Microchip Technology 24LC21T-I/SN

Overview

24LC21T-I/SN 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 24LC21T-I/SN datasheet, 24LC21T-I/SN pinout, 24LC21T-I/SN application, or 24LC21T-I/SN 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 SOIC-8 industrial-grade packaging.

Technical Context

The 24LC21T-I/SN implements two distinct operational protocols sharing SDA but using separate clocks: VCLK drives unidirectional bitstream output of full memory contents, while SCL enables standard I²C bidirectional byte-addressable read/write. Mode switching occurs only on valid high-to-low SCL edge; return to Transmit-only requires power cycle.

Its internal architecture includes a page latch buffer for up to 8-byte writes, Schmitt-trigger inputs on SCL/SDA/VCLK for noise immunity, VCC threshold detector disabling programming below 1.5V, and open-drain SDA requiring external pull-up. Write protection is enforced by VCLK logic level: high enables write, low disables all writes including in Bidirectional mode.

Key Specifications

Pinout & Package

24LC21T-I/SN is housed in an 8-pin narrow SOIC package (SOIC-8, 150 mil body, JEDEC MS-012), rated for industrial temperature range (–40°C to +85°C). Pin 1 is marked with a dot or notch; pins are numbered 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 24LC21T-I/SN, the 24LCS21A-I/SN offers enhanced reliability and ongoing support, while the AT24C01C-SSHM-T provides basic I²C EEPROM functionality without DDC-specific features-making it unsuitable for monitor identification where Transmit-only mode is mandatory.

Availability

24LC21T-I/SN is available at Aetrix Electronics and suitable for monitor identification systems, industrial HMI configuration storage, legacy VGA/DVI EDID applications, and embedded display controller bootloader parameter retention requiring stable component supply across extended temperature ranges.

Supply support for 24LC21T-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 components, and memory solutions, headquartered in Chandler, Arizona, with global design and manufacturing operations.

The 24LC21T-I/SN belongs to Microchip's legacy serial EEPROM product line, specifically engineered for display interface compliance (DDC1/DDC2) and dual-mode operation in monitor and graphics subsystems.

FAQ

What is the primary function of the VCLK pin on the 24LC21T-I/SN?

The VCLK pin on the 24LC21T-I/SN serves two distinct roles: in Transmit-only mode, it acts as the clock input that synchronizes serial bitstream output on SDA; in Bidirectional mode, it functions as a hardware write-enable control-logic high permits writes, logic low disables all write operations including page and byte writes. This dual functionality is central to the device's DDC compliance and robust configuration protection.

How does the 24LC21T-I/SN enter Bidirectional mode from power-up?

The 24LC21T-I/SN 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 write enable). No software command or register write is needed-mode transition is purely hardware-event-driven.

Can the 24LC21T-I/SN be used as a standard I²C EEPROM without utilizing the Transmit-only mode?

Yes, the 24LC21T-I/SN fully supports standard I²C Bidirectional mode operation with slave address 1010xxx0/1, byte and page writes (up to 8 bytes), current/random/sequential reads, and ACK polling. Its I²C timing meets standard (100 kHz) and fast-mode (400 kHz) specifications. The Transmit-only mode is optional and disabled by default-users may ignore VCLK entirely if only standard EEPROM functionality is required.

What is the significance of the "T" and "I/SN" suffixes in 24LC21T-I/SN?

In 24LC21T-I/SN, "T" denotes tape-and-reel packaging (standard for automated assembly), "I" specifies industrial temperature grade (–40°C to +85°C), and "/SN" indicates the 8-pin narrow SOIC package (JEDEC MS-012, 150 mil body width). Together, they define the exact orderable variant: tape-and-reel packaged, industrial-temp, SOIC-8 version of the 24LC21.

Why does the 24LC21T-I/SN require nine VCLK cycles before valid data output in Transmit-only mode?

The 24LC21T-I/SN requires nine VCLK cycles after VCC stabilization to perform internal synchronization and reset its output state machine. During these cycles, SDA remains in high-impedance. On the rising edge of the tenth VCLK cycle, the first valid MSB of the first memory byte appears on SDA-ensuring deterministic alignment between clock and data for reliable DDC handshake timing without host-side delay calibration.