deployments.

Supply support for 24LC21A-I/SN 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 leading provider of microcontrollers, analog devices, and memory solutions, headquartered in Chandler, Arizona, with global design and manufacturing operations.

The 24LC21A belongs to Microchip's DDC-optimized serial EEPROM product line, engineered specifically for VESA-compliant display identification and configuration in monitors, projectors, and video infrastructure equipment.

FAQ

What is the primary function of the VCLK pin on the 24LC21A-I/SN?

The VCLK pin on the 24LC21A-I/SN serves two critical functions: in Transmit-Only mode, it acts as the clock input that drives continuous EDID bitstream output on SDA; in Bidirectional mode, it functions as a hardware write-enable control - logic high permits writes, logic low locks the memory as read-only ROM. This dual role enables both automatic plug-and-play operation and secure field updates without software intervention.

How does the 24LC21A-I/SN ensure DDC1 and DDC2 compliance?

The 24LC21A-I/SN ensures DDC1/DDC2 compliance by fully implementing the VESA DDC Standard flowchart (excluding Access.bus), including automatic recovery to Transmit-Only mode after 128 VCLK pulses if no SCL activity is detected, precise timing for VCLK and SCL signals per DDC specifications, and correct response to control byte 1010000X. Its dual-mode architecture natively satisfies both DDC1 (unidirectional streaming) and DDC2 (bidirectional I²C) requirements without external logic.

Can the 24LC21A-I/SN be used in systems operating at 3.3V?

Yes, the 24LC21A-I/SN supports operation from 2.5V to 5.5V, making it fully compatible with 3.3V systems. At 3.3V, it maintains 100 kHz I²C speed and meets all DC/AC specifications including VIH (≥2.31V), VIL (≤0.99V), and ICCS standby current (≤30 μA). Its Schmitt-trigger inputs on SDA, SCL, and VCLK provide noise immunity essential for stable operation in mixed-voltage display interfaces.

What happens if power is removed while the 24LC21A-I/SN is in Bidirectional mode?

If power is removed while the 24LC21A-I/SN is in Bidirectional mode, it automatically resets to Transmit-Only mode upon next power-up. This is a hardwired behavior: the device always initializes in Transmit-Only mode, streaming EDID from address 00h on VCLK. The only way to enter Bidirectional mode is via a valid high-to-low transition on SCL - ensuring safe, predictable startup behavior for display enumeration regardless of prior state.

Does the 24LC21A-I/SN support page write operations, and what is the page size?

Yes, the 24LC21A-I/SN supports page write operations with an 8-byte page buffer. Up to eight data bytes can be loaded into the buffer before issuing a Stop condition, after which all bytes are written simultaneously in a self-timed cycle. Physical page boundaries align to 8-byte intervals (addresses 00h–07h, 08h–0Fh, etc.), and crossing a boundary causes wraparound within the current page - requiring application-level address validation to prevent unintended overwrites.