24LC21A pdf datasheet

The Microchip Technology Inc. 24LC21A is a 128 x 8-bit
dual-mode Electrically Erasable PROM. This device is
designed for use in applications requiring storage and
serial transmission of configuration and control information.
Two modes of operation have been implemented:
Transmit-Only mode and Bidirectional mode. Upon
power-up, the device will be in the Transmit-Only mode,
sending a serial bit stream of the memory array from 00h
to 7Fh, clocked by the VCLK pin. A valid high-to-low
transition on the SCL pin will cause the device to enter
the transition mode, and look for a valid control byte on
the I2C bus. If it detects a valid control byte from the
master, it will switch into Bidirectional mode, with byte
selectable read/write capability of the memory array
using SCL. If no control byte is received, the device will
revert to the Transmit-Only mode after it receives 128
consecutive VCLK pulses while the SCL pin is idle. The
24LC21A is available in a standard 8-pin PDIP and
SOIC package in industrial temperature range.