24Cxx I2C EEPROM: addressing, page writes and safe reading

The 24Cxx family of I2C EEPROMs stores settings, calibration data and IDs everywhere: monitor and TV mainboards, car dashboards, industrial controllers, and countless microcontroller projects. In repair you'll often read, back up or rewrite one; in design you'll use one when the microcontroller's own EEPROM isn't enough.

Pinout (DIP-8 / SOIC-8)

Device address

The 7-bit I2C address is 1010 A2 A1 A0 — 0x50 to 0x57 — followed by the read/write bit. That lets up to eight chips share one bus. The exception is the small parts: in the 24C04, 24C08 and 24C16, some of the A-bits are used as extra memory-address bits (“block select”), so fewer chips fit on a bus and one 24C16 answers on all eight addresses.

Sizes, address bytes and page sizes

The page size differs between manufacturers for the same part number — always check the datasheet of the chip that's actually on the board.

Writing

1. START, device address + W, the memory address (one or two bytes), then up to one page of data bytes, then STOP.
2. The chip now programs internally for up to 5 ms (10 ms on some older parts) and ignores the bus.
3. Instead of a fixed delay, use ACK polling: keep sending START + device address; the chip stays silent (NACK) while busy and acknowledges when it's done.

The classic bug: a write that crosses a page boundary doesn't continue into the next page — the address wraps to the start of the same page and overwrites it. Split writes at page boundaries.

Reading

A random read is a “dummy write” of just the memory address, then a repeated START with the device address + R. After that the chip streams bytes as long as the master keeps acknowledging — a sequential read can run through the whole memory, wrapping at the end.

Hardware tips

• Pull-ups of 4.7 kΩ suit 100 kHz; use around 2.2 kΩ for 400 kHz or long wiring.
• Tie WP to GND while writing, to VCC to protect data in the field.
• Reading a chip in circuit with a USB programmer (a CH341A or similar) often works, but other devices on the same bus can interfere. If readings differ between attempts, desolder the chip or isolate its VCC.
• Before changing anything, read twice, compare, and keep the backup. Mismatched reads mean a bad connection — writing in that state can brick the device.