24C64 pins: an I2C EEPROM and WP write protection

The board's 'memory': a small store that doesn't forget when unplugged. A TV keeps its volume and channel settings and panel type here; monitors and car dashboards keep their settings. Tap a pin first, then try out how WP works below.

Where you'll meet it TV and monitor mainboards, car dashboards, printers, routers, boiler boards (family: 24C02, 24C04, 24C08, 24C16, 24C32, 24C64, 24C128, 24C256; 24LC64, AT24C64 and M24C64 share the same pinout)

Control / signalGroundInput / outputClock (crystal)Supply

Pin 1 · Control / signal

One of the address select bits. With several EEPROMs on one bus, each has its A2-A1-A0 wired differently (up to 8). On single-EEPROM boards all three are usually at GND, giving the address 0x50 (write 0xA0, read 0xA1).

🔧 How to measure

DC to GND, or continuity to GND on Ω.

Expected on a working boardUsually 0 V (tied to GND)

On the scope

⚠ If the reading is wrongIf the programmer can't find the chip, check the address pins: the chip may sit at another address.

📋 What order to measure in?

VCC (8): is 3.3 V or 5 V present?
Are SDA (5) and SCL (6) at VCC when idle? If not, the pull-ups or a short on the bus.
Do clock bursts appear on SCL at power-up? (is the processor reading the EEPROM?)
If settings aren't saved: does WP (7) drop to 0 V while saving?
Writing with a programmer: have you taken WP to GND and read a backup of the old contents first?

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How does the WP pin work in a circuit?

✍ WP (write protect): how does it work?

Choose how WP is wired, then press Write. Watch the bits travel on the I2C bus and see whether the memory cell really changes.

WP:

I2C write frame (blue: sent by the master, green: EEPROM's reply)

S1010 000 0ACK0x00ACK0x10ACK0x5AACKP5 ms

Press Write to send the frame.

Memory (read back)

0x0010

0x32

0x0011

0x0F

0x0012

0xFF

0x0013

0x00

0x0014

0x48

0x0015

0x12

0x0016

0x9C

0x0017

0x01

🔧 In repair work

TV, monitor and car-dashboard boards keep settings, panel type and hours in this chip. Before writing with a programmer (e.g. CH341A), always read and save a backup first; then take WP to GND, write, read back and compare.

🔒 This content is in the repair pack

Repair pack: 5,000 ₺, members get 10% off → 4,500 ₺. Or 1-week access for 1,000 ₺.

Go further

Lesson: the TV mainboard (processor, memory, EEPROM)ATmega328P pins