Microchip Technology 24LC21A/P

Overview

24LC21A from Microchip Technology Inc. is a 128 × 8-bit dual-mode I²C-compatible serial EEPROM designed for DDC1™/DDC2™ monitor identification. 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 1M erase/write cycles with >200-year data retention - deployed in display interface circuits for real-time EDID transmission and configuration storage.

For engineers reviewing the 24LC21A datasheet, 24LC21A pinout, 24LC21A application, or 24LC21A equivalent, this page provides verified functional identity, dual-mode timing behavior, VCLK/SCL role separation, write-protection via VCLK logic level, and confirmed industrial-temperature package options - all critical for display subsystem design, EDID compliance validation, and legacy DDC1-to-DDC2 transition support.

Technical Context

The 24LC21A implements two physically distinct communication protocols sharing SDA: Transmit-Only mode uses VCLK as clock input to stream EDID contents continuously from address 00h to 7Fh (MSB-first, 9th null bit), requiring nine VCLK cycles for initialization and reverting after 128 idle pulses if no SCL transition occurs. Bidirectional mode activates only upon valid high-to-low SCL edge, then responds exclusively to slave address 1010000X, with VCLK held high to enable writes and ignored during self-timed programming.

Its internal architecture includes separate control logic paths for each mode, Schmitt-triggered VCLK/SCL/SDA inputs for noise immunity, HV generator for EEPROM programming, and page latches supporting up to eight-byte page writes. Mode persistence is absolute: once in Bidirectional mode, power cycle is the sole reset mechanism - no software or bus command can revert it.

Key Specifications

Pinout & Package

Available in 8-pin PDIP (300 mil) and SOIC (3.90 mm) packages - both RoHS-compliant, lead-free, and rated for industrial temperature range (−40°C to +85°C).

Key Features

Applications

Equivalent & Alternatives

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

Compared with 24LC21 and AT24C01C-PU, the 24LC21A uniquely combines DDC1 transmit capability with I²C bidirectional access in a single device - making it irreplaceable in systems requiring both legacy EDID streaming and modern configuration flexibility.

Availability

24LC21A is available at Aetrix Electronics and suitable for display interface design, EDID compliance validation, and industrial HMI configuration storage requiring stable component supply, long-lifecycle assurance, and RoHS-compliant sourcing.

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 U.S.-based semiconductor company specializing in microcontrollers, analog devices, and memory products, with a focus on embedded control and connectivity solutions.

The 24LC21A belongs to Microchip's DDC-compliant serial EEPROM product line, engineered specifically for display industry standards - balancing EDID streaming simplicity with I²C configurability for monitor, graphics, and interface applications.

FAQ

What is the primary function of the VCLK pin on the 24LC21A?

The VCLK pin serves two distinct roles: in Transmit-Only mode, it clocks out EDID data bits on SDA (rising-edge triggered); in Bidirectional mode, it acts as a hardware write-enable - logic high permits writes, logic low disables all programming. This dual functionality enables seamless DDC1/DDC2 interoperability without external control logic. The 24LC21A relies on VCLK state to enforce write protection and mode-specific timing.

How does the 24LC21A switch between Transmit-Only and Bidirectional modes?

The 24LC21A powers up in Transmit-Only mode and remains there until a valid high-to-low transition is detected on the SCL pin. Upon that edge, it enters Transition mode and waits for its I²C slave address (1010000X). If received, it locks into Bidirectional mode permanently until power is cycled. If no address arrives within 128 VCLK pulses, it reverts to Transmit-Only mode. This behavior is hardwired - no register or command controls it. The 24LC21A enforces strict mode persistence per VESA DDC flowchart compliance.

Can the 24LC21A be used as a standard I²C EEPROM without DDC functionality?

Yes - the 24LC21A fully supports standard I²C byte and page writes/reads in Bidirectional mode using slave address 1010000X, with ACK polling and 400 kHz operation at 5V. Its I²C implementation complies with Philips I²C specification, including Start/Stop generation, addressing, and acknowledge timing. However, unlike generic EEPROMs, it lacks a write-protect pin - instead using VCLK for hardware write enable. For non-DDC applications, the 24LC21A functions as a robust, industrial-grade I²C EEPROM with added noise immunity and endurance.

What happens if VCLK is held low while the 24LC21A is in Bidirectional mode?

If VCLK is held low during Bidirectional mode operation, all write attempts (byte or page) will be blocked - the 24LC21A ignores incoming data and does not initiate programming cycles. Reads remain fully functional. This behavior effectively converts the 24LC21A into a read-only serial ROM without modifying firmware or adding external logic. The 24LC21A uses this feature for secure configuration lockdown in field-deployed displays where accidental writes must be prevented.

Is the 24LC21A pin-compatible with other Microchip EEPROMs like the 24LC01B?

No - the 24LC21A has a unique 8-pin pinout with dedicated VCLK and NC pins not present on standard I²C EEPROMs such as the 24LC01B (which uses 8-pin SOIC/PDIP but assigns pins differently: e.g., WP instead of VCLK). Attempting pin-for-pin replacement would cause functional failure due to mismatched clock inputs and unconnected pins. The 24LC21A is only pin- and function-compatible with the earlier 24LC21, as explicitly stated in its datasheet features. Always verify pin mapping before board reuse.