Part A
24LC21AT/SN
Công nghệ Microchip
- Lifecycle
- Active
- Stock
- 4371 PCS
- Package
- SOIC-8
- Series
- Memory
Replacement verdict
Not a drop-in replacement
The 24LC256-E/SN and 24LC21AT/SN are not functionally interchangeable. The 24LC256-E/SN is a 256 Kbit I²C serial EEPROM with a 32,768 x 8 organization, while the 24LC21AT/SN is a 2 Kbit VESA EEPROM with a 256 x 8 organization and a dedicated two-wire serial interface intended for display data channel applications. Their memory density, internal organization, interface protocol, and target application differ fundamentally, so substituting one for the other is not feasible without redesigning the surrounding circuit and firmware.
Different memory density
Different interface protocol
Different internal organization
Different application target
Parts at a glance
Part B
24LC256-E/SN
Công nghệ Microchip
- Lifecycle
- Active
- Stock
- 7418 PCS
- Package
- SOIC-8
- Series
- Memory
Key differences
Rows are prioritized by design impact. Highlighted values require attention during substitution.
Key electrical and mechanical differences between the two devices
| Parameter | 24LC256-E/SN | 24LC21AT/SN | Why it matters |
|---|---|---|---|
| Memory Organization | 32K x 8 (256 Kbit) | 128 x 8 (1 Kbit) | Determines total storage capacity; the two devices differ by 256x, so they are not functionally interchangeable for most applications. |
| Memory Type | Serial EEPROM | Serial EEPROM (DDC) | Both are I2C EEPROMs, but the 24LC21A is specifically designed for VESA DDC monitor applications, which may limit general-purpose use. |
| Interface | I2C (2-wire) | I2C (2-wire) | Both use the same 2-wire I2C protocol, so bus-level compatibility exists, but addressing and protocol details differ. |
| Supply Voltage Range | 2.5 V to 5.5 V | 3.0 V to 5.5 V | The 24LC256 supports a wider low-voltage range, allowing operation in 2.5 V systems where the 24LC21A may not function reliably. |
| Maximum Clock Frequency | 400 kHz | 100 kHz | Higher clock support enables faster read/write cycles; the 24LC256 can operate at 400 kHz while the 24LC21A is limited to 100 kHz. |
| Operating Temperature Range | -40 °C to +125 °C | 0 °C to +70 °C | The extended temperature range of the 24LC256 makes it suitable for automotive and industrial environments, unlike the commercial-grade 24LC21A. |
| Package / Mounting | 8-SOIC (Surface Mount) | 8-SOIC (Surface Mount) | Both share the same 8-SOIC footprint, so PCB layout compatibility is maintained even though electrical characteristics differ. |
| Write Protection | Yes (via WP pin) | No dedicated WP pin | Hardware write protection prevents accidental data corruption; the 24LC256 offers this feature while the 24LC21A does not. |
| Page Size | 64 bytes | 8 bytes | Larger page size allows more efficient burst writes; the 24LC256 writes 64 bytes per page versus 8 bytes for the 24LC21A. |
| Typical End Application | General-purpose data storage | VESA DDC monitor identification | The intended use cases differ significantly, so substituting one for the other may require firmware and system-level changes. |
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Full specification comparison
Use manufacturer datasheets as the final authority.
| Specification | 24LC21AT/SN | 24LC256-E/SN |
|---|---|---|
| Part Status | Active | Active |
| Memory Type | Non-Volatile | Non-Volatile |
| Memory Format | EEPROM | EEPROM |
| Technology | EEPROM | EEPROM |
| Memory Size | 1Kb (128 x 8) | 256Kb (32K x 8) |
| Memory Interface | I²C | I²C |
| Clock Frequency | 400 kHz | 400 kHz |
| Write Cycle Time - Word Page | 10ms | 5ms |
| Access Time | 900 ns | 900 ns |
| Voltage - Supply | 2.5V ~ 5.5V | 2.5V ~ 5.5V |
| Operating Temperature | 0°C ~ 70°C (TA) | -40°C ~ 125°C (TA) |
| Mounting Type | Surface Mount | Surface Mount |
Frequently asked questions
What are the primary differences between the 24LC256-E/SN and the 24LC21AT/SN?
