3

AT24C256B

5080A–SEEPR–9/04

Pin Description

SERIAL CLOCK (SCL): The SCL input is used to positive-edge clock data into each

EEPROM device and negative-edge clock data out of each device.

SERIAL DATA (SDA): The SDA pin is bidirectional for serial data transfer. This pin is open-

drain driven and may be wire-ORed with any number of other open-drain or open-collector

devices.

DEVICE/PAGE ADDRESSES (A2, A1, A0): The A2, A1, and A0 pins are device address

inputs that are hardwired (directly to GND or to Vcc) for compatibility with other AT24Cxx

devices. When the pins are hardwired, as many as eight 256K devices may be addressed on

a single bus system. (Device addressing is discussed in detail under “Device Addressing,”

page 8.) A device is selected when a corresponding hardware and software match is true. If

these pins are left floating, the A2, A1, and A0 pins will be internally pulled down to GND. How-

ever, due to capacitive coupling that may appear during customer applications, Atmel

recommends always connecting the address pins to a known state. When using a pull-up

resistor, Atmel recommends using 10k

Ω or less.

WRITE PROTECT (WP): The write protect input, when connected to GND, allows normal write

operations. When WP is connected directly to Vcc, all write operations to the memory are

inhibited. If the pin is left floating, the WP pin will be internally pulled down to GND. However,

due to capacitive coupling that may appear during customer applications, Atmel recommends

always connecting the WP pins to a known state. When using a pull-up resistor, Atmel recom-

mends using 10k

Ω or less.

Memory

Organization

AT24C256B, 256K SERIAL EEPROM: The 256K is internally organized as 512 pages of 64

bytes each. Random word addressing requires a 15-bit data word address.