The Microchip Technology Inc. 24LCS21A 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, a valid high-to-low transition on the SCLpin will cause the device to enter the Transition mode and look for a valid control byte on the I2C™Compatible 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 24LCS21A also enables the user to write-protect the entire memory array using its write-protect pin. The 24LCS21A is available in a standard 8-pin PDIP and SOIC package in industrial temperature range.