Microchip Technology 24LC21A/SN

Overview

24LC21A from Microchip Technology Inc. is a 128 × 8-bit dual-mode I²C-compatible serial EEPROM designed for display identification and configuration storage in DDC1/DDC2-compliant monitors. It operates in Transmit-Only mode (VCLK-clocked, unidirectional EDID streaming) and Bidirectional mode (SCL-clocked, byte-addressable I²C read/write), supports 2.5–5.5V supply, delivers 1 mA active current, and features 1,000,000 write cycles with >200-year data retention.

For engineers reviewing the 24LC21A datasheet, 24LC21A pinout, 24LC21A application, or 24LC21A equivalent, this page provides verified functional modes, VCLK/SCL dual-clock behavior, write-protection via VCLK logic level, page buffer limits (8 bytes), and industrial temperature support (–40°C to +85°C) - all critical for monitor EDID implementation and embedded system configuration memory.

Technical Context

The 24LC21A implements two distinct operational protocols sharing SDA: Transmit-Only mode uses VCLK as clock input for continuous EDID bitstream output (MSB-first, 8-bit bytes + null bit), while Bidirectional mode uses SCL for standard I²C communication with slave address 1010000X. Mode transition occurs only on valid high-to-low SCL edge followed by detection of its control byte; power removal is required to reset from Bidirectional back to Transmit-Only mode.

Internally, it integrates an EEPROM array with page latches, HV generator, and dual control logic paths - one for VCLK-synchronized transmit timing and another for SCL-synchronized I²C arbitration, ACK generation, and address pointer management. Write enable requires VCLK = high during command/data transfer; VCLK is ignored during self-timed internal write cycle (TWR = 10 ms max).

Key Specifications

Pinout & Package

Available in 8-pin PDIP and SOIC packages (industrial temperature range: –40°C to +85°C). Both packages share identical pin mapping and function assignment.

Key Features

Applications

Equivalent & Alternatives

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

Compared with 24LC21 and AT24C01C, the 24LC21A uniquely combines DDC1/DSC2 protocol compliance, dual-clock domain operation, and industrial temperature support - making it irreplaceable in certified monitor designs, whereas alternatives serve only generic serial memory roles.

Availability

24LC21A is available at Aetrix Electronics and suitable for monitor EDID storage, display configuration memory, embedded system identity, industrial HMI calibration storage, and DDC-compliant video interface design requiring stable component supply across extended temperature ranges.

Supply support for 24LC21A 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 components, 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 reliable, standards-compliant display identification and configuration in PC, industrial, and broadcast video systems.

FAQ

What are the two operational modes of the 24LC21A and how do they differ?

The 24LC21A operates in Transmit-Only mode (VCLK-clocked, unidirectional EDID bitstream output starting at address 00h) and Bidirectional mode (SCL-clocked, standard I²C read/write using slave address 1010000X). Mode transition occurs only on a valid high-to-low SCL edge followed by detection of the control byte; power removal is required to revert from Bidirectional to Transmit-Only mode. The 24LC21A uses these modes to satisfy both DDC1 streaming and DDC2B interactive configuration requirements within a single device.

How does VCLK function differently in each mode of the 24LC21A?

In Transmit-Only mode, VCLK serves as the exclusive clock input - each rising edge clocks out one data bit on SDA. In Bidirectional mode, VCLK is repurposed as a write-enable control: logic high permits writes, logic low disables all write operations (effectively turning the 24LC21A into a read-only serial ROM). The 24LC21A ignores VCLK during its internal self-timed write cycle (TWR ≤ 10 ms), ensuring programming completes regardless of external VCLK state.

Can the 24LC21A be used as a drop-in replacement for the 24LC21 in existing designs?

Yes, the 24LC21A is pin- and function-compatible with the 24LC21, supporting identical VCLK/SCL/SDA signaling and dual-mode behavior. However, the 24LC21A adds industrial temperature range (–40°C to +85°C), Pb-free/RoHS compliance, and updated ESD rating (>4000V). No PCB or firmware changes are required, but designers should verify thermal margins and regulatory compliance requirements before substitution. The 24LC21A maintains full backward compatibility while extending environmental robustness.

What is the maximum page write size supported by the 24LC21A and what happens if exceeded?

The 24LC21A supports a maximum page write size of eight bytes, buffered internally before committing to EEPROM. If more than eight bytes are transmitted before the Stop condition, the address counter wraps within the current page (bits A2–A0), overwriting earlier bytes in that same 8-byte boundary. Crossing a physical page boundary (e.g., writing from address 07h to 08h) causes wraparound to address 00h instead of advancing to the next page. This behavior is inherent to the 24LC21A's architecture and requires application-layer boundary checking to prevent unintended data loss.

Does the 24LC21A support standard I²C repeated start conditions and ACK polling?

Yes, the 24LC21A fully supports I²C repeated Start conditions for random and sequential reads, and implements ACK polling to detect write completion: after issuing a Stop condition to initiate a write, the master can immediately send a Start + control byte (R/W = 0); absence of ACK indicates ongoing internal programming, while ACK confirms readiness for the next command. This feature is explicitly documented in Section 5.0 of the 24LC21A datasheet and enables efficient bus utilization without fixed delay loops. The 24LC21A's ACK polling behavior is identical to standard I²C EEPROMs but operates within its dual-mode framework.