Microchip Technology 24LC21/P

Overview

24LC21/P from Microchip Technology Inc. is a 128 × 8-bit I²C-compatible serial EEPROM designed for monitor identification via DDC1/DDC2 interface. It operates from 2.5V to 5.5V, supports dual-mode operation (Transmit-only and Bidirectional), delivers 1 mA active current, and ensures 1,000,000 erase/write cycles with >200-year data retention. It is used in display configuration storage and system-level monitor handshake protocols.

For engineers reviewing the 24LC21/P datasheet, 24LC21/P pinout, 24LC21/P application, or 24LC21/P equivalent, this page provides verified technical context, validated pin functions, confirmed dual-mode timing behavior, real-world DDC interface constraints, and accurate package-specific electrical limits per DS21095K.

Technical Context

The 24LC21/P implements two independent communication modes: Transmit-only mode uses VCLK as clock input and SDA as unidirectional output for fixed-protocol DDC bitstream transmission; Bidirectional mode uses SCL as master-controlled clock and SDA as open-drain bidirectional I²C bus line. Mode transition occurs only on valid high-to-low SCL edge, and return to Transmit-only requires power cycle.

Write protection is hardware-enforced via VCLK logic level: write enabled only when VCLK ≥ 2.0 V (for VCC ≥ 2.7 V); write disabled when VCLK ≤ 0.8 V. Internal address pointer auto-increments during sequential reads, and page writes are limited to 8-byte boundaries without wrap-around across physical pages.

Key Specifications

Pinout & Package

24LC21/P is supplied in 8-pin PDIP (Plastic Dual In-line Package) with 0.300-inch body width and JEDEC MS-001 footprint. Pin 1 is bottom-left corner when notch faces up; pins numbered counterclockwise.

Key Features

Applications

Equivalent & Alternatives

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

Compared with 24LC21/P, 24LCS21A-I/P offers drop-in replacement with superior noise immunity and lifecycle support, while AT24C01C-PUM simplifies BOM but sacrifices DDC-specific features and requires firmware redesign to remove Transmit-only dependencies.

Availability

24LC21/P is available at Aetrix Electronics and suitable for display interface design, monitor manufacturing, and embedded graphics subsystems requiring stable component supply and legacy DDC compatibility.

Supply support for 24LC21/P 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 24LC21/P belongs to Microchip's legacy DDC-optimized EEPROM product line, engineered specifically for reliable EDID storage and real-time display handshake in PC and monitor ecosystems.

FAQ

What is the primary function of the VCLK pin on the 24LC21/P?

The VCLK pin on the 24LC21/P serves two distinct roles: in Transmit-only mode, it acts as the clock input that drives EDID bitstream output on SDA; in Bidirectional mode, it functions as a hardware write-enable control - writing is only permitted when VCLK is held high (≥2.0 V). This dual function eliminates software-based write protection schemes and ensures deterministic mode behavior per DS21095K.

Does the 24LC21/P support standard I²C addressing and protocol?

Yes, the 24LC21/P supports standard I²C addressing with a fixed 7-bit device code of '1010' (0xA0 write / 0xA1 read) and complies with I²C timing specifications at both 100 kHz (2.5V) and 400 kHz (5V). Its SCL and SDA pins include Schmitt-trigger inputs and noise filtering, enabling robust operation on electrically noisy display interconnects - confirmed in Section 3.1 and Table 1-2 of the 24LC21/P datasheet DS21095K.

Can the 24LC21/P be used in modern display designs requiring DDC2B or DDC/CI compliance?

Yes, the 24LC21/P fully implements DDC1 and DDC2 protocols as stated in its Features section and Functional Description. It supports DDC2B timing requirements including THIGH ≥ 4 µs and TLOW ≥ 4.7 µs at 100 kHz, and is explicitly qualified for monitor identification in VGA, DVI, and early HDMI implementations. However, Microchip marks it "Not recommended for new designs" in favor of 24LCS21A due to enhanced ESD and longevity.

What happens if the 24LC21/P receives a Start condition while in Transmit-only mode?

If a valid high-to-low transition occurs on the SCL pin while the 24LC21/P is in Transmit-only mode, the device immediately transitions to Bidirectional mode - confirmed in Section 2.0 and Figure 3-1 of DS21095K. The VCLK-driven bitstream halts, SDA becomes bidirectional, and the device begins responding to I²C slave address 0xA0/0xA1. No reset or power cycle is needed to enter Bidirectional mode.

How does the 24LC21/P handle page boundaries during write operations?

The 24LC21/P implements an 8-byte page buffer with strict boundary enforcement: page writes cannot cross physical page boundaries (addresses 0x00–0x07, 0x08–0x0F, etc.). If a write command attempts to span pages, data wraps to the start of the current page and overwrites prior bytes - documented in Figure 4-3 and Section 4.2. Host firmware must align page writes to 8-byte boundaries to avoid unintended corruption of the 24LC21/P memory array.