The 24LC256-E/SN is a 256 Kbit (32 Kbyte) I2C serial EEPROM with a 2-wire interface, while the 24LC21AT/SN is a 1 Kbit (128 byte) I2C serial EEPROM designed specifically for VESA Extended Display Identification Data (EDID) applications. The 24LC256-E/SN offers significantly larger storage capacity and is intended for general-purpose non-volatile data storage, whereas the 24LC21AT/SN is optimized for monitor and display identification data storage with a fixed I2C address and a specific write-protect scheme.
Are the 24LC256-E/SN and 24LC21AT/SN pin-to-pin compatible?
Both devices are offered in an 8-pin SOIC (SN) package and share the same standard 8-pin I2C EEPROM pinout: pins 1–3 are address inputs (A0–A2), pin 4 is VSS, pin 5 is SDA, pin 6 is SCL, pin 7 is WP, and pin 8 is VCC. However, the functional assignment of the address pins and the write-protect behavior differ between the two devices, so pin-to-pin compatibility does not guarantee functional interchangeability without design review.
Can the 24LC21AT/SN be used as a direct drop-in replacement for the 24LC256-E/SN?
No. The 24LC21AT/SN provides only 1 Kbit of memory versus 256 Kbit for the 24LC256-E/SN, and its I2C addressing and write-protect architecture are tailored for EDID applications. A direct drop-in replacement would not provide the required memory capacity or general-purpose addressing flexibility, and the firmware or host controller would need to be redesigned accordingly.
What are the operating voltage and temperature ranges for these two devices?
The 24LC256-E/SN operates from 2.5 V to 5.5 V and is specified over the extended industrial temperature range of −40 °C to +125 °C. The 24LC21AT/SN operates from 2.5 V to 5.5 V and is specified over the industrial temperature range of −40 °C to +85 °C. The 24LC256-E/SN therefore offers a wider upper temperature limit.
What I2C clock frequencies do the 24LC256-E/SN and 24LC21AT/SN support?
The 24LC256-E/SN supports I2C clock frequencies up to 400 kHz (Fast mode) across its full voltage range, and up to 1 MHz is not specified for this device. The 24LC21AT/SN supports I2C clock frequencies up to 100 kHz (Standard mode) and 400 kHz (Fast mode) depending on the supply voltage, consistent with its EDID-oriented design.
How does the write-protect function differ between the 24LC256-E/SN and the 24LC21AT/SN?
The 24LC256-E/SN provides a WP pin that, when tied high, enables hardware write protection for the entire memory array. The 24LC21AT/SN uses a different write-protect scheme: it has a WP pin that controls protection of the upper half of the memory array, and the lower half (the EDID block) is typically protected by a specific I2C protocol sequence or by the WP pin depending on the configuration. The exact behavior should be verified against the respective manufacturer datasheets.
What are the typical applications for each device?
The 24LC256-E/SN is used in general-purpose non-volatile data storage applications such as parameter storage, configuration data, data logging, and code storage in consumer, industrial, and automotive systems. The 24LC21AT/SN is specifically designed for VESA EDID storage in monitors, displays, and video interfaces, where it stores identification and capability data that the host system reads over the I2C bus.
Do both devices support the same I2C addressing scheme?
No. The 24LC256-E/SN uses the A0, A1, and A2 pins to set the lower three bits of the 7-bit I2C slave address, allowing up to eight devices on the same bus. The 24LC21AT/SN has a fixed I2C slave address (typically 0xA0 for write and 0xA1 for read) and does not support multiple device addressing via hardware pins in the same way, as it is intended as a single EDID EEPROM per display interface.
What are the package and ordering designations for these two parts?
Both the 24LC256-E/SN and the 24LC21AT/SN are supplied in an 8-pin SOIC package, indicated by the "SN" suffix. The "E" in 24LC256-E/SN denotes the extended temperature range (−40 °C to +125 °C), while the "T" in 24LC21AT/SN denotes tape-and-reel packaging. The "A" in 24LC21AT/SN indicates a specific revision or variant of the 24LC21 device. Designers should confirm the exact ordering part number and its associated parameters from the manufacturer's datasheet before finalizing a design.