Pinout

24LC256 I2C EEPROM pinout

Pins

1: A0 — Address-select input; sets the low bit of the chip's 3-bit I2C address along with A1/A2, and must be tied to VCC or GND (never left floating).
2: A1 — Address-select input; middle bit of the address-select trio, letting up to 8 of these EEPROMs share one I2C bus at once.
3: A2 — Address-select input; high bit of the address-select trio — with all three grounded the device answers at the default 0x50.
4: VSS — Ground reference for the chip.
5: SDA — I2C serial data line, open-drain; needs an external pull-up resistor (commonly 2.2–10 kΩ) since the chip has none built in.
6: SCL — I2C serial clock line, open-drain like SDA and needing its own external pull-up.
7: WP — Write-protect input; tie to VCC to make the whole 32 KB array read-only, tie to GND for normal read/write — don't leave it floating.
8: VCC — Supply voltage, 2.5–5.5 V on the 24LC256 (the 24AA256/24FC256 variants extend down to about 1.7 V); draws a few mA when active and next to nothing in standby.

SDA and SCL are open-drain and won't work at all without external pull-up resistors — many breakout boards and MCU dev boards don't include them, so a 'dead' I2C bus is often just a missing pull-up.
Writes are buffered in 64-byte pages; a single write that crosses a page boundary wraps around and overwrites the start of the same page instead of spilling into the next one, so multi-page writes must be split in software at 64-byte boundaries.
After each write the chip needs up to about 5 ms to internally commit the data (self-timed write cycle) and won't acknowledge on the bus during that time — either wait it out or poll for an ACK before the next access.
Rated for roughly 1,000,000 write cycles per page and over 200 years of data retention, which is why it's commonly chosen over a microcontroller's internal EEPROM for data that's rewritten often.