Microchip Technology 24LCS21A/SN

Overview

24LCS21A/SN from Microchip Technology Inc. is a 128 × 8-bit dual-mode I²C™-compatible serial EEPROM designed for monitor identification (DDC1™/DDC2™) in display systems. It operates from 2.5V to 5.5V, supports both Transmit-Only (VCLK-clocked) and Bidirectional (SCL-clocked) modes, and features hardware write-protection via WP pin and VCLK-level control. Its 1 M erase/write cycles and >200-year data retention make it suitable for EDID storage in industrial-grade monitors and embedded display interfaces.

For engineers reviewing the 24LCS21A/SN datasheet, 24LCS21A/SN pinout, 24LCS21A/SN application, or 24LCS21A/SN equivalent, key selection considerations include DDC1/DDC2 mode switching behavior, VCLK vs. SCL clock domain separation, WP fuse activation logic at address 7Fh, and SOIC-8 package compatibility with legacy display controller footprints.

Technical Context

The 24LCS21A/SN implements two independent 2-wire protocols sharing SDA: Transmit-Only mode uses VCLK as clock input and outputs memory contents sequentially from 00h to 7Fh (MSB-first), while Bidirectional mode uses SCL and responds to I²C control byte 1010000X. Mode transition requires a valid high-to-low SCL edge followed by detection of that control byte; absence triggers automatic reversion to Transmit-Only after 128 VCLK pulses.

Write protection is implemented in two layers: factory-default unlocked state (VCLK = high enables writes), and post-activation fused mode where both WP (active-low) and VCLK level jointly determine write capability per Table 6-1. Internal address pointer auto-increments during sequential reads, and page write buffer holds up to eight bytes before self-timed 10 ms programming cycle.

Key Specifications

Pinout & Package

24LCS21A/SN is housed in an 8-pin Plastic Small Outline Integrated Circuit (SOIC) package, 150 mil body width, surface-mount compatible with IPC-7351B SOIC8_150.

Key Features

Applications

Equivalent & Alternatives

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

Compared with AT24C128C-SSHL-T and 24LC128-I/SN, the 24LCS21A/SN uniquely delivers autonomous EDID streaming without host CPU involvement, enabling lower-power and simpler display subsystems - a capability absent in standard I²C EEPROMs.

Availability

24LCS21A/SN is available at Aetrix Electronics and suitable for industrial display manufacturing, KVM switch production, and embedded HMI development requiring stable component supply and long-term lifecycle assurance.

Supply support for 24LCS21A/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 U.S.-based semiconductor company specializing in microcontrollers, analog devices, and nonvolatile memory solutions for industrial, automotive, and consumer applications.

The 24LCS21A/SN belongs to Microchip's DDC-optimized serial EEPROM product line, engineered specifically for VESA-compliant display identification and configuration storage in resource-constrained video interfaces.

FAQ

What distinguishes the 24LCS21A/SN from standard I²C EEPROMs like the 24LC128?

The 24LCS21A/SN integrates dual-mode operation - Transmit-Only mode (VCLK-clocked EDID streaming) and Bidirectional mode (SCL-clocked I²C reads/writes) - whereas the 24LC128 supports only standard I²C communication. This makes the 24LCS21A/SN uniquely capable of autonomous EDID broadcast without host processor involvement, a requirement for DDC1™ compliance. The 24LCS21A/SN also includes VCLK-based write-enable logic and fuse-controlled WP pin behavior not found in the 24LC128.

How does the 24LCS21A/SN enter and exit Transmit-Only mode?

The 24LCS21A/SN powers up in Transmit-Only mode and remains there as long as SCL is held high. A valid high-to-low transition on SCL initiates mode transition; if the 24LCS21A/SN detects its I²C control byte (1010000X) within 128 VCLK pulses, it enters Bidirectional mode permanently until power cycle. If no control byte is received, it reverts to Transmit-Only mode after the 128th VCLK pulse. Power removal is the only way to reset from Bidirectional back to Transmit-Only mode.

What is the function of the WP pin on the 24LCS21A/SN, and when does it become active?

The WP pin on the 24LCS21A/SN provides hardware write protection but is inactive by default. It becomes functional only after any data is written to memory address 7Fh, which sets an internal fuse. Before fuse setting, writes are enabled solely by VCLK = high. After fuse setting, write access requires both VCLK = high AND WP = high (or open). This two-stage mechanism allows field-programmable lock-down of EDID content post-initialization.

Can the 24LCS21A/SN be used in systems operating at 3.3V, and what are the timing implications?

Yes, the 24LCS21A/SN operates down to 2.5V and is fully specified at 3.3V. At 3.3V, its I²C interface runs in Standard Mode (100 kHz max), with minimum clock high time of 4000 ns and bus free time of 4700 ns. Its VCLK timing remains valid: TVHIGH ≥ 4000 ns, TVLOW ≥ 4700 ns. All DC parameters - including VOL ≤ 0.4V at 3 mA load - are guaranteed across the 2.5V–5.5V range, ensuring robust operation in mixed-voltage display systems.

What happens if a page write crosses a physical page boundary on the 24LCS21A/SN?

If a page write command attempts to cross a physical page boundary on the 24LCS21A/SN, data wraps around to the beginning of the current page instead of advancing to the next page. Since the 24LCS21A/SN has an 8-byte page buffer, boundaries occur every 8 bytes (addresses 00h–07h, 08h–0Fh, etc.). Writing nine bytes starting at 00h will overwrite bytes 00h–07h with the first eight, then overwrite 00h again with the ninth. Application firmware must enforce page-aligned writes to prevent unintended data loss